Invertebrates - Advanced

Douglas Wilkin, Ph.D. Jennifer Blanchette

Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required) AUTHORS Douglas Wilkin, Ph.D. To access a customizable version of this book, as well as other Jennifer Blanchette interactive content, visit www.ck12.org EDITOR Douglas Wilkin, Ph.D.

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Printed: January 24, 2016 www.ck12.org Chapter 1. Invertebrates - Advanced

CHAPTER 1 Invertebrates - Advanced

CHAPTER OUTLINE 1.1 Invertebrates - Advanced 1.2 Invertebrate Classification - Advanced 1.3 Invertebrate Evolution - Advanced 1.4 Sponges - Advanced 1.5 Sponge Structure and Function - Advanced 1.6 Sponge Reproduction - Advanced 1.7 Sponge Ecology - Advanced 1.8 Cnidarians - Advanced 1.9 Cnidarian Structure and Function - Advanced 1.10 Cnidarian Reproduction - Advanced 1.11 Cnidarian Ecology - Advanced 1.12 Flatworms - Advanced 1.13 Flatworm Classification - Advanced 1.14 Flatworm Diseases - Advanced 1.15 Roundworms - Advanced 1.16 Roundworm Classification - Advanced 1.17 Roundworm Diseases - Advanced 1.18 Mollusks - Advanced 1.19 Mollusk Classification - Advanced 1.20 Mollusk Structure and Function - Advanced 1.21 Mollusk Nervous System and Reproduction - Advanced 1.22 Mollusk Body Plans - Advanced 1.23 Mollusk Ecology - Advanced 1.24 Annelids - Advanced 1.25 Annelid Classification - Advanced 1.26 Annelid Structure and Function - Advanced 1.27 Annelid Reproduction - Advanced 1.28 Annelid Ecology - Advanced 1.29 Arthropods - Advanced 1.30 Arthropod Structure and Function - Advanced 1.31 Arthropod Growth and Development - Advanced 1.32 Arthropod Evolution - Advanced 1.33 Arthropod Classification - Advanced

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1.34 Crustaceans - Advanced 1.35 Arachnids - Advanced 1.36 Insects - Advanced 1.37 Insect Structure and Function - Advanced 1.38 Insect Flight - Advanced 1.39 Insect Reproduction and Development - Advanced 1.40 Insect Behavior - Advanced 1.41 Humans and Insects - Advanced 1.42 Echinoderms - Advanced 1.43 Echinoderm Structure and Function - Advanced 1.44 Echinoderm Classification - Advanced 1.45 Echinoderm Ecology - Advanced 1.46 Nonvertebrate Chordates - Advanced 1.47 Tunicates - Advanced 1.48 Lancelets - Advanced 1.49 Nonvertebrate Chordate Evolution - Advanced 1.50 References

Introduction

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How’d you like this staring at you? This may look like a scary creature from your worst nightmare, but it wouldn’t hurt a fly. In fact, it is a fly! The picture shows the charming portrait of a horsefly, up close and personal. Those big, striped, colorful orbs are its eyes. Did you ever look through a kaleidoscope? If so, then you have an idea of what the world looks like to a horsefly. What other organs do insects like this horsefly have? Besides sensing their environment, what other functions do their organs serve? In this chapter, you will find out. You will read not only about fly eyes. You’ll also read about octopus ink, spider fangs, and other fascinating features of invertebrates.

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1.1 Invertebrates - Advanced

• Describe the defining characteristics of invertebrates.

What adaptations occurred during invertebrate evolution? The vast majority of animals on earth today are invertebrates. They make up 97% of all animal species and all but one of the animal phyla. The invertebrate phyla include a wide array of body plans and adaptive strategies. The evolutionary changes that took place among the animals within the invertebrate phyla were extremely important steps in the development of more complex organisms.

Characteristics of Invertebrates

Invertebrates are defined as animals that do not have a spinal, or vertebral, column. Invertebrates also lack a cartilagi- nous or bony internal skeleton. Some invertebrates have either exoskeletons or internal skeletons ( endoskeletons), but they are not made of bone or cartilage. Generally, invertebrate skeletons are made of calcium carbonate or, in some cases, organic molecules such as complex carbohydrates. As members of the animal kingdom, all invertebrates are made up of eukaryotic, heterotrophic cells that do not have a cell wall. Beyond that feature, the physical characteristics of invertebrates vary widely. They include such diverse species as jellyfish and tarantulas, both of which are shown in the Figure 1.1.

Digestion

Invertebrates have two general types of digestive systems. They either have a digestive cavity with one opening or a digestive tract with two openings. In the first case, the opening serves as both the mouth and the anus. This is the case with the earliest invertebrate phyla, such as Porifera (which contains sponges) and Cnidaria (which contains corals and jellyfish). These two phyla will be discussed in more detail in additional concepts. A digestive tract with two openings uses one opening for the mouth and the other for the anus, allowing food to be processed in stages as it progresses along the tract. Many invertebrate phyla have a digestive tract with two openings.

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FIGURE 1.1 A jellyfish, also known as a sea jelly (left), and a tarantula (right) are members of the phyla Cnidaria and Arthropoda respec- tively. These organisms demonstrate the large diversity of invertebrate species.

Movement

All invertebrates are capable of movement during at least one of their stages in life. There is a great deal of variation in the types and complexity of these movements. For some invertebrates, movement consists of a rhythmic pulse that propels the animal along water currents in the ocean. These animals, such as jellyfish, are not able to move with any real sense of direction. Invertebrates with a worm-like body plan, such as roundworms, can usually move in a directed manner, either forward or backward. This movement does not depend on water currents. Some invertebrates have appendages that allow them to exhibit very precise movement in all directions, including the ability to navigate an uneven surface.

Nervous System

Most invertebrates have a nervous system that allows them to detect aspects of their environment and elicit responses to external stimuli. In some cases, this consists of a simple nerve net throughout the body wall of the animal that can detect touch. The Figure below shows a diagram of coral, illustrating the nerve net lining the body wall. However, many invertebrates have primitive brains that are capable of much more sensitive perception. There are even some invertebrates, such as octopi and squids, that have both complex eyes capable of forming images and a well-developed brain.

FIGURE 1.2 The nervous system in invertebrates.

Reproduction

Although invertebrates are all capable of sexual reproduction, most phyla include species that are also capable of asexual reproduction. Unlike sexual reproduction, asexual reproduction does not involve the formation or fusion of gametes. One type of asexual reproduction is called fission. In this process, the animal simply divides itself into two parts. Each part then regenerates the missing region to form a whole organism. Another method of asexual reproduction is called budding. In budding, the parent usually forms a small protrusion that remains attached to the parent while developing into a new individual. In some colonial invertebrates, the new individuals may remain attached to the parent throughout their lifetime, forming large interconnected colonies.

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Vocabulary

• asexual reproduction: Reproduction involving only one parent; it occurs without a fusion of gametes and produces offspring that are all genetically identical to the parent.

• budding: A form of asexual reproduction in which a new organism develops from an outgrowth, or bud, on another one; the bud may stay attached or break free from the parent.

• endoskeleton: An internal skeleton that provides support and protection.

• exoskeleton: A non-bony skeleton that forms on the outside of the body of some invertebrates and provides protection and support.

• fission: Asexual reproduction in which a parent separates into two or more individuals of about equal size.

• invertebrate: An animal that lacks a vertebral column, or backbone.

• nerve net: The primitive nervous system of cnidarians; it is non-centralized and consists of interconnected neurons spanning the body wall.

• sexual reproduction: Reproduction involving the joining of haploid gametes, producing genetically diverse individuals.

Summary

• Invertebrates are animals that do not have a spinal column, or backbone. In addition, invertebrates lack an internal cartilaginous or bony skeleton. • As members of the animal kingdom, all invertebrates are made up of eukaryotic, heterotrophic cells that do not have a cell wall. • All invertebrates have digestive systems and most have nervous systems as well.

Practice

Use this resource to answer the questions that follow.

• Facts About Invertebrates at http://animals.about.com/od/invertebrates/a/tenfactsinvertebrates.htm .

1. Why is it so hard to determine when invertebrates first evolved? 2. What comprises the largest group of invertebrates? 3. What is the simplest type of animal?

Practice Answers

1. The first animals had soft bodies and, for this reason, they left little fossil evidence of their existence. Scientists have discovered fossilized burrows and tracks in sediments that date back nearly 1 billion years. The oldest fossil of an invertebrate dates back to the late Precambrian, about 600 million years ago. 2. Insects comprise the largest group of invertebrates. Scientists estimate that there are 1 to 30 million species of insects. 3. Sponges, a type of invertebrate, are the simplest type of animal. Although they have specialized cells, they do not form true tissues like other animals.

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Review

1. Describe the major defining characteristic of invertebrates. 2. How prevalent are invertebrates? 3. Give two examples of asexual reproduction. 4. What is one of the most primitive nervous systems that are present in invertebrates?

Review Answers

1. Invertebrates are defined as animals that do not have a spinal, or vertebral, column. Invertebrates also lack a cartilaginous or bony internal skeleton. 2. The vast majority of animals on earth today are invertebrates. They make up 97% of all animal species and all but one of the animal phyla. 3. Fission is one type of asexual reproduction, where the animal simply divides itself into two parts. Each part then regenerates the missing region to form a whole organism. Budding is another form of asexual reproduction, where the parent usually forms a small protrusion that remains attached to the parent while developing into a new individual. 4. The most simple nervous system is a nerve net that is non-centralized and consists of interconnected neurons spanning the body wall.

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1.2 Invertebrate Classification - Advanced

• Differentiate between the different phyla of invertebrates, and give a brief overview of each.

With so many differences, how do you classify an invertebrate? Sponges, jellyfish, worms, snails, and squids. These are just a few of the many types of invertebrates. But there are lots more. In fact, most animals are invertebrates, so their classification is very important.

Classification of Invertebrates

There are over 30 phyla dedicated to invertebrates. All but one of the animal phyla are exclusively invertebrates. The phylum Chordata is divided into three subphyla, two of which are made up of invertebrates. The third subphylum, Vertebrata, includes the vertebrate animals. In addition, there are a great many invertebrate species that are extinct. Evidence of their existence is found only in the fossil record. The major exclusively invertebrate phyla that are still in existence today are the following:

TABLE 1.1:

Phylum (includes) Notable Characteristics Example Porifera (sponges) multicellularity, specialized cells sponges but no tissues, asymmetry, incom- plete digestive system Cnidaria (jellyfish, corals) radial symmetry, true tissues, in- jellyfish complete digestive system

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TABLE 1.1: (continued)

Phylum (includes) Notable Characteristics Example Platyhelminthes (flatworms, tape- cephalization, bilateral symmetry, flatworm worms, flukes) mesoderm, complete digestive sys- tem Nematoda (roundworms) pseudocoelom, complete digestive roundworm system Mollusca (snails, clams, squids) true coelom, organ systems, some snail with primitive brain Annelida (earthworms, leeches, segmented body, primitive brain earthworm marine worms) Arthropoda (insects, spiders, crus- segmented body, jointed insect (dragonfly) taceans, centipedes) appendages, exoskeleton, brain Echinodermata (sea stars, sea complete digestive system, coelom, sea urchin urchins, sand dollars, sea cucum- spiny internal skeleton bers)

The organization of invertebrates into each of these phyla is based on their evolutionary relationships to each other. Within each phyla, the organisms share certain traits and a certain level of structural organization. Generally, this organization becomes increasingly complex as invertebrate species diverged and new phyla were formed. The details of this increased complexity will be discussed in the Invertebrates: Evolution (Advanced) concept. In this concept we will summarize the types of organisms found within each phylum and briefly list the features that distinguish them from species of other phyla. Each of these phyla is further discussed in later concepts.

Porifera and Cnidaria

The phylum porifera contains the earliest invertebrates: the sponges. Sponges lack true tissues. Instead of tissues, sponges have specialized cells that carry out functions such as digestion and reproduction. Sponges are both extremely simple organisms and very well-adapted in an evolutionary sense. They have been around in a similar form for well over 600 million years. The phylum cnidaria includes jellyfish, hydrozoans, and corals. They are radially symmetrical and have true tissues. Many cnidarian species are critical members of the vast coral reefs found in tropical marine regions. An example of a sponge species is shown in the Figure 1.3.

Platyhelminthes and Nematoda

Flatworms make up the phylum platyhelminthes. They develop from three embryonic cell layers called germ layers and exhibit bilateral symmetry. Both flatworms and the roundworms of the phylum nematoda include many parasitic species, a number of which are infectious to humans or livestock. These two phyla therefore have an enormous impact on the health and economy of humans. Unlike flatworms, roundworms have an incomplete body cavity and a complete digestive tract. The Figure 1.4 shows examples of a non-parasitic flatworm and a parasitic roundworm species.

Annelida and Arthropoda

Annelida consists of segmented worms, such as the familiar earthworm and leeches. They exhibit a closed circu- latory system, a nervous system with a primitive brain, and a specialized digestive system. The closed circulatory system pumps blood throughout the length of the worm’s body. Annelids generally have a well-developed body cavity, an excretory system, and a nervous system with a primitive brain.

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FIGURE 1.3 A sponge in its natural marine environ- ment.

FIGURE 1.4 A parasitic roundworm (left) and a non- parasitic flatworm (right).

Arthropoda is an extremely large phylum consisting of over 80% of all species alive on earth today. They include insects, arachnids, and crustaceans. Arthropods have segmented bodies with jointed appendages and an open circulatory system with several hearts. Some species within the phylum arthropoda were the first animals to leave the aquatic environment of their ancestors and venture onto land. Arthropods have a tough exoskeleton made of a complex carbohydrate called chitin, and some species have gills for gas exchange. Arthropods also have an excretory system and a nervous system with a primitive brain. Members of the phyla annelida and arthropoda are shown in the Figure 1.5.

Mollusca and Echinodermata

The phylum Mollusca is a highly diverse phylum that includes clams, octopi, and squids. One distinguishing characteristic of mollusks is the presence of a muscular foot called a mantle that can be used for locomotion. Many mollusks have a solid exoskeleton made of calcium carbonate. The exoskeleton is secreted by the mantle. Another

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FIGURE 1.5 An arthropod species (left) and an an- nelid, the earthworm (right).

unique feature of mollusks is a specialized feeding structure called the radula that is located within the mouth. The radula contains teeth made of chitin that are used to chew or scrape food. One class of mollusks that includes the squid and octopus has the most highly evolved nervous system of all invertebrates. Echinodermata is considered the closest related phylum to Chordata. Like chordates, echinoderms have a type of embryonic development in which the opening for the anus is formed prior to the opening that becomes the mouth. The characteristic feature of echinoderms is a spiny surface. They have an internal skeleton made of calcium spines that lies underneath a thin ectodermal layer of tissue. Another interesting trait of echinoderms is that they are bilaterally symmetrical in their juvenile stages but then develop into radially symmetrical adults. In addition, echinoderms have a vascular system that pumps water instead of blood. Echinoderms include sea stars (starfish), sea urchins, and sand dollars. Examples of mollusks and echinoderms are shown in the Figure 1.41.

FIGURE 1.6 This figure shows some of the more com- mon and familiar mollusks.

Vocabulary

• bilateral symmetry: A body plan of an organism with a distinct head and a distinct tail region; a cut along the middle of the anterior-posterior axis divides the animal into two equal halves.

• chitin: The tough carbohydrate (polysaccharide) that makes up the cell walls of fungi and the exoskeletons of insects, crustaceans, and other arthropods.

• closed circulatory system: A circulatory system in which the blood is enclosed at all times within vessels.

• germ layer: A group of cells formed during animal embryogenesis which eventually gives rise to all of an animal’s tissues and organs; the germ layers are the ectoderm, endoderm, and mesoderm.

• mantle: A distinguishing characteristic of mollusks; a dorsal layer of tissue that lies between the body and the shell which secretes layers of calcium carbonate that form the shell; it is also a muscular foot that can be used for locomotion.

• radial symmetry: Symmetry of a body plan in which there is a distinct top and bottom but no distinct head or tail ends, so the body can be divided into two halves like a pie.

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• radula: A specialized feeding structure located within the mouth of mollusks; it contains teeth made of chitin that are used to chew or scrape food.

Summary

• There are over 30 phyla dedicated to invertebrates. • Major invertebrate phyla that you should know include porifera, cnidaria, platyhelminthes, nematoda, mol- lusca, annelida, arthropoda, and echinodermata.

Practice

Use this resource to answer the questions that follow.

Practice Answers

Review

1. Which phylum contains the earliest invertebrates? 2. Which two phyla have an enormous impact on human health? 3. How large is the arthropoda phylum? 4. Which phylum of invertebrates is the closest related to chordates?

Review Answers

1. The phylum porifera contains the earliest invertebrates: the sponges, which lack true tissues. 2. The phyla platyhelminthes and nematoda contain many parasitic species that infect humans or livestock. 3. Arthropoda is an extremely large phylum consisting of over 80% of all species alive on earth today. 4. Echinodermata is considered the closest related phylum to Chordata. Like chordates, echinoderms have a type of embryonic development in which the opening for the anus is formed prior to the opening that becomes the mouth.

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1.3 Invertebrate Evolution - Advanced

• Learn about the different evolutionary steps taken from one phyla of invertebrates to another. • Understand how these evolutionary steps made strides toward ever more complex animals.

A red sea urchin. Why did this animal evolve? Sea urchins are members of the phylum echinodermata. They have features such as a complete digestive system, a coelom, and a spiny internal skeleton, all of which are important steps in evolution. Yes, those spines are part of an internal skeleton.

Invertebrate Evolution

The evolution of invertebrates from the earliest sponge species to the more recent echinoderms established a number of fundamental features of higher organisms. The organization of the eight major phyla of invertebrates into a phylogenetic tree is shown in the Figure 1.7. Each branch point in the tree represents a significant evolutionary development. These developments led to an overall step-by-step increase in the level of complexity of invertebrates.

True Tissue

The first multicellular animals, the sponges, exhibit a very simple level of organization. Sponges do not possess true tissues or organs. This is referred to as parazoa. Instead, sponges have what is called cellular-level organization. In cellular-level organization, specialized cells carry out certain functions within the organism, such as digestion and reproduction. The phylum Cnidaria contains what are considered the first eumetazoans, meaning that they possess true tissues. A tissue is a group of similarly structured cells that all work together to carry out a particular function. This is a more complex level of organization, termed tissue-level organization, than the cell-level organization seen with the sponges. The development of primitive tissues is the first step in the development of complex organ systems, such as

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FIGURE 1.7 This phylogenetic tree shows where dif- ferent phyla split off and the different char- acteristics that each developed.

the mammalian respiratory system. Cnidaria were also the first animals to show symmetry in their body plan. They are radially symmetrical which means that if they are divided into two halves from top to bottom, those two halves will be identical no matter where the division is made. Finally, cnidarians are diploblasts meaning that all of their tissues develop from two primary embryonic tissue layers called germ layers. These two layers are the ectoderm (outer layer) and the endoderm (inner layer). The presence of only two germinal tissue layers limits the number of distinct tissues that can be formed within an organism.

FIGURE 1.8 Symmetry in Invertebrates. Sponges lack symmetry. Radial symmetry evolved first. This was followed by bilateral symmetry. How do the two types of symmetry differ?

Bilateral Symmetry and Cephalization

The next major evolutionary event within the invertebrate phyla is the transition from radial to bilateral symmetry. This is first observed in the various worm phyla. A radially symmetrical animal often has a clearly discernible top and bottom. In the case of cnidarians, the bottom can be distinguished in some life stages by the presence of the mouth. However, each “side” of the organism is exactly the same. This makes it difficult for the animal to sense direction. In contrast, a bilaterally symmetrical animal has a distinct head and a distinct tail region. The head region is referred to as the anterior end, and the tail region is referred to as the posterior end. Bilateral symmetry means that a cut along the middle of the anterior-posterior axis is the only way to divide the animal into two equal halves. Bilaterally symmetrical animals are able to distinguish forward from reverse. The differences between asymmetry (sponges), radial symmetry (cnidarians), and bilateral symmetry (worms) are illustrated in Figure 1.8.

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As bilateral symmetry evolves, invertebrate species begin to show a concentration of nervous tissue in the anterior region. This is called cephalization. The process of cephalization occurred slowly over time but it ultimately leads to the formation of the brain encased within a true head. This is a prominent feature of vertebrates within the phylum chordata.

The Mesoderm

Another evolutionary advancement that arose with bilateral symmetry was the presence of a third tissue, or germ layer, called the mesoderm. The mesoderm lies between the ectoderm and the endoderm.

FIGURE 1.9 The three layers of tissue, or germ layers, are illustrated.

Animals with three germ layers are referred to as triploblasts. A number of tissues are derived from the mesoderm, including muscles and the tissues of the vascular system in higher animals. The evolution of the mesoderm is an important step towards the establishment of these organ systems.

FIGURE 1.10 Three Cell Layers in a Flatworm. A flat- worm has three cell layers.

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Complete Digestive System

Nematodes were some of the first invertebrates to have a complete digestive system with separate openings for the mouth and anus. Porifera, Cnidaria, and Platyhelminthes all have a digestive cavity with a single opening that serves as both a mouth and an anus. The presence of separate openings allows specialization of regions along the digestive tract.

Body Cavity: Coelom

The first animals to show a well-developed coelom were the annelids. The coelom is a fluid-filled cavity that forms between the digestive cavity and the body wall. The cavity is lined on all sides by mesodermal tissue. It is important to realize that the coelom, or body cavity, is distinct from the digestive cavity. The Figure 1.11 illustrates the relative position of the coelom and the digestive cavity.

FIGURE 1.11 Cross Section of an Invertebrate with a coelom. The coelom forms within the mesoderm.

The fluid-filled space of the body cavity allows room for internal organs to develop. The organs suspended within the coelom are often attached to the mesodermal lining, allowing them to remain in a particular place within the body. When the animal is moving, the cushioning of the fluid in the cavity protects the organs. The pressure from this fluid also helps soft-bodied invertebrates hold their shape. This is referred to as a hydrostatic skeleton. The hydrostatic skeleton plays an important role in the movement of many invertebrates. Vertebrates contain an internal skeleton and their movements are mediated by the force of muscle contractions on the attached skeletal components. With a hydrostatic skeleton, the contractile force of muscles surrounding the body cavity are opposed by the hydrostatic pressure of the fluid within that cavity. The opposing forces cause shape changes in the animal that produce movement. This evolutionary adaptation allowed coelomates, organisms with a true coelom, to move in an efficient and coordinated fashion without the benefit of an internal, bony skeleton.

Protostome vs. Deuterostome

The bilateral phyla are divided into two groups based on their patterns of embryonic development: protostomes and deuterostomes. See http://www.mun.ca/biology/scarr/Protostomes_vs_Deuterostomes.html for additional in- formation. The characteristics of protostomes and deuterostomes are summarized in the Table Protosomes vs. Deuterostomes.

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TABLE 1.2: Protostomes vs. Deuterostomes

Protostomes Deuterostomes

Spiral cleavage Radial cleavage

Determinant cleavage Indeterminant cleavage

Blastopore = mouth Blastopore = anus

Schizocoelous Enterocoelous

In the first few cell divisions of a developing embryo, protostomes generally exhibit a type of cell division called spiral cleavage, while deuterostomes exhibit radial cleavage. Radial cleavage means that the cell divisions occur parallel to or perpendicular to the axis of the embryo. Spiral cleavage means that the cell divisions occur at an angle to the axis of the embryo. This is shown more clearly in the Figure 1.12. Spiral cleavage is considered to be determinant, meaning that the fate of each cell is fixed as it is formed. In contrast, radial cleavage is considered to be indeterminant, and the fate of a cell can change under certain circumstances. Indeterminant cleavage also means that each cell retains the ability to form a whole organism during these early cell divisions.

FIGURE 1.12 This diagram shows a comparison of cell cleavage at the 8 cell stage embryo and mesoderm formation later in embryogen- esis between protostomes and deuteros- tomes. In the drawings of early embry- onic cleavage, the black line indicates the axis of the embryo prior to cleavage. In deuterostomes cell divisions normally take place along this axis. In protostomes the cell divisions take place at an angle from the embryo axis, indicated in this figure by a red dashed line. This results in an embryo that looks spirally twisted. The bottom drawings show mesoderm forma- tion. In protostomes, the mesoderm de- velops from a cluster of cells between the endoderm and the ectoderm. This cluster then divides in the middle to form an opening that becomes the coelom. This is called schizocoelous. In deuterostomes, the mesoderm forms from invaginations of pouches of endodermal tissue. This is termed enterocoelous.

A second distinction between protostomes and deuterostomes is the formation of the mesoderm. In protostomes,

17 1.3. Invertebrate Evolution - Advanced www.ck12.org the mesoderm forms from a group of cells between the endoderm and the ectoderm. This mass of cells then divides down the middle to form the body cavity. The mesoderm of deuterostomes forms from invaginated pouches of endodermal tissue. This is depicted in the Figure 1.12. The third major distinction between protostomes and deuterostomes concerns the fate of the blastopore. As animal embryos develop, they go through several stages, two of which are blastula formation and gastrulation. A blastula is a hollow ball of cells. During gastrulation, one area of this ball invaginates to form a pocket inside of the blastula. The opening formed by this invagination is called the blastopore. In protostomes, the blastopore eventually becomes the mouth of the organism, while in deuterostomes the blastopore becomes the anus, and a second opening formed later in development becomes the mouth. Platyhelminthes, mollusca, annelida, nematoda, and arthropoda are all protostomes. Echinodermata and chordata are deuterostomes.

Ecdysozoa and Lophotrochozoa

As we explored in the Animals: Classification concept, there are two general approaches to categorizing organisms: traditional and molecular. The introduction of molecular methods has led to some changes in our understanding of the evolutionary relationships between a number of invertebrate phyla. One example of this is seen with the phyla annelida and arthropoda. Earlier classifications considered these two groups to be closely related based on their segmented body plans. However, the information gathered from molecular studies of phylogeny has made it clear that these two groups are more distantly related. It is likely that their segmented body plans evolved separately, an example of convergent evolution. The fact that a segmented body evolved independently multiple times suggests a strong evolutionary advantage. One advantage of a segmented body is that it allows more flexible movement, particularly in an animal with a solid skeleton. Flexibility around the joint areas connecting the segments facilitates this greater ease of movement. The most recent classification schemes divide protostomes into two major groups: ecdysozoa and lophotrochozoa. The ecdysozoa includes the phyla nematoda and arthropoda. The species within these phyla have a thick organic layer called a cuticle which surrounds the outer surface of their bodies. The cuticle serves as a protective layer, however, it is not able to grow or stretch as the animal increases in size. As the animal grows, it sheds the cuticle in a process called molting. This process is shown in the Figure 1.13. Platyhelminthes, mollusca, and annelida make up the lophotrochozoa. Although members of this group may have one of the two features not seen in the ecdysozoa, namely a specialized larval stage called a trochophore larva or a specialized feeding organ called a lophophore, there is no single physical feature that unites these three phyla. This explains why traditional classification did not identify their close evolutionary relationships. However, molecular data strongly suggest that they are close relatives with a shared evolutionary history. In this lesson we have outlined the invertebrate phyla within the animal kingdom, highlighting some of the critical evolutionary advances that were achieved throughout invertebrate evolution. In the following concepts, we will examine each of these phyla in more detail to gain a better understanding of the incredible world of invertebrates.

Vocabulary

• bilateral symmetry: A body plan of an organism with a distinct head and a distinct tail region; a cut along the middle of the anterior-posterior axis divides the animal into two equal halves.

• cephalization: A concentration of nervous tissue in the anterior region.

• coelom: A fluid-filled cavity that forms between the digestive cavity and the body wall, lined on all sides by mesodermal tissue.

• cuticle: A thick organic layer surrounding the outer surface of nematodes and arthropods; a waxy waterproof covering over the aerial surfaces of a plant.

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FIGURE 1.13 A cicada shedding its cuticle through the process of molting.

• deuterostome: An animal in which the first opening formed during development (the blastopore) becomes the anus.

• diploblast: An animal that develops from two basic germ layers: an ectoderm, or outer layer, and an endo- derm, or inner layer.

• eumetazoan: Possess true tissues.

• hydrostatic skeleton: A structure consisting of a fluid-filled cavity, the coelom, surrounded by muscles; this skeleton is used to change an organism’s shape and produce movement.

• mesoderm: The third tissue or germ layer; it lies between the ectoderm and the endoderm and develops into cells such as muscles, bones, teeth, and blood.

• molting: Shedding of the cuticle.

• parazoa: Without true tissues or organs; a very simple level of organization which is characteristic of sponges.

• protostome: An animal in which the first opening formed during development (the blastopore) becomes the mouth.

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• radial cleavage: The initial cell divisions that occur parallel to or perpendicular to the axis of the embryo.

• spiral cleavage: The initial cell divisions that occur at an angle to the axis of the embryo.

• triploblast: Animal with three germ layers: an endoderm, a mesoderm, and an ectoderm.

Summary

• The evolution of invertebrates from the earliest sponge species to the more recent echinoderms established a number of fundamental features of higher organisms. • The development of primitive tissues was the first step in the development of complex organ systems, such as the mammalian respiratory system. • Bilateral symmetry allows animals to distinguish direction better. • Nematodes were some of the first invertebrates to have a complete digestive system with separate openings for the mouth and anus. • The bilateral phyla are divided into two groups based on their patterns of embryonic development: protostomes and deuterostomes. • Molecular methods have discovered new relationships between different invertebrates.

Practice

Use this resource to answer the questions that follow.

Practice Answers

Review

1. What are parazoa and what phylum is considered to be the first eumetazoans? 2. What is a limitation of being a diploblast? What animals are diploblasts? 3. What advantages does bilateral symmetry give? What happened to the nervous tissue as bilateral symmetry evolved? 4. Which phylum evolved the first complete digestive system? 5. In humans, movement is achieved by the force of muscle contractions on the attached skeletal components. How do soft-bodied invertebrates move? 6. Are humans protostomes or deuterostomes? 7. How has molecular methods of classifying animals changed scientists’ conceptions about different phyla?

Review Answers

1. Parazoa are animals without true tissue or organs (characteristic of sponges). Cnidaria were the first eumeta- zoans that possed true tissue. 2. A diploblast’s tissue develops from just two primary embryonic tissue layers (germ layers). The presence of only two germ tissue layers limits the number of distinct tissues that can be formed within a diploblast. Cnidarians are diploblasts. 3. Bilaterally symmetric animals are able to distinguish forward and reverse. Radially symmetric animals often don’t have this sense of direction. As bilateral symmetry evolved, animals began the process of cephalization which ultimately led to the formation of a brain. 4. Nematodes were some of the first invertebrates to have a complete digestive system with separate openings for the mouth and anus. Porifera, Cnidaria, and Platyhelminthes all have a digestive cavity with a single opening that serves as both a mouth and an anus.

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5. Soft-bodied invertebrates have a hydrostatic skeleton that helps hold their shape and aid in movement. The contractile force of muscles are opposed by the hydrostatic pressure of the fluid located in the cavities that form the hydrostatic skeleton. 6. Humans are part of the chordata phylum, so they are deuterostomes. Platyhelminthes, mollusca, annelida, nematoda, and arthropoda are all protostomes. Echinodermata and chordata are deuterostomes. 7. Scientists used to believe that annelida and arthropoda were closely related because of their segmented bodies, but molecular methods have proved that they are more distantly related. Molecular methods have also proven that platyhelminthes, molluscaa, and annelida are closely related even though they do not share many physical features.

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1.4 Sponges - Advanced

• Describe the general characteristics of sponges and how the various classes of sponges are distinguished.

So what exactly is a sponge? Here we have a giant pink barrel sponge adorned with tube sponges and gorgonian sea fans. How can something that looks like that be considered an animal? Where’s the head? Where are the legs? Where’s the mouth? Sponges are aquatic, invertebrate animals that make up the phylum Porifera. The word Porifera means “pore- bearing,” and a highly porous body is one of the most striking features of sponges. Sponges are the simplest multicellular animals found in the fossil record. It is difficult to imagine that an organism as complicated as a human being could be related to such a remarkably simple animal. However, sponges represent an essential step in the evolution of complex animals: the transition from simple protists to multi-celled, complex animals. In this concept we will consider the characteristics and classification of sponges, their specific structural features, how they reproduce, and the environments that they inhabit.

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Characteristics and Classification of Sponges

The phylum Porifera contains many beautifully colored sponge species (see http://www.pbs.org/kcet/shapeoflife/ animals/porifera.html ) that range in size from one centimeter (about the width of a pinky finger) to over one meter (about the arm span of a human) in diameter. An example of a sponge is shown in the Figure 1.14. Sponges arose roughly 500 million years ago, and there are currently over 5,000 different species. Adult sponges are sessile, meaning that they are not able to move from place to place. This characteristic makes sponges seem superficially plant-like, but sponges do not share other features of plants. In particular, sponges are not capable of photosynthesis. See The First Animal at http://www.pbs.org/kcet/shapeoflife/episodes/origins.html for an introduction to sponges.

FIGURE 1.14 The sponge species Aplysina aerophoba.

Sponges do not have organs or true tissues, however, they do have specialized cells that can carry out distinct functions within the organism. This is generally referred to as cellular-level organization. Cell specialization is one of the major advantages that multicellular animals, or metazoans, have over single-celled organisms. This is the first step in the evolution of tissue and organ systems, such as the muscular and nervous systems. These developments ultimately allow higher organisms to have complex interactions with their environments. There are three classes within the phylum porifera: Calcerea, Desmospongia, and Hexactinellida. Sponges are divided into these classes based primarily on the composition of their spicules and skeletal fibers. Spicules are rod- shaped cellular projections that make up the skeleton of sponges. Sponges within the class Calcerea have skeletal spicules made up of calcium carbonate. Species within the class Hexactinellida are also referred to as glass sponges because their skeletons consist of spicules made of silica, the primary component of glass. The skeletons of the class Desmospongia are composed of spicules made up of silica and skeletal fibers made from spongin, a type of collagen protein. Desmospongia is the most abundant class of sponges alive today. More than 90% of all known sponge species are found within the class desmospongia. Because their skeletons are often composed primarily of spongin fibers that are less rigid than spicules, it is desmospongia species that have been used to make the cleaning sponges we commonly think of when we hear the word “sponge.” This will be discussed in greater detail in the Sponges: Ecology (Advanced) concept. Sponges are filter-feeders that pump water in via their porous surface and through a system of internal canals where bacteria and nutrients can be trapped and digested.

Vocabulary

• metazoan: A multicellular animal.

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FIGURE 1.15 Stove-pipe Sponge (Aplysina archeri) and corals on a tropical reef in Bonaire, Netherlands Antilles.

FIGURE 1.16 Yellow Tube Sponges.

FIGURE 1.17 Colorful red finger sponge and brown tube sponges on Belize reef.

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• sessile: Not able to move from place to place.

• spicule: Rod-shaped cellular projections that make up the skeleton of sponges.

• spongin: A type of collagen protein found in the skeletal fibers of the class Desmospongia.

Summary

• Sponges are the simplest multicellular organisms, classified by their primitive cellular-level of organization, their porous bodies, and their filter-feeding system. • Sponges have specialized cells that can carry out distinct functions within the organism. • Sponges are divided into three classes based primarily on the composition of their spicules and skeletal fibers.

Practice

Use this resource to answer the questions that follow.

• Proifera sponges at http://animaldiversity.ummz.umich.edu/site/accounts/information/Porifera.html .

1. What function do choanocytes perform within sponges? 2. What are the canals and pores in sponges called? 3. Where are sponges usually found?

Practice Answers

1. Choanocytes have flagella that move water through the sponge’s pores. 2. The system of pores and canals in a sponge is known as the ostia. 3. Sponges are found in virtually all aquatic habitats, although they are most common and diverse in marine environments.

Review

1. How many different species of sponges are there? How are they characterized? 2. How are sponges divided into different classes? 3. What class of sponges is most common?

Review Answers

1. There are currently over 5,000 different species of sponges. Sponges are the simplest multicellular organisms, classified by their primitive cellular-level of organization, their porous bodies, and their filter-feeding system. 2. Sponges are divided into three classes based primarily on the composition of their spicules and skeletal fibers. 3. More than 90% of all known sponge species are found within the class Desmospongia. Because their skeletons are often composed primarily of spongin fibers that are less rigid than spicules, it is desmospongia species that have been used to make the cleaning sponges we commonly think of when we hear the word “sponge.”

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1.5 Sponge Structure and Function - Advanced

• Identify specific structures of sponges and how they function.

Does it just sit there, or does it do something? As an animal, a sponge must "do something" to stay alive. The sponge is constantly obtaining energy and nutrients. Notice the simple structure of this animal. How does it eat?

Structure and Function in Sponges

Sponges are diploblasts meaning that they develop from two basic germ layers: an ectoderm, or outer layer, and an endoderm, or inner layer. Most sponges are asymmetric. Asymmetry means that if the animal is divided into two halves along any axis, the halves will not be equal or identical. In this concept we will consider sponge structure and examine the functions of the major types of specialized sponge cells.

Body Plan

There are three different body plans found among sponges, and they are depicted in the Figure 1.18. The main difference between each body plan is the complexity of the canal system that pumps water through the animal. The most basic body plan is called asconoid. In asconoid sponges the two major cell layers surround a fluid-filled cavity called the spongocoel, the large central cavity of sponges . Water is pumped directly through pores, called ostia, into the spongocoel and then out of the sponge through an opening called the osculum (plural oscula). The spongocoel is lined with specialized digestive cells called choanocytes that filter and take in food. Synconoid is a more complex body plan. In synconoid sponges the ostia lead to a network of canals that are lined with choanocytes. Water is pumped into the ostia and through these canals before arriving at the spongocoel. There are no choanocytes lining the spongocoel of synconoid sponges so digestion takes place in the canals. The most complex sponge body plan is called leuconoid. In these sponges the canal system forms a more elaborate branched network, and the

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FIGURE 1.18 These drawings show cross-sections of asconoid, synconoid, and leuconoid sponge body walls. Water flows through the ostia/ostium into the spongocoel.

canals lead to digestive chambers instead of a spongocoel. In leuconoid sponges the choanocytes line the digestive chambers and not the canals. Once water has passed through the digestive chambers it is released into an exit canal that leads to the osculum. There is no real spongocoel in leuconoid sponges. One feature that is common to all three types of body plan is the presence of a holdfast at the base of each animal. The holdfast is what the sponge uses to anchor itself to a solid surface, such as a rock. This prevents the sponge from being transported by water currents.

Specialized Cell Types

Sponge feeding is critically dependent on several specialized cells within the organism. The organization of various cell types within the sponge body wall is depicted in the Figure 1.19. One of the main digestive cell types is the choanocyte. These cells are commonly called “collar cells” because they have a collar of microvilli at the cell surface that is used to trap food particles flowing through the organism. Microvilli are long, thin extensions of a cell’s outer membrane. In the middle of this collar of microvilli is a large flagellum projection. The flagella are long, whip-like structures that move back and forth to create a flow of current through the sponge body. Once bacteria and other food particles have been trapped by the ciliated collar, the choanocytes engulf them and digest them. Some of the engulfed material may be distributed to another type of cell called an amoebocyte. The amoebocyte assists in the digestive process and in the distribution of nutrients to other cells of the body. Amoebocytes are also totipotent, meaning that they can change into other sponge cell types. In some species, amoebocytes are able to become egg or sperm cells for sexual reproduction. Pinacocytes are the epidermal or “skin” cells on the outer surface of sponges. Porocytes are cylindrical cells that make up the pores or ostia. Water enters the animal through the pores formed by these cells. The flow of water into the sponge body is not only crucial for feeding and digestion, but also for circulation within the sponge. Since sponges do not have organ systems they do not posses a respiratory or circulatory system. They obtain oxygen by diffusion from the water flowing through their bodies, and waste is expelled by diffusion into the same pool of water. As mentioned above, sponges are diploblasts and consist of essentially two cell layers. Between these two layers there is a gelatinous substance called the mesohyl. This matrix encases the sponge’s skeletal elements as well as scattered amoebocytes. Sponge skeletons are made up of hard, rod-like projections called spicules and a protein called collagen. As discussed in Sponges: Characteristics and Classification, sponge classes are based on the composition of the spicules. Spicules made of calcium carbonate or silica are secreted by cells called sclerocytes. Skeletal elements made of spongin are often referred to as spongin fibers because they are more flexible than calcium or silica spicules. Spongin is a protein and it is secreted by cells called spongocytes.

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FIGURE 1.19 Sponge Anatomy. A sponge lacks tissues and organs, but it has several types of specialized cells.

FIGURE 1.20 Collar Cell. The collar cells of sponges trap and digest food.

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Although sponges have no muscle tissue and are sessile organisms, they do have muscle-like cells called myocytes. Myocytes surround canal openings and porocytes. These cells are able to contract in order to regulate water flow through the body. For an interactive tour of sponge structure, visit http://www.phschool.com/atschool/phsciexp/active_art/structure_o f_a_sponge/index.html .

Vocabulary

• amoebocyte: A cell that assists in the digestive process and in the distribution of nutrients to other cells of the sponge body.

• asconoid: The most basic body plan of a sponge; the two major cell layers surround a fluid-filled cavity called the spongocoel.

• choanocytes: Specialized digestive cells that filter and take in food; they line the spongocoel and are also known as collar cells.

• diploblast: An animal that develops from two basic germ layers: an ectoderm, or outer layer, and an endo- derm, or inner layer.

• ectoderm: The outer embryonic cell layer in animals; it develops into cells such as nerves, skin, hair, and nails.

• endoderm: The inner embryonic cell layer in animals; it develops into cells such as lungs, liver, pancreas, and gall bladder.

• leuconoid: A body plan in which the canal system forms a more elaborate branched network than in the synconoid body plan; the canals lead to digestive chambers instead of a spongocoel.

• mesohyl: A gelatinous substance between the endoderm and ectoderm; it encases the sponge’s skeletal elements as well as scattered amoebocytes.

• myocytes: A type of cell found in muscle tissue; they are also found as muscle-like cells in sponges.

• ostia: Pores through which water is pumped into the spongocoel.

• pinacocytes: The epidermal, or "skin," cells on the outer surface of sponges.

• porocytes: Cylindrical cells that make up the pores, or ostia; water enters the animal through the pores formed by these cells.

• sclerocytes: Cells that secrete spicules made of calcium carbonate or silica.

• spongocytes: Cells that secrete the spongin protein.

• synconoid: A body plan in which the ostia lead to a network of canals that are lined with choanocytes.

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Summary

• Sponges have a number of specialized cells that carry out distinct functions within the organism, but they do not have true tissues or organs. • There are three different body plans found among sponges: asconoid, synconoid, and leuconoid. • Sponge feeding is critically dependent on several specialized cells within the organism including the following: choanocytes, amoebocytes, and porocytes.

Practice

Use this resource to answer the questions that follow.

• Structure of Sponges at http://www.mesa.edu.au/porifera/porifera01.asp .

1. What is the jelly-like mesohyl in sponges made up of? 2. Does water flow through a sponge only in one direction like blood in a human circulatory system? 3. If you cut a piece of a sponge off, can that piece regenerate into another sponge?

Practice Answers

1. The mesohyl is made mainly of collagen and reinforced by a dense network of fibers also made of collagen sandwiched between two thin layers of cells. 2. The flow of water through the sponge is in one direction only, driven by the beating of flagella which line the surface of chambers connected by a series of canals. 3. Sponges can regenerate from fragments that are broken off by currents or predators, although this only works if the fragments include the right types of cells.

Review

1. How many germ layers do sponges develop from? 2. Which body plans of sponges include spongocoels? Which ones don’t? 3. What is the difference between the asconoid body plan and the synconoid body plan? 4. What role do porocytes play? 5. Why is water so crucial for sponges?

Review Answers

1. Sponges are diploblasts and develop from two basic germ layers: ectoderm and endoderm. 2. Sponges with an asconoid or synconoid body plan have spongocoels. Sponges with a leuconoid body plan do not have spongocoels. Instead, they have digestive chambers lined with choanocytes. 3. In asconoid sponges, water is pumped through the ostia into the spongocoel that is lined with choanocytes. Synconoid sponges have a more comlex body plan where water is pumped into the ostia and through canals where digestion takes place before arriving at the spongocoel. 4. Porocytes are cylindrical cells that make up the pores, or ostia. Water enters the animal through the pores formed by these cells. 5. The flow of water into the sponge body is not only crucial for feeding and digestion, but also for circulation within the sponge because they do not have a respiratory or circulatory system. Sponges obtain oxygen by diffusion from the water flowing through their bodies, and waste is expelled by diffusion into the same pool of water.

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1.6 Sponge Reproduction - Advanced

• Explain how sponges reproduce sexually and asexually.

Does this animal reproduce? Of course it does. But how? Recall that all living organisms must reproduce. Is this sea sponge male or female? Or neither? Or both?

Reproduction of Sponges

Despite the fact that adult sponges cannot move, they are capable of sexual reproduction. Sexual reproduction is the formation of a new organism by the fusion of gametes - an egg and a sperm cell. The sperm is usually produced by the male parent and the egg is produced by the female parent. However, most species of sponges are hermaphroditic, meaning that each individual can produce both eggs and sperm ( hermaphrodite). Gametes contain half of the genetic information of each parent (half of their chromosomes), and their fusion during fertilization provides a complete set of chromosomes to the offspring. Sponges are also able to reproduce asexually. Asexual reproduction does not involve the formation of gametes. The offspring are often formed by budding from a single parent organism. In this concept, we will discuss how sponges carry out each of these types of reproduction.

Sexual Reproduction of Sponges

Most sponges are hermaphrodites, but an individual will usually only make one type of gamete at a time, so they are not able to self-fertilize. The general life cycle of a sponge is depicted in the Figure below. Gametes develop from the differentiation of either choanocytes or amoebocytes, depending on the species. Sperm produced by the “male” sponge (one that is producing sperm at the time of reproduction) is concentrated and released into the aquatic environment through the oscula. The sperm floating in the water reach the “female” sponge (one that is producing eggs at the time of reproduction) by the pumping action of choanocytes. In the same way that food is obtained, the choanocytes trap sperm cells as they flow through the interior of the organism. The sperm are then delivered to the eggs by the amoebocytes. Eggs are stored within the mesohyl, and that is where fertilization takes place to form a zygote. Once the zygote develops into a larva, it is usually released back into the water. A larva is a juvenile stage of an organism that is structurally very different from the adult stage. In the case of sponges, this

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FIGURE 1.21 The sponge life cycle includes sexual re- production. Sponges may also reproduce asexually.

is certainly true since the larvae of many species are able to swim using tiny hair-like projections called cilia that beat repeatedly to move the organism through the water. As the larvae continue to develop in the water, they become structurally more similar to adult sponges and lose their locomotive ability. At this point they settle and attach to a solid support where they complete development to the adult stage. For a video of sponges producing sexually, see http://www.youtube.com/watch?v=mVavqt4Sbyo (1:16)

MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/139360

Asexual Reproduction of Sponges

Sponges are also capable of asexual reproduction. This is accomplished by one of two mechanisms: external budding or internal budding. External budding is when a portion of the sponge breaks off and then regenerates into a complete organism. Internal budding takes place by the formation of internal buds called gemmules. Gemmules are a small collection of several different types of cells that are enclosed by a protective covering. This is an important survival mechanism in unfavorable conditions where the adult may not survive. The protected gemmule can withstand harsh conditions and can begin to develop into an adult sponge when conditions are more favorable. One fascinating feature of some sponge species has been demonstrated by forcing individual animals through a sieve. This causes all of the cells of the animal to separate from each other. If the separated cells are incubated together in

32 www.ck12.org Chapter 1. Invertebrates - Advanced an aqueous environment, they begin to move and function somewhat independently. Over time the cells will come together to reform the original animal. This finding has strong implications for how multicellular animals may have arisen from the aggregation of single-celled organisms.

Vocabulary

• budding: A form of asexual reproduction in which a new organism develops from an outgrowth, or bud, on another one; the bud may stay attached or break free from the parent.

• gemmules: A small collection of several different types of cells that are enclosed by a protective covering.

• hermaphrodite: An animal that can produce both eggs and sperm.

Summary

• Sponges are able to reproduce both sexually using gametes and asexually by budding. • Even though sponges are hermaphroditic, individuals will only make one type of gamete at a time. • There are two forms of asexual reproduction that sponges can go through: external budding and internal budding.

Practice

Use this resource to answer the questions that follow.

• Reproduction Sponges at http://www.iaszoology.com/reproduction-sponges/ .

1. How can sponges reproduce through fission and fragmentation? 2. What are reproduction bodies? 3. How are larva different between syconoid sponges and asconoid sponges?

Practice Answers

1. In some sponges multiplication takes place by developing a line of fission and throwing off parts of the body which later can develop into a new sponge. Sponges can break into several pieces along several lines of weakness, breaking into fragments that are capable to tide over unfavorable environmental conditions and grow into complete sponges in the following favorable season. 2. Reproduction bodies are small round balls of amoebocytes covered by pinacoderm and spicules. When favorable conditions return, reproduction bodies grow back into full sponges. 3. Syconoid sponge larva are called stomoblastula, since it has a mouth and feeds on nurse cells within mesogloea and grows for a few days. Asconoid sponge larva are called coeloblastula as it does not possess a mouth but has a blastocoel and flagella on the surface of its body. This larva escapes from the sponge body and swims about freely in water.

Review

1. What does it mean to be hermaphroditic? 2. Can sponges self-fertilize themselves? 3. What kind of cells do sponge gametes develop from?

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4. Name some differences between the larva stage and the adult stage of sponges. 5. What are some advantages of forming gemmules? 6. What fascinating phenomenon of sponges points to how multicellular animals may have arisen?

Review Answers

1. Most sponges are hermaphroditic, meaning each individual can produce both eggs and sperm. 2. Most sponges are hermaphrodites, but an individual will usually only make one type of gamete at a time, so they are not able to self-fertilize. 3. Gametes develop from the differentiation of either choanocytes or amoebocytes, depending on the species. 4. The larvae of many species are able to swim using tiny hair-like projections called cilia that beat repeatedly to move the organism through the water. As the larvae continue to develop in the water, they become structurally more similar to adult sponges and lose their locomotive ability. 5. Gemmules are a small collection of several different types of cells that are enclosed by a protective covering. This is an important survival mechanism since the gemmule can withstand harsh conditions and begin to develop into an adult when conditions are more favorable. 6. If the cells of a sponge are forced through a sieve, they begin to move and function somewhat independently, but eventually reform the original animal. This supports the theory that multicellular animals evolved form aggregations of single-celled organisms.

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1.7 Sponge Ecology - Advanced

• Describe the environments inhabited by sponges. • Discuss how humans have made use of sponges for their unique characteristics.

Do sea sponges interact with other organisms? As shown in this coral reef, sea sponges are an important part of coral reef ecosystems. As members of an ecosystem, like all animals, they have an important role in their environment.

Ecology of Sponges

Sponges are strictly aquatic organisms as you may expect considering that their feeding mechanism is based on the filtration of inflowing water from the environment. Almost all sponge species are found in the ocean with the exception of about 150 freshwater species. Sponges can be found throughout the marine world, in both cold polar waters and warmer tropical regions and at both great depths and in shallower areas. Because sponges generally require a solid support for attachment, they are often found in rocky marine areas or on the ocean floor. Sponges are critical components of the ecosystems of coral reefs, where they provide shelter for a variety of organisms including shrimp, crabs, and algae. They are also a source of food for many sponge-eating fish species. Many sponge species form large colonies or aggregates of individual organisms.

Symbiosis of Sponges

Sponges form symbiotic relationships with a variety of microorganisms, including bacteria and algae. A symbiotic relationship between organisms is a close ecological association between two species that may be mutually beneficial or may benefit one partner at the expense of the other. In the case of sponges, the benefit for the microorganisms may be a sheltered surface area on which to grow, and the benefit for the sponge may be nutrients provided by the metabolism of the microorganisms. Although most sponges are filter-feeders that consume microorganisms or obtain nutrients by symbiosis, there is one family of sponges that is actually carnivorous. These sponges capture and consume small crustaceans using their spicules. One such sponge, Chondrocladia lyra, has been called the "harp

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sponge," because its structure resembles a harp or lyre turned on its side (see http://news.discovery.com/earth/ocea ns/carnivorous-sponge-121112.htm ).

Human Use of Sponges

The term sponge is commonly used to refer to a porous cleaning or scrubbing tool. An example of bath sponges made from actual sponge organisms is shown in the Figure 1.22. It is the combination of the extensive pore system and the relatively soft spongin skeletal structure that has made some species of the class Desmospongia useful to humans as cleaning or bath sponges.

FIGURE 1.22 Natural bath sponges.

Nowadays it is rare for us to use a cleaning sponge derived from an actual organism. Most cleaning sponges are made of synthetic materials, but the porous structure is modeled after the bodies of these animals. The commonly used bath accessory called a loofah looks similar to a natural sponge, but it is actually made from a plant species. Another aspect of sponge biology that is of great use to humans is their chemical defense mechanisms. As sessile animals, sponges are vulnerable to a variety of predators. The pointed sponge spicules function as one method of defense against predators. Sponges also defend themselves by producing chemically active compounds. Some of these compounds are antibiotics that prevent pathogenic bacterial infections, and others are toxins that are poisonous to predators that consume the sponge. Many of these chemicals have been isolated and studied by scientists.

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A number of them have been found to have beneficial uses in humans; for example, some can be used as anti- inflammatory and anti-cancer drugs.

Vocabulary

• symbiotic relationship: A close ecological association between two species in which at least one species benefits; this is also known as symbiosis.

Summary

• Most sponges are marine animals that may live singly or in colonies and often participate in symbiotic relationships with a variety of microorganisms. • The porous structure of sponges and the toxins that they produce have proven useful for humans.

Practice

Use this resource to answer the questions that follow.

• Toxic Sponges Pharmaceutical Properties at http://www.qm.qld.gov.au/Find+out+about/Animals+of+Q ueensland/Sea+Life/Sponges/Toxic+sponges+and+pharmaceutical+properties .

1. Why has sponge research increased in recent years? 2. Dysinosin A, found in the sponge Citronia inhibits blood clotting enzymes. What diseases can this molecule treat? 3. How can sponges sometimes out-compete other marine species for space? 4. Why are sponges sometimes called "living fossils"?

Practice Answers

1. The major reason why our knowledge of sponges has escalated over the past few decades is due to the increasing interest in their pharmaceutical properties. 2. Dysinosin A has potential applications to treating human cardiovascular disorders such as stroke and throm- bosis. 3. In highly crowded communities such as coral reefs, space and other resources are limited, and being toxic gives an advantage to out-compete other marine invertebrates for space. 4. Many species of sponges alive today are morphologically identical to those that lived at the beginning of the Cretaceous Period (more than 150 million years ago).

Review

1. How many species of sponges live in freshwater? 2. Give an example of a symbiotic relationship that sponges have with microorganisms. 3. How have the toxins that sponges produce proven helpful for humans?

Review Answers

1. Almost all sponge species are found in the ocean with the exception of about 150 freshwater species.

37 1.7. Sponge Ecology - Advanced www.ck12.org

2. Microorganisms are provided a sheltered surface on the sponge with which to grow on, while the sponge is provided with nutrients by the metabolism of the microorganisms. 3. Chemically active compounds produced by sponges have been utilized in antibiotics, anti-inflammatory drugs, and anti-cancer drugs.

38 www.ck12.org Chapter 1. Invertebrates - Advanced

1.8 Cnidarians - Advanced

• Describe the general characteristics of cnidarians and how those characteristics differ between the various classes of cnidarians.

How are jellyfish related to coral? Did you know that coral was even an animal? The phylum Cnidaria contains a wide variety of aquatic invertebrates that are primarily found in marine environ- ments. Cnidarians evolved roughly 500 million years ago and have a body structure that is slightly more complex than the sponges. The representatives of the phylum cnidaria seen above are coral, giant green anemone, hydrozoa, and jellyfish.

Characteristics and Classification of Cnidarians

Cnidarians are a large phylum containing over 10,000 different species. The species within this phylum are highly diverse and range from the immobile corals that make up tropical coral reefs to the soft-bodied, free-swimming jelly- fish. Despite this wide variation, all cnidarians have one important structural feature in common: the nematocyst. A nematocyst is a long, thin, coiled stinger that can be propelled out of the cnidarian to attack prey or to defend against predators. The name cnidarian comes from the word “cnidos” which means stinging needle. It is the presence of the nematocyst that indicates all cnidarians arose from a common ancestor and therefore belong in the same phylum. Nematocysts are generated by a specialized cell called a cnidocyte, which is depicted in the Figure below. The major evolutionary step that occurred with the phylum cnidaria was the development of tissue-level organization. Recall that sponges exhibit cellular-level organization but have no true tissues. A tissue is an aggregation of similar cells that work together to carry out a specific function within the body. This increased organization allows

39 1.8. Cnidarians - Advanced www.ck12.org

FIGURE 1.23 This diagram shows cnidocytes present on the surface of a cnidarian ten- tacle. The nematocyst is shown at different stages of release. In the first cnidocyte the nematocyst is contained as a tight coil encapsulated within the cell. The cnidocil functions as a trigger to release the nematocyst upon contact. The second cnidocyte has been activated and is beginning to be released. Barbed ends, often laced with venom, contribute to the potency of a nematocyst sting. The operculum is a flap of tissue that covers the opening through which the nematocyst exits when released. The final cnidocyte in the diagram has an extended nematocyst.

cnidarians to have a simple nervous system and muscle tissue. Another feature that emerges with cnidarians is radial symmetry. Radial symmetry means that the animal can be cut in half from top to bottom at any angle to produce two identical sections. A bicycle wheel is an example of something that has radial symmetry. The phylum cnidaria is divided into four classes. The most common members of each class are listed in the Cnidarian Classes Table. TABLE 1.3: Cnidarian Classes

Class Members Anthozoa True corals, sea anemones, and sea pens Hydrozoa Siphonophores, hydroids, and fire corals Scyphozoa True jellyfish Cubozoa Box jellyfish

The class Anthozoa contains roughly 6,000 species and includes the true corals, sea anemones, and sea pens. There are approximately 3,000 species, including siphonophores and hydroids, within the hydrozoan class. The class Scyphozoa contains 200 species of true jellyfish. A small class, Cubozoa, contains only 20 species, and these are the deadly box jellyfish.

KQED: Amazing Jellies

Jellyfish. They are otherworldly creatures that glow in the dark, lack brains and bones, and are sometimes more than 100 feet long. And there are many different types. Jellyfish are free-swimming members of the phylum Cnidaria. Jellyfish are found in every ocean, from the surface to the deep sea. To find out more about jellyfish, see http://w ww.kqed.org/quest/television/amazing-jellies–siphonophores2 (10:53).

MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/5735

Cindarian Features

Cnidarians40 are the earliest multicellular animals to have tissue-level organization. Common features of cnidarians include radially symmetrical diploblasts with true tissues and nematocysts. Cnidarians reproduce both sexually and asexually. They are classified based on the presence of a specialized cell called the cnidocyte that releases the thin, harpoon-like structure called a nematocyst to attack prey. Features of cnidarians will be discussed in the additional Cnidarians concepts.

TABLE 1.4:

Anthozoa Hydrozoa Scyphozoa Cubozoa Class members Corals, anemones Hydra, Jellyfish Box Jellyfish siphonophores Predominant body Polyp Polyp Medusa Medusa type Alternation of gen- No Yes Yes Yes erations www.ck12.org Chapter 1. Invertebrates - Advanced

TABLE 1.4: (continued)

Anthozoa Hydrozoa Scyphozoa Cubozoa Colony formation Common for corals Common, can be Less common Less common highly sophisticated Nervous System Nerve net Nerve net Rhopalia, ocelli Rhopalia, complex eyes Ecology Marine, major Mostly marine but Marine, mostly soli- Marine, mostly soli- components of coral some freshwater, tary free-floating tary free-floating reefs but can be both sessile and found in all ocean free-floating regions colonies

For more information on cnidarians, visit the PBS Shape of Life series: Cnidarians: Life on the Move at http://w ww.shapeoflife.org/video/cnidarians-life-move and Cnidarians: Deep Sea Research at http://www.shapeoflife.org/v ideo/cnidarians-deep-sea-research .

Vocabulary

• cnidocyte : A specialized cell that generates a nematocyst.

• nematocyst: A long, thin, coiled stinger that can be propelled out of the cnidarian to attack prey or to defend against predators.

• radial symmetry: Symmetry of a body plan in which there is a distinct top and bottom but no distinct head and tail ends, so the body can be divided into two halves like a pie.

• tissue: An aggregation of similar cells that work together to carry out a specific function within the organ- ism/body.

Summary

• Common features of cnidarians include radially symmetrical diploblasts with true tissues and nematocysts. • The presence of nematocysts indicates that all cnidarians arose from a common ancestor. • The major evolutionary step that occurred with the phylum cnidaria was the development of tissue-level organization.

Practice

Use this resource to answer the questions that follow.

• Cnidaria at http://www.mesa.edu.au/cnidaria/ .

1. Do cnidarians have respiratory or excretory systems? 2. How complex is the digestive system of cnidarians? 3. Do cnidarians have a centralized nervous system? 4. What kind of predators do cnidarians encounter? 5. How do some cnidarians form colonies? 6. How do corals get their energy?

41 1.8. Cnidarians - Advanced www.ck12.org

Practice Answers

1. Cnidarians do not have respiratory or excretory systems. Respiration and excretion occur by diffusion. 2. Cnidarians have a gastrovascular cavity with a mouth but no anus. Both food and waste pass through this one opening. 3. No, cnidarians do not have a centralized nervous system. They do, however, have a primitive nervous system consisting of nerve nets. 4. Predators of Cnidaria include sea slugs, sea stars (for example, the Crown of Thorn which can devastate coral reefs), nudibranchs, fish (including butterfly fish and parrot fish which eat corals), and marine turtles and sunfish (which eat jellyfish). 5. Many Cnidaria live in colonies made up of large numbers of individuals joined together in some way. These individuals (called zooids) can either be directly connected by tissues or share a common exoskeleton made from chitin or calcium carbonate. 6. Corals form a symbiotic relationship with zooxanthellae (a type of algae). The algae produces oxygen and energy (sugars) that the coral polyp needs to live and, in return, the polyp produces carbon dioxide and other substances the algae needs.

Review

1. Describe the structural feature that all cnidarians have in common and explain its function. 2. What major evolutionary step occred within the phylum cnidaria that differentiates the phylum from porifera? 3. Describe the differences between each of the four classes of cnidarians.

Review Answers

1. All cnidarians have neamtocysts, which are long, thin, coiled stingers that can be propelled out to attack prey or defend against predators. 2. Cnidarians differ from sponges because they have true tissue-level organization. This major evolutionary step allowed for simple nervous systems and muscle tissue. 3. The anthozoa and hydrozoa classes have predominantly polyp bodies and commonly form colonies, whereas the scyphozoa and cubozoa have predominantly medusa bodies and do not usually form colonies. Hydrozoas are the only class that can be found in freshwater. Cubozoa have cube-shaped bodies, whereas scyphozoa have bell-shaped bodies.

42 www.ck12.org Chapter 1. Invertebrates - Advanced

1.9 Cnidarian Structure and Function - Ad- vanced

• Identify specific structural features of cnidarians and their functions. • Understand the different anatomical structures of the four classes of cnidarians.

How would one characterize this structure? "Bell-shaped" (or medusoid) to start. The body of a jellyfish allows it to propel itself using muscle contractions - or float along water currents.

Structure and Function in Cnidarians

Similar to sponges, cnidarians are diploblasts, meaning that they develop from two basic germ (cell) layers: an ectoderm, or outer layer, and an endoderm, or inner layer. Between the ectoderm, which contains the cnidocysts, and the endoderm there is a non-cellular substance called the mesoglea. The mesoglea is a gelatinous matrix that contains fibers made up of the protein collagen. There are usually cells loosely scattered throughout the mesoglea but not in a defined layer. The basic body plan of all cnidarians consists of the two cell layers enclosing a digestive cavity.

Polyps and Medusae

There are two distinct cnidarian body forms: polypoid and medusoid. These are both depicted in the Figure 1.24. As you can see from the Figure 1.24, the polyp has a tubular shaped body. Polyps are usually sessile, with the bottom attached to a solid surface and the mouth opening at the top. The attachment region at the base of the animal is called the basal plate. The medusa is more of an umbrella or bell shape, with the mouth facing down. The body of the medusa is often called the bell. Medusae are usually free-swimming and either propel themselves using muscle contractions or float along water currents like plankton. Many cnidarian species exhibit what is called an “alternation

43 1.9. Cnidarian Structure and Function - Advanced www.ck12.org

FIGURE 1.24 Both polyp and medusa forms consist of a digestive sac, the coelenteron, sur- rounded by two layers of tissue, the en- doderm and the ectoderm. A gelatinous matrix called the mesoglea lies between the two layers and may contain loose aggregations of cells.

of generations” meaning that they alternate between polyps and medusae. This will be further discussed for each class in another concept. The cnidarian digestive cavity, called the coelenteron, has a single mouth opening through which food/prey enters and waste is expelled. The coelenteron is considered a gastrovascular cavity because it is where both digestion and gas exchange between the organism’s cells and water in the cavity take place. This cavity can be either one large chamber, several smaller chambers, or a branched network of canals. The mouth opening is usually surrounded by tentacles. Cnidarians are carnivorous animals. They use cnidocytes on the surface of their tentacles to release nematocysts for attacking and capturing prey. The immobilized prey can then be brought into the coelenteron through the mouth. Medusae also have oral “arms” that assist in capturing and ingesting prey. Digestion of prey takes place extracellularly within the coelenteron. The endodermal lining of the coelenteron, called the gastrodermis, absorbs nutrients derived from the digestive process. In addition to a primitive digestive cavity, cnidarians have a decentralized nervous system, muscle tissue, repro- ductive tissues, and a hydrostatic skeleton. A hydrostatic skeleton is maintained by the internal pressure of fluids within the organism. This hydrostatic pressure allows the animals to hold their shape without bones or solid skeletal elements. The force of muscle contractions against the hydrostatic pressure allows the organism to move from place to place. The primitive nervous system of cnidarians is non-centralized and consists of a network of nerve cells, called a nerve net, spanning the body wall. The nerve net is capable of sensing touch. There is no circulatory or respiratory system within cnidarians. As mentioned above, the coelenteron functions as vascular tissue, and both circulation and respiration occur by simple diffusion between the cells and the water within and surrounding the animal. Reproductive tissues are usually located in the mesoglea of polyps or in the gastric cavity of medusae.

Anthozoan Anatomy

Species within the class Anthozoa include the corals and sea anemones. Most anthozoan species exist exclusively in the polyp form and do not exhibit alternation of generations. As polyps, the Anthozoa are primarily sessile. An example from the class Anthozoa is depicted in the Figure 1.25. The coral head depicted in the Figure 1.25 is actually a colony made up of many small, interconnected anthozoan polyps. These colonies form by asexual reproduction in which the developing bud forms a polyp that remains attached to the parent. In addition to the hydrostatic skeleton discussed above, several coral species secrete an exoskeleton. In these species, the ectodermal cells at the base of the polyp secrete the cup-shaped exoskeleton called

44 www.ck12.org Chapter 1. Invertebrates - Advanced

FIGURE 1.25 The anthozoan species, Acropora vari- abilis.

the calicle or basal plate. The basal plate is made up of calcium. As the polyp grows, the calicle size increases and, over time, becomes the major constituent of coral reefs.

Hydrozoan Anatomy

The class Hydrozoa contains species that include the siphonophores and hydroids. Unlike the anthozoans, many Hydrozoa alternate between polyp and medusa forms. However, the polyp form normally dominates in the hy- drozoan life cycle. The medusa form is generally small and short-lived. Its primary function is to carry out sexual reproduction and to allow the species to disperse to different locations. Hydrozoa are classified based on the presence of a membrane called the velum that lines the inside edge of the bell in the medusa forms. Hydrozoa of the order Siphonophora are fascinating creatures. These animals can form large, sophisticated colonies of interconnected and interdependent individual polypoids and medusoids called zooids. The term zooid refers to the fact that the individual can usually only function as part of the whole colony. This is because the colony often divides up tasks with some individuals becoming specialized for certain functions. Zooids specialized for a particular function usually lose the ability to perform other functions. For example, nectophores are medusoid zooids that function to move the colony through the water. Nectophores however, cannot feed. They are dependent on specialized feeding polyps within the colony to absorb and deliver nutrients. This is a distinguishing feature of the hydrozoans. Although some jellyfish also form colonies, it is neither as prevalent nor as sophisticated among species within the Scyphozoa and Cubozoa classes as it is in the Hydrozoa class. The Portugese Man O’ War pictured in the Figure 1.26 is probably one of the most notorious hydrozoan siphonophores. A sting from the Portugese Man O’ War can be extremely painful due to the venom present on the nematocysts and has even caused several human deaths. The animal floats at the surface of the ocean using a specialized organ called

45 1.9. Cnidarian Structure and Function - Advanced www.ck12.org

FIGURE 1.26 The hydrozoan species Portugese Man O’ War. Note the air bladder at the top of the animal and the long feeding tentacles hanging below.

an air bladder. Long tentacles, sometimes several meters in length, dangle below the surface where they sting and capture prey.

Scyphozoan and Cubozoan Anatomy

Members of the class Scyphozoa (true jellyfish) and Cubozoa (box jellyfish) alternate between polyp and medusa forms. One significant difference between these two classes and the Hydrozoa is the predominant form, the form in which the organism spends the majority of its lifetime. In most hydrozoan species the polyp form is predominant, with the medusa form being small, short-lived or never detaching from the polyp colony. The opposite is true for the Scyphozoa and Cubozoa. In these classes the medusa stage is generally fairly large, while the polyp stage is small and short-lived. In addition, scyphozoans and cubozoans lack a velum on the inside edge of the bell. The Figure 1.27 shows examples of scyphozoans and cubozoans. Although scyphozoans and cubozoans are called jellyfish, they are actually not fish at all. Fish are vertebrates and cnidarians are invertebrates, but the term jellyfish is commonly used to refer to these classes of cnidarians. Species of both Scyphozoa and Cubozoa are very similar structurally with a few key differences. Box jellies, as the name implies, have cube-shaped bodies. There are also differences in the venom present on their nematocysts. One interesting feature of both classes is the presence of small sensory structures called rhopalia. Rhopalia typically

46 www.ck12.org Chapter 1. Invertebrates - Advanced

FIGURE 1.27 (a) A scyphozoan jellyfish and (b) a cubo- zoan box jellyfish. These photographs illustrate the clear difference in the body shape between true jellies, which have a bell-shaped body, and box jellies, which have a cube-shaped body.

have visual structures called ocelli and gravity-sensing structures called statoliths. The ocelli allow both species to sense light. Cubozoa have an even more complex vision system with several sets of eyes that may be capable of seeing blurred images. These sophisticated eye pairs contain a lens, retina, cornea, and iris. It is not known how clear of an image these eyes are capable of capturing, particularly since the animals lack a centralized nerve system to process the information sensed by the eyes. This feature of cubozoans makes them particularly difficult to observe in their natural habitat because they tend to swim away when they “see” divers approaching.

Vocabulary

• basal plate: A cup-shaped exoskeleton also known as the calcite; they are found in class Anthozoa.

• calicle: A cup-shaped exoskeleton also known as the basal plate; they are found in class Anthozoa.

• coelenteron: The digestive sac/cavity with a single mouth opening through which food/prey enters and waste is expelled.

• gastrodermis: The endodermal lining of the coelenteron which absorbs nutrients derived from the digestive process.

• hydrostatic skeleton: A structure consisting of a fluid-filled cavity, the coelom, surrounded by muscles; it is used to change an organism’s shape and produce movement.

• medusa: The cnidarian body form with an umbrella or bell shape and the mouth facing down.

• mesoglea: A gelatinous matrix that contains fibers made up of the protein collagen; it lies between the ectoderm and the endoderm.

• nerve net: The primitive nervous system of cnidarians; it is non-centralized and consists of interconnected neurons spanning the body wall.

• ocelli: Simple light sensing tissues.

• polyp: The cnidarian body form with a tubular shaped body.

• rhopalia: Small sensory structures found in both scyphozoan and cubozoan jellyfish.

47 1.9. Cnidarian Structure and Function - Advanced www.ck12.org

• statoliths: Gravity-sensing structures typically found in rhopalia; they are plastids filled with starch which enable their cells (statocytes) to detect gravity.

• velum: The membrane that lines the inside edge of the bell in the medusa forms; they are found in Hydrozoa.

• zooids: Large, sophisticated colonies of interconnected and interdependent individual polypoids and medu- soids; they are characteristic of the order Siphonophora.

Summary

• The anatomy of cnidarians is based on two distinct body types called the polyp (polypoid), which is normally sessile, and the medusa (medusoid), which is normally free-floating. • Often the cnidarian life cycle involves an alternation of generations, in which the organisms can switch between the polyp and medusa forms. • Cnidarians have a primitive digestive cavity and a decentralized nervous system. • Most anthozoan species exist exclusively in the polyp form and are therefore primarily sessile. • Hydrozoa are classified based on the presence of a membrane called the velum that lines the inside edge of the bell in the medusa forms. • Species of both Scyphozoa and Cubozoa are very similar structurally with a few key differences: Cubozoa are cube-shaped while Scyphozoa are bell-shaped, and they have different venom as well.

Practice

Use this resource to answer the questions that follow.

• Cnidarian Characteristics at http://animals.about.com/od/cnidarians/ss/cnidarians.htm .

1. What are the three kinds of cnidae? What are the functions of each? 2. How do cnidarians digest their food? 3. Describe the life cycle of jellyfish. 4. What is the difference between stony coral and soft coral?

Practice Answers

1. The three kinds of cnidae are nematocysts, spirocysts, and ptychocysts. Nematocysts consist of a capsule containing a coiled thread and barbs known as stylets; they are used to capture prey. Spirocysts are cnidae found in some corals and sea anemones that consist of sticky threads, and they help the animal capture prey and adhere to surfaces. Ptychocysts help a group of cnidarians known as the Ceriantaria establish a secure hold on substrates. 2. Cnidarians use their tentacles to draw the food into their mouth and gastrovascular cavity. Once in the gastrovascular cavity, enzymes secreted from the gastrodermis break down the food. Small hair-like flagella that line the gastrodermis beat, mixing enzymes and food until the meal has been fully digested. 3. A jellyfish begins its life as a free-swimming planula which, after a few days, drops to the sea floor and attaches itself to a hard surface. It then develops into a polyp which buds and divides to form a colony. After further development, the polyps shed tiny medusae which mature into the familiar adult jellyfish form which goes on to reproduce sexually to form new planulae and complete their life cycle. 4. Stony corals produce a skeleton of calcium carbonate crystals which they secrete from the epidermis of the lower part of their stalk and basal disc. Soft corals do not produce calcium carbonate skeletons like those of stony corals. Instead, they contain tiny cacareous spicules and grow in mounds or mushroom shapes.

48 www.ck12.org Chapter 1. Invertebrates - Advanced

Review

1. Explain the general nervous system of cnidarians, and describe several features specific to the nervous system of jellyfish. 2. In considering the features of both body forms of cnidarians, can you think of some of the advantages for an organism that is capable of switching between the two forms? 3. Describe the two distinct cnidarian body forms, and explain what is meant by “alternation of generations.” 4. What functions does the coelenteron have? 5. How do coral colonies form? 6. What is particular about hydrozoan colonies? 7. What are some differences between scyphozoans and cubozoans?

Review Answers

1. The primitive nervous system of cnidarians is non-centralized and consists of a network of nerve cells, called a nerve net, spanning the body wall. The nerve net is capable of sensing touch. The nervous system of jelly fish contains rhopalia which allow jellyfish to sense light (using ocelli) and gravity (using statoliths). 2. An organism that is capable of switching between two forms can live in one area as a sessile organism when conditions are favorable and become mobile when it needs to reproduce sexually. Thus, its second form can afford it greater genetic variation. An organism might also switch to its mobile form to find more habitable environments. 3. The polyp is the sessile cnidarian body form that is tubular in shape and has an attachment region known as the basal plate. The medusa is more of an umbrella or bell shape with the mouth facing down, and they are usually free-swimming. “Alternation of generations” means that cnidarians alternate between polyps and medusae. 4. The coelenteron is a single mouth opening where both digestion and the exchange of gases takes place. Thus, it is considered a gastrovascular cavity. 5. Coral colonies form by asexual reproduction in which the developing bud forms a polyp that remains attached to the parent. 6. Hydrozoa can form large, sophisticated colonies of interconnected and interdependent individual polypoids and medusoids called zooids. Some individuals may become specialized and lose the ability to perform other functions. 7. Species of both Scyphozoa and Cubozoa are very similar structurally with a few key differences: Cubozoa are cube-shaped while Scyphozoa are bell-shaped, and they have different venom as well. Cubozoans also have more complex vision systems with several sets of eyes that may be capable of seeing blurred images.

49 1.10. Cnidarian Reproduction - Advanced www.ck12.org

1.10 Cnidarian Reproduction - Advanced

• Explain how cnidarians reproduce, and illustrate their general life cycle.

What color is a male jellyfish? Are males blue? Is there a male and female jellyfish? In fact, there are. Some cnidarians reproduce sexually, meaning they produce eggs or sperm.

Reproduction of Cnidarians

In general, polyps primarily reproduce asexually by budding, however, some produce gametes (eggs and sperm) and reproduce sexually. Medusae usually reproduce sexually using eggs and sperm. Depending on the species, cnidarians can be monoecious (also called hermaphroditic), with individuals capable of producing both eggs and sperm, or they can be dioecious, with individuals of separate sexes for gamete production. Typically the hermaphroditic species cannot self-fertilize, so sexual reproduction requires at least two individuals. Jellyfish of the class Scyphozoa are dioecious. A generalized life cycle of a cnidarian that alternates between polyp and medusa forms is outlined in the Figure 1.28. For an interesting article on how hydrozoans are able to go through their life cycle several times, visit Can a Jellyfish Unlock the Secret of Immortality? at http://www.nytimes.com/2012/12/02/magazine/can-a-jel lyfish-unlock-the-secret-of-immortality.html?pagewanted=all&_r=0 Cnidarians switch from the polyp to the medusa stage by a form of asexual reproduction in which the polyp develops a stack of medusoid structures that can then bud off to become independent medusae. This process is called strobilation and is depicted in the Figure 1.29. The polyp stage can be regenerated when medusae reproduce sexually to form a ciliated larva called a planula. The planula can then develop into a polyp and continue the cycle. There are many variations on this general life cycle, depending on the species. For example, exclusively polyp species in the class Anthozoa will obviously exhibit a truncated version of the life cycle omitting the medusa stages. There are also some species that are exclusively medusae and do not form polyps. Another variable in the sexual reproduction of cnidarians is whether fertilization takes place internally or externally. In some species, sperm released by the males must be ingested by the females in order to reach eggs within the female body for fertilization. In other species, both sperm and eggs are released into the aquatic environment by the organisms, and fertilization takes place externally.

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FIGURE 1.28 A generalized life cycle of a cnidarian. Medusae primarily reproduce by sexual reproduction with the formation of a larval stage called the planula. The planula then develops into a polyp that can reproduce either sexually or asexually. One type of asexual reproduction by polyps leads to the formation of new medusae.

FIGURE 1.29 Polyps forming medusae: strobilation. The process of strobilation is when a polyp asexually reproduces to form medusae. The cartoons labeled 1 through 3 depict a polyp developing the medusae. The cartoons labeled 4 through 6 show the continued development of the medusa once it is released from the polyp.

Vocabulary

• dioecious: Having individuals of separate sexes for gamete production.

• monoecious: Having individuals capable of producing both eggs and sperm; these animals are hermaphroditic.

• planula: The larval stage formed by sexual reproduction; the planula then develops into a polyp that can reproduce either sexually or asexually.

• strobilation: The process by which cnidarians switch from the polyp to the medusa stage; a form of asexual reproduction in which the polyp develops a stack of medusoid structures that can then bud off to become independent medusae.

Summary

• Reproduction of cnidarians can be either asexual by budding or sexual using gametes. • Depending on the species, cnidarians can be monoecious or dioecious. • Cnidarians usually cycle between a medusa stage and a polyp stage during their life cycle.

51 1.10. Cnidarian Reproduction - Advanced www.ck12.org

Practice

Use this resource to answer the questions that follow.

• The Reproductive System in Cnidaria at http://mgmreproductivesystem.weebly.com/cnidaria.html .

1. What happens to the fertilized eggs of jellyfish? 2. Why does synchronous spawning occur in many corals? 3. Which cnidarians reproduce sexually through internal fertilization? Which from external fertilization?

Practice Answers

1. After jellyfish eggs are fertilized, they are stored in brooding pouches along the oral arms of the female or in her stomach. They eventually become tiny planulae that detach from the mother’s body. 2. Synchronous spawning means that the polyps release eggs and sperm at the same time. This spawning method disperses eggs over a larger area. 3. Jellyfish reproduce sexually through internal fertilization. Anemones reproduce sexually through external fertilization. Corals can reproduce either through internal or external fertilization.

Review

1. What does it mean to be monoecious or dioecious? Can cnidarians self-fertilize during sexual reproduction? 2. During which stage of a cnidarians life cycle do individuals usually reproduce sexually? 3. How does the polyp switch to the medusa stage? 4. In cnidarians, where does fertilization take place?

Review Answers

1. Monoecious means that individuals are capable of producing eggs and sperm, whereas dioecious means that individuals can only produce on type of gamete during sexual reproduction. The hermaphroditic species of cnidarians typically cannot self-fertilize. 2. The medusa primarily reproduces through sexual reproduction. The polyp phase can reproduce either sexually or asexually. 3. Cnidarians switch from the polyp to the medusa stage by a form of asexual reproduction in which the polyp develops a stack of medusoid structures that can then bud off to become independent medusae (strobilation). 4. In some species, sperm released by the males must be ingested by the females in order to reach eggs within the female body for fertilization. In other species, both sperm and eggs are released into the aquatic environment by the organisms, and fertilization takes place externally.

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1.11 Cnidarian Ecology - Advanced

• Describe the habitats of each class of cnidarians.

The sea anemone. Plant or animal? It may look like a plant, but it’s not. Sea anemones are a group of water-dwelling, predatory animals in the phylum Cnidaria. A sea anemone is a polyp attached at the bottom to the surface beneath it. They can have anywhere from a few tens of tentacles to a few hundred tentacles. And they eat small fish and shrimp.

Ecology of Cnidarians

As with reproduction and anatomy, cnidarian ecology varies greatly depending upon the class and species. Overall, cnidarians can be found in almost all marine environments, cold and warm, shallow and deep. Several species can also inhabit freshwater. In the following sections we will consider the various niches that are inhabited by each class of cnidarians.

Anthozoan Ecology

Cnidarians within the class Anthozoa make up what are likely the most complex and rich ecosystems on earth since this class contains reef-forming corals. Coral reefs are found in shallow waters of warm, tropical oceans. An example of a coral reef is shown in the Figure 1.30.

FIGURE 1.30 The Great Barrier Reef in Australia.

Reefs are essentially large structures made up of coral skeletons and calcium carbonate deposited by algae. They function to provide food and shelter to many organisms, including fish, invertebrates, and microorganisms such

53 1.11. Cnidarian Ecology - Advanced www.ck12.org as algae. They also serve to protect the shoreline from erosion. As major constituents of reefs, corals contribute to the sheltering capacity of reefs and also provide important defense against predators, using their tentacles and nematocysts. Reef corals usually form colonies of large groups of individuals growing attached to one another. Although they can feed by catching plankton with their tentacles, they often engage in symbiotic relationships with microorganisms such as algae. Living within the coral body, photosynthetic algae harness the energy of sunlight to produce oxygen and carbon-rich molecules that can be absorbed by the corals for nutrition. Although reefs are located exclusively in shallow, warm waters, not all corals are reef-forming, and Anthozoa also includes other species such as sea anemones, some of which associate with reefs and some of which do not. Anthozoans that are not associated with reefs live in marine waters all over the world, including deep or cold habitats. As sessile polyps, most of these species are found attached to the ocean floor.

Hydrozoan Ecology

As both polyp form and medusa forms, Hydrozoa occupy varied niches. They can be found in almost all aquatic environments. The sessile polyps are usually attached to solid surfaces, including the ocean floor. The less prominent medusa forms are free-swimming or free-floating. The most striking feature of hydrozoan ecology is the formation of interconnected and interdependent colonies described in a previous concept on hydrozoan anatomy. These colonies are usually free-floating, or planktonic. Many of the polyp forms have symbiotic relationships with either microorganisms such as algae that live inside the polyps or with larger organisms such as crustaceans that may be used by the polyps as attachment surfaces. Both polyp and medusa forms feed on a wide variety of microorganisms and animals. Polyp feeding is somewhat passive and dependent on water currents to bring prey into close proximity. In contrast, free-swimming medusa forms can cover large distances to find prey. There is one species of Hydrozoa, Polypodium hydriforme, that has an extremely unusual habitat. It lives as a parasite inside the oocytes of certain fish species. This is the only multicellular animal that is also an intracellular parasite. Hydrozoa can also inhabit freshwater and there are a number of well-studied species within the freshwater genus Hydra. Hydra are found in ponds, lakes, and streams of temperate and tropical regions. They feed on small invertebrates and, like many other polyp forms of cnidarians, they form symbiotic relationships with algae. Hydra are primarily sessile, but they are also capable of an unusual type of movement when they are seeking prey. This movement is best described as a somersault or a back flip in which the organism bends over and attaches to a surface nearby using the mouth and tentacles. The Hydra then releases the adhesive foot that it normally uses for attachment and flips over to reattach the foot to a new location.

Scyphozoan and Cubozoan Ecology

Most scyphozoan and cubozoan species are free-swimming and live throughout the open ocean. There are scypho- zoan species found in both shallow ocean waters and at great depths. They can also be found in tropical and polar regions. Cubozoans are more restricted to tropical and subtropical waters. Jellyfish generally live as solitary organisms, but some species form large aggregates, or swarms. These swarms can sometimes become enormous and problematic as they compete with other organisms within the ecosystem. Most jellyfish feed on crustaceans, but many also consume plankton, other invertebrates, and fish eggs and larvae. Species that feed on fish eggs and larvae can sometimes have an indirect impact on the human food supply by causing changes in fish population levels. This can be an especially significant problem when jellyfish swarms are formed. Scyphozoans have an impact on humans in several additional ways. Their stings can be quite painful to humans swimming in the ocean, although they are not often life-threatening. Additionally, in several Asian countries some scyphozoan species are used as a food source. Cubozoans are unusual in that they are capable of active swimming and will actually hunt and catch their prey. Cubozoan species are also interesting in that the venom found on their nematocysts can be extremely deadly. In the last 130 years they have been responsible for over 5,000 human deaths. For more information on cubozoans and their habitats, see Introduction to Cubozoa: The Box Jellies! at http://www.ucmp.berkeley.edu/cnidaria/cubozoa.ht

54 www.ck12.org Chapter 1. Invertebrates - Advanced ml .

Vocabulary

• symbiotic relationship: A close ecological association between two species in which at least one species benefits; this is also known as symbiosis.

Summary

• Cnidarians inhabit all regions of the world’s oceans and some freshwater environments. They are major constituents of coral reefs. • Cnidarians are integral parts of the marine ecosystem where they may engage in symbiotic relationships with other organisms and where their predatory activities contribute to the delicate balance of the oceanic food chain. • The habitats of cnidarians vary greatly depending on whether they take on predominantly polyp body forms or medusa body forms.

Practice

Use this resource to answer the questions that follow.

• Cnidaria: Life History and Ecology at http://www.ucmp.berkeley.edu/cnidaria/cnidarialh.html .

1. What symbiotic relationship do many cnidarians have with zooxanthellae. 2. What two major niches do cnidarians occupy? 3. What is bleaching of coral? 4. When do cnidarian fossils date back to? 5. Which class of cnidarians do scientists believe are the most primitive cnidarians?

Practice Answers

1. Zooxanthellae are symbiotic dinoflagellates that carry out photosynthesis within cnidarian tissues, and pass on the carbon compounds they fix to their hosts. Many cnidarians depend on zooxanthellae to produce energy. 2. Some cnidarians occupy a predatory niche because they may use their cnidocysts to trap prey. Other cnidarians occupy an almost photosynthetic niche due to their symbiotic relationships with zooxanthellae. 3. Bleaching is the loss of symbionts by coral. It usually appears as white areas on the coral reef when it is exposed at low tide. This is deadly to coral reefs. 4. Cnidarian fossils date back to the time when animals first appear in the fossil record, the Vendian period. 5. Scientists now believe that the Anthozoa are the most primitive, in part due to the fact that they completely lack the medusoid stage of cnidarian life cycles. Further studies are needed to determine which class is truly the most primitive.

Review

1. Where are corals usually found? 2. What is so particular regarding the habitat of Polypodium hydriforme? 3. Which class of cnidarians can be found in freshwater? 4. Where are cubozoans usually found? How does this differ from scyphozoans? 5. Which classes of cnidarians usually exhibit symbiotic relationships with other organisms?

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Review Answers

1. Reef corals are found exclusively in shallow, warm waters of tropical oceans. Not all corals are reef corals, however, and some are associated with deeper, cooler habitats on the ocean floor. 2. This particular species of Hydrozoa lives as a parasite inside the oocytes of certain fish species. This is the only multicellular animal that is an intracellular parasite. 3. Hydrozoa can inhabit freshwater, and there are a number of well-studied species within the freshwater genus Hydra. Hydra are found in ponds, lakes, and streams of temperate and tropical regions. 4. Cubozons are restricted to tropical and subtropical waters, whereas scyphozoans are found in both shallow ocean waters and at greater depths, from tropical to polar regions. 5. Anthozoans such as corals usually exhibit symbiotic relationships with algae. The polyp forms of hydrozoans also form symbiotic relationships with algae, other microorganisms, and crustaceans.

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1.12 Flatworms - Advanced

• Describe the structural features that arose with the evolution of flatworms and what role those features played in the evolution of higher organisms.

Would you believe that this gold-dotted creature is a flatworm? No? Well it is. There are more than 25,000 different types of flatworms, so they can be very different in how they appear. And many don’t even look like your typical worm. Within the animal kingdom there are several phyla dedicated to worms. When people think of worms in everyday life they usually think of earthworms that wash up on the sidewalk after a rainstorm. In fact, there are many thousands of different worm species that live in the world today, and scientists believe there may be over half a million worm species that have yet to be discovered. Many of these worms are either very tiny or they live inside of other animals as parasites so we cannot see them. We rarely consider the critical steps that were achieved during worm evolution and how those steps contributed to the body plan of higher organisms such as humans. Nor do many people realize the enormous impact that parasitic worms have on the lives of millions of humans throughout the world. In this lesson we will discuss the distinguishing features of worms in two phyla: Platyhelminethes (flatworms) and Nematoda (roundworms), and we will consider the role these animals play in infection and disease.

Characteristics of Flatworms (Platyhelminthes)

Worms are invertebrates, but they have a more complex body structure and life cycle than the invertebrates dis- cussed in the previous lessons of this chapter. The phylogenetic tree in the Figure 1.31 illustrates the evolutionary relationships between sponges, cnidarians, flatworms, and roundworms. In the following sections we will examine the features that first appeared in the flatworm phylum and persist throughout the evolution of higher organisms. These characteristics are listed in the Structural Features Table. Then, we will discuss the specific traits of flatworms that place them in a phylum distinct from roundworms.

57 1.12. Flatworms - Advanced www.ck12.org

FIGURE 1.31 A phylogenetic tree of the invertebrate phyla in the kingdom Animalia. The evo- lutionary changes that occurred between phyla are in red text, and phyla names are in blue text.

TABLE 1.5: Structural Features Distinguishing Flatworms from Cnidarians

Flatworms Cnidarians Triploblast (3 germ layers) Diploblast (2 germ layers) Organ systems No clear organ systems Cephalization (head region) No head Bilateral symmetry Radial symmetry

Mesoderm

The appearance of an additional layer of cells in the developing organism marks a major evolutionary advance between flatworms and lower invertebrates. Flatworms are considered to be triploblasts because their organs develop from three germ layers: ectoderm, mesoderm, and endoderm. This contrasts with diploblasts such as cnidarians that develop from only two germ layers: ectoderm and endoderm. Germ layers are primary tissues that arise early in the development of an organism (in the embryo) and eventually differentiate to give rise to specific organs within the body. Each germ layer or tissue is initially made up of a sheet or mass of cells defined by its relative position in the developing embryo. As the names imply, the ectoderm is the outer cell layer, the endoderm is the inner cell layer, and the cells of the mesoderm lie between the ectoderm and endoderm. The presence of a third distinct primary tissue, the mesoderm, allows flatworms and higher animals to develop distinct organ systems. For example, the mesoderm ultimately forms muscular, reproductive, and excretory systems in adult animals. The muscular system allows the worms to move from place to place, while a reproductive system allows them to carry out sexual reproduction using eggs and sperm. The excretory system allows them to maintain a proper balance of water and salts within their bodies, much like the kidney does in humans. Flatworms also have a simple nervous system (derived from the ectoderm) that allows them to sense environmental conditions, such as the presence of food.

Cephalization

The next evolutionary change found in flatworms is termed cephalization. Cephalization refers to the concentration of nervous tissue to one end of the body, ultimately forming the head. The distinction between a “head,” or anterior region, and a “tail,” or posterior region, results in a shift from radial to bilateral symmetry, an additional feature of animals that first arose in worms and is shown in the Figure 1.32. Bilateral symmetry means that if the worm were sliced from top to bottom along the anterior-posterior midline, both sides (lateral halves) would be identical. Bilateral symmetry allows animals to move more efficiently and in a more directed manner. 58 FIGURE 1.32 Radial versus bilateral symmetry. Radial symmetry means that cutting the animal in half anywhere from top to bottom will result in two identical sections. Bilateral symmetry is more restrictive in that the animal must be cut from top to bottom along the anterior-posterior axis to pro- duce two identical halves.

As mentioned previously, the features described so far arose in flatworms and were retained during the evolution of higher organisms. Within both the flatworm and nematode phyla there is a great deal of diversity of species. www.ck12.org Chapter 1. Invertebrates - Advanced

However, there are also some similarities that are shared by all worms in each phylum which allows them to be classified together. In the following section we will examine the characteristics that are unique to flatworms, placing them in their own phylum.

Anatomy of Flatworms

Flatworms make up the phylum Platyhelminthes. There are more than 25,000 different species of flatworm, and they range in size from a millimeter (about the size of a comma on this page) to greater than 20 meters (the width of a basketball court measures about 18 meters). The flatworm phylum is further subdivided into four classes that will be described below. The features that distinguish flatworms from worms in other phyla include their flat body (dorso-ventrally, or top- to-bottom, flattened as if someone had stepped on their backs), their primitive digestive cavity (gut) with only one opening, and their lack of a body cavity, or coelom. The coelom is a fluid-filled cavity between the gut and the outer body wall that contains and supports internal organs. The coelom is usually formed when the mesoderm splits into two sheets of tissue that separate to form an internal cavity lined on both the ectodermal and endodermal side by mesodermal tissue. In acoelomates, such as flatworms, there is no space surrounding the internal organs and this area is instead filled with mesodermal tissue, as shown in the Figure 1.33.

FIGURE 1.33 A comparison of a cross-sectioned acoelomate (flatworm), pseudocoelomate (roundworm), and coelomate (higher organism).

Flatworms do not have an enclosed circulatory system, and they do not have a respiratory system. Their cells release waste by diffusion across the cell membrane directly into the environment, and they absorb oxygen by diffusion through the surface of their bodies. Sexual reproduction in flatworms is interesting in that most flatworms are hermaphrodites. That means that a single animal produces both eggs and sperm within its body. Even though an individual animal could fertilize its eggs using its own sperm (self-fertilization), there are some flatworms that must be cross-fertilized by the sperm of another individual. A number of characteristics of flatworms, including the habitats they live in, the food they eat, and the details of their life cycles, depend on the particular class of the flatworm. Flatworms are subdivided into four main classes: Turbellaria, Trematoda, Monogenia, and Cestoda. Despite the current classification, there is growing controversy and debate about exactly how flatworms should be subdivided. These conflicts are based on more recent research comparing both the anatomical differences between different flatworm species and differences identified in their DNA sequences that suggest several classes of flatworms may not have all arisen from the same ancestor and should therefore not be in a phylum together. This demonstrates the way that science itself evolves and adapts to additional

59 1.12. Flatworms - Advanced www.ck12.org information gained by active research. In the following sections we will consider the different types of flatworms in each of the four classes mentioned above.

Vocabulary

• cephalization: A concentration of nervous tissue in the anterior region.

• coelom: A fluid filled cavity formed within the mesoderm; the coelom forms between the digestive cavity and the body wall.

• mesoderm: The third tissue or germ layer; it lies between the ectoderm and the endoderm and develops into cells such as muscles, bones, teeth, and blood.

Summary

• The appearance of an additional layer of cells in the developing organism, the mesoderm, marks a major evolutionary advancement between flatworms and lower invertebrates. • Cephalization allows animals to move more efficiently and in a more directed manner. • The features that distinguish flatworms from worms in other phyla include their flat body, their primitive digestive cavity with only one opening, and their lack of a body cavity, or coelom. • There is growing controversy over exactly how flatworms should be subdivided.

Practice

Use this resource to answer the questions that follow.

• Flat Worms (Phylum Platyhelminthes) at http://www.animalcorner.co.uk/insects/worms/worms_flat.html .

1. What kind of a digestive system do flatworms possess? 2. How do flatworms propel themselves? 3. What kind of nervous system do flatworms possess?

Practice Answers

1. Flatworms possess what is known as a "blind gut." Flatworms have a mouth but no opening at the lower end of the alimentary canal for solid waste to be eliminated. Instead, they have protonephridial excretory organs. 2. Flatworms have tiny bristles called cilia that help them move. There are also two layers of muscle under their skin which they use to throw their bodies into undulating waves. 3. Flatworms have a very simple nervous systems with two nerve cords running down either side. They have two simple brains called ganglia, which are simple bundles of nerves. Flatworms also posses two eyespots that sense light.

Review

1. What traits first appeared in the flatworm phylum and led to the evolution of higher organisms? 2. Why is the emergence of the mesoderm germ layer so important to evolution? 3. What features distinguish platyhelminthes from other worms? 4. What organ systems do flatworms lack? 5. Why is there currently controversy over how flatworms are subdivided?

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Review Answers

1. Traits such as the appearance of mesoderm, organ systems, cephalization, and bilateral symmetry first ap- peared in platyhelminthes and distinguishes them from cnidarians. 2. The presence of a third distinct primary tissue, the mesoderm, allows flatworms and higher animals to develop distinct organ systems. The mesoderm ultimately forms the muscular, reproductive, and excretory systems in adult animals. 3. The features that distinguish flatworms from worms in other phyla include their flat body, their primitive digestive cavity with only one opening, and their lack of a body cavity, or coelom. There is no space surrounding the internal organs. The area is filled with mesodermal tissue. 4. Flatworms do not have an enclosed circulatory system, and they do not have a respiratory system. Their cells release waste by diffusion across the cell membrane directly into the environment, and they absorb oxygen by diffusion through the surface of their bodies. 5. More recent research suggests that several classes of flatworms may not have all arisen from the same ancestor and therefore should not be in a phylum together.

61 1.13. Flatworm Classification - Advanced www.ck12.org

1.13 Flatworm Classification - Advanced

• Differentiate between the four classes of Platyhelminthes and their special characteristics.

How would you classify this worm? Would you even begin by guessing this is a worm? Well, it is. Flatworms come in many shapes and sizes (and colors), though they all are more or less somewhat flat.

Classification of Flatworms (Platyhelminthes)

Turbellaria

Turbellaria are free-living flatworms. This means that they are able to find and digest their own food, and they do not depend on a host organism. An example of a turbellarian is shown in the Figure 1.34. Turbellaria are carnivores, and they eat other small invertebrates and dead or decaying animals. They are mostly found in aquatic environments although some species live in moist soil. Turbellaria propel themselves through the water using cilia. Cilia are small hair-like projections on the surface of the body that repeatedly flap in one direction and function like oars on a boat to move the animal through the water. One interesting feature of some turbellarian species is that, in addition to being capable of sexual reproduction, they have the ability to reproduce asexually by simply splitting their body into two halves, an anterior half and a posterior half. This is called transverse fission, and each half then regenerates the missing half to form a whole organism. Planaria is a common term often used to refer to turbellarians, however, planaria are actually a genus of one particular family of turbellaria. They are often used in biological experiments to study transverse fission.

Trematoda

Trematoda are obligate parasitic flatworms commonly called flukes. They cannot survive without a host. There are roughly 20,000 species of Trematoda. Trematoda have all of the general anatomical characteristics described

62 www.ck12.org Chapter 1. Invertebrates - Advanced

FIGURE 1.34 A turbellarian of the genus Thysanozoon.

above for flatworms plus two important, additional structures that enable their parasitic lifestyle: an oral sucker surrounding the mouth and a ventral sucker on the ventral surface. The suckers can be used to attach securely to the host and to assist in feeding off of the host tissue. Typically, adult flukes inhabit the circulatory system or the liver of a host organism. One example member of the Trematoda class, a liver fluke of the species F. magna, is shown in the Figure 1.35. Most worms in the class Trematoda have a complex life cycle that involves two or more hosts. The final host is called the primary host, and all other hosts are called intermediate hosts. Prior to reaching adulthood, trematodes develop through several different juvenile stages that are structurally very different from the adult form. These are termed larval stages. It is usually in the first larval stage that the worms enter their initial intermediate host, which is often a mollusk. Within the intermediate hosts, trematodes will continue to develop and may pass through several different larval stages before exiting the host in order to seek out the next intermediate host or the primary host. Once they have entered the primary host they complete development into the adult form. Some trematodes do not actively seek out a primary host and instead reach a cyst-like larval stage in an intermediate host (this host is said to be “encysted” with the worm). These species only reach the primary host if the primary host consumes the encysted intermediate host. As reproductive adults in the host tissue, the worms lay eggs that are then shed in the host’s feces. Following hatching, the offspring become larvae, and they seek out their first intermediate hosts to begin the life cycle again. This complicated pathway is depicted in the Figure 1.36. Several species of Trematoda are the cause of numerous human infections that will be discussed in the Flatworms: Diseases (Advanced) concept.

Monogenea

Members of the class Monogenea are very similar to trematodes. In fact, they were once considered a subclass of Trematoda. Monogeneans are also obligate parasites, however, they are ecto-parasites. They attach and feed off of the outside epidermal layer of their hosts. In addition, they have only one host per lifetime (this is the meaning of the word monogenea). Most worms in the monogenean class infect fish, so their impact on humans is minimal.

Cestoda

Cestoda are another class of parasitic flatworms, commonly called tapeworms. There are several species of tapeworm that are infectious to humans. Tapeworms can grow up to 18 meters long. These worms do not have a mouth or a digestive cavity because they live in the intestinal tract of vertebrates and feed by absorbing nutrients from food

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FIGURE 1.35 A liver fluke (species F. magna). These parasites often infect deer.

digested by the host. Nutrients are absorbed through cells on the surface of their bodies. Like trematodes, cestodes have a complex life cycle involving intermediate hosts. One difference is that they do not actively seek hosts at any stage. Hosts become infected only when they ingest an encysted intermediate host or egg. Although they have all of the other organ systems common to flatworms (nervous system, excretory system, and reproductive system), the anatomy of cestodes is very different from that of other flatworms. This is primarily due to an elaborate reproductive system made up of proglottids. The proglottids are repeated segment-like regions that are produced by and reside behind a “neck” structure. Each proglottid contains both male and female reproductive organs, and a single animal can produce up to thousands of proglottids. A single proglottid can produce thousands of eggs, making the reproductive capacity of the cestode enormous. In addition, they have a distinctive head region termed the scolex, which usually has several hooks and suckers for attachment to the host’s intestinal wall. Photographs highlighting the proglottid and scolex of two cestode species are shown in the Figure 1.37. Tapeworms in the order Cyclophyllidea have the greatest impact on humans due to their use of both humans and livestock as hosts. We will discuss this impact in another concept.

Vocabulary

• cilia (singular, cilium): Short, hairlike projections that are similar to flagella and allow some cells to move.

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FIGURE 1.36 The life cycle of Fasciola hepatica (sheep liver fluke). The sheep liver fluke has a complicated life cycle with two hosts. How could such a complicated way of life evolve?

FIGURE 1.37 Proglottids from the tapeworm species D.latum (a) and the scolex from the tape- worm species T.solium (b).

• flukes: Obligate parasitic flatworms, also known as Trematoda.

• intermediate host: A host to a parasite that harbors the parasite for only a transitional period.

• larval stages: The early, developing stages of various animals in which the organism differs from its adult form.

• primary host: A host to a parasite which allows it to reach maturity; the final host of a parasite.

65 1.13. Flatworm Classification - Advanced www.ck12.org

• proglottid: A repeated segment-like region behind the neck of a tapeworm that contains both male and female reproductive organs.

• scolex: The anterior end of a tapeworm, which has suckers or hooklike parts for attachment to the host.

• transverse fission: A form of asexual reproduction where the body of an animal splits into two halves, an anterior half and a posterior half, which then regenerate into two organisms.

Summary

• Turbellaria are free-living, carnivorous flatworms that eat other small invertebrates and dead or decaying animals. • Trematoda, or flukes, are obligate parasitic flatworms that cannot survive without a host. • Most flatworms in the class Trematoda have a complex life cycle that involves two or more hosts. • Monogenea are ecto-parasites that have only one host per lifetime. • Cestoda, or tapeworms, do not have a mouth or a digestive cavity because they live in the intestinal tract of vertebrates and feed by absorbing nutrients from food digested by the host.

Practice

Use this resource to answer the questions that follow.

• Platyhelminthes at http://www.encyclopedia.com/topic/Platyhelminthes.aspx .

1. How are turbellarians subdivided? 2. Describe the reproductive cycle of Cestoda. 3. Where can monogenetic flukes be found?

Practice Answers

1. Turbellarians are generally divided into five groups based primarily on differences in the form of their di- gestive cavity, a structure that is readily observable through their transparent body wall. The most primitive turbellarians, the acoels, have no digestive cavity. 2. The proglottids of tapeworms constantly bud off and gradually enlarge. As they mature they become filled with male and female reproductive organs. Cross-fertilization takes place with adjacent worms or neighboring proglottids; in some cases self-fertilization occurs. Eggs are then shed through feces. 3. Monogenetic flukes spend their entire life cycle as parasites on a single host, often on the gills and skin of fish.

Review

1. Which class of Platyhelminthes are not parasitic? 2. Describe what occurs during transverse fission. 3. In what organ systems are Trematoda usually found? 4. Give a brief description of the life cycle of a typical fluke. 5. How is the anatomy of Cestoda different from other flatworms?

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Review Answers

1. The class Turbellaria are free-living flatworms that eat other small invertebrates and dead or decaying animals. 2. Transverse fission is a form of asexual reproduction where the body splits into two halves and then regenerates into two whole organisms. 3. Typically, adult flukes inhabit the circulatory system or the liver of a host organism. 4. After the egg of a fluke is fertilized, the first larval stage usually involves the worm entering its initial intermediate host. The fluke develops and passes through different stages within the intermediate host before exiting to find another intermediate host or a primary host. The fluke completes development into the adult form inside the primary host and lays eggs that are shed in the host’s feces. 5. Although they have all of the other organ systems common to flatworms (nervous system, excretory system, and reproductive system), tapeworms do not have a mouth or digestive cavity because they absorb nutrients from the host directly. They also have both proglottids that each contain male and female productive organs and a specialized scolex for attaching to hosts.

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1.14 Flatworm Diseases - Advanced

• Explain several types of parasitic worm infections and the impact of those infections on human health.

Where can you find tapeworms? This tapeworm is inside a human intestine. Both the flatworm and roundworm phyla contain a number of parasitic species that are infectious to humans. Many of these infections can be prevented by thoroughly cooking food (especially meat) and practicing good hygiene.

Parasitic Flatworms

Most parasitic flatworm species are contained within the classes Trematoda (flukes) and Cestoda (tapeworms), described in the Flatworms: Classification concept. The most common types of human flatworm infections are summarized in the Common Human Flatworm Infections Table. TABLE 1.6: Common Human Flatworm Infections

Disease(s) Worm type Worm species Host entry method Infected tissue Schistosomiasis Blood fluke Several Skin Blood Fascioliasis, Tissue fluke Several Ingestion Liver, Lung Clonorchiasis, Paragonimiasis Taeniasis, Cysticer- Tapeworm T. solium, T. sagi- Ingestion Various tissues cosis nata

Trematodes, or flukes, have an enormous impact on the health of humans by infecting over 200 million people worldwide. There are two general types of flukes: blood flukes and tissue flukes.

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There are several different species of blood flukes that infect humans, and they are predominantly found in tropical countries. They feed on blood cells and cause a disease called schistosomiasis. The symptoms vary, depending on which parts of the body the worms inhabit, but can include fever, abdominal pain, coughing, diarrhea, and enlargement of the liver and spleen. The disease is contracted by swimming in freshwater contaminated with infected snails (recall that mollusks are a common intermediate host for flukes). Larvae that have left the snails enter the human body by penetrating the skin and migrating to the circulatory system. A good summary of schistosomiasis can be found in The Blood Fluke at http://www.youtube.com/watch?v=VnlYU e57Lr0 (1:36).

MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/139363

The tissue flukes include seven species that are infectious to humans. They typically infect the liver or gastrointestinal tract. Unlike blood flukes, tissue flukes are not able to actively seek out and penetrate the skin of their primary host. Humans become infected when they ingest encysted plant or animal intermediate hosts (usually fish). Tissue flukes are a major cause of disease in both humans and livestock and have severe impacts both on the health and economy of humans. The most important parasitic tapeworms for humans are the species T. solium (the pork tapeworm) and T. saginata (the beef tapeworm). Both species cause a disease called taeniasis, but only T. solium can lead to the disease cysticercosis. Taeniasis is a classical tapeworm infection where humans are the primary hosts and become infected by consuming either undercooked pork from an encysted pig infected with T. solium or undercooked beef from an encysted cow infected with T. saginata. The consumed larvae reach adulthood in the intestine. The adults attach to the intestinal lining and feed on digested material in the human intestine. The symptoms of taeniasis can include loss of appetite, nausea, vomiting, and diarrhea, but there are often no symptoms until the person has been infected for a long time. Following a long infection, the symptoms can include malnutrition and possibly intestinal blockage. Cysticercosis is another, more serious, disease that is caused by T. solium. It is believed to result in roughly 50,000 deaths each year worldwide. T. solium is able to cause cysticercosis because it has the unusual ability to utilize humans as an intermediate host in addition to a primary host. The infection occurs when humans consume T. solium eggs that were shed in the feces of an infected individual. The result is that the eggs hatch and develop into a larval form that is capable of penetrating the intestinal wall and migrating through the body to form cysts in various tissues. These tissues include striated muscle tissue, brain, liver, or a number of other tissues. This infection can be quite serious and even fatal, particularly if the organism forms cysts in the brain (called neurocysticercosis). For more information on the pork tapeworm, see Monsters Inside Me: Pork Tapeworm at http://www.youtube.com/w atch?v=bb32g02IIs8 (1:56).

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Unlike T. solium, T. saginata cannot use humans as an intermediate host. If humans consume T. saginata eggs, these

69 1.14. Flatworm Diseases - Advanced www.ck12.org eggs are not able to develop into cyst-forming larvae, and there is no risk of cysticercosis. Both tapeworm and fluke infections can generally be treated using drugs that specifically target flatworms.

Vocabulary

• cysticercosis: An infection of the body by a pork tapeworm; this infection causes cysts in muscle tissue, brain, liver, or a number of other tissues.

• schistosomiasis: An infection of the body by blood flukes; the symptoms include fever, abdominal pain, coughing, diarrhea, and enlargement of the liver and spleen.

• taeniasis: An infection of the body by a tapeworm; this infection causes loss of appetite, nausea, vomiting or diarrhea, malnutrition, and possibly intestinal blockage.

Summary

• Both the flatworm and roundworm phyla contain a number of parasitic species that are infectious to humans. • Most parasitic flatworm species are contained within the classes Trematoda (flukes) and Cestoda (tapeworms). • Trematodes, or flukes, have an enormous impact on the health of humans by infecting over 200 million people worldwide. • The most important parasitic tapeworms for humans are the pork tapeworm and the beef tapeworm.

Practice

Use this resource to answer the questions that follow.

• Parasitic Worms at http://parasitology.com/worms/ .

1. Give an example of how parasitism changed flatworm anatomy. 2. Where are blood flukes mainly found? 3. How do blood flukes manage to disperse their eggs? 4. What is "swimmer’s itch?" 5. How do the larvae of tapeworms get into the human body?

Practice Answers

1. Parasitic flatworms have reduced nervous systems because there is little need for sensory feedback when they infest their host. The tapeworm does not have a nervous system. 2. Blood flukes are mainly found in Southeast Asia, North Africa, and other tropical areas. 3. Blood flukes disperse their eggs by laying large numbers of eggs in small blood vessels near the host’s intestine. The large number of eggs sometimes forces the blood vessel to break open into the intestine, and the eggs are then transported out with feces. 4. "Swimmer’s itch" occurs when flukes try to penetrate the skin of humans. Blood flukes in the US cannot live in human bodies, and the itch usually goes away with time. 5. The larvae of tapeworms burrow into the muscle of its intermediate host and become cysts. Humans that eat meat that is not fully cooked may ingest these cysts and become infected by tapeworms.

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Review

1. Which species of Platyhelminthes causes schistosomiasis? 2. How do parasitic flatworms enter the human body? 3. What are the symptoms of taeniasis? 4. Why is infection by the pork tapeworm more serious than infection by the beef tapeworm?

Review Answers

1. Blood flukes feed on blood cells and cause schistosomiasis. 2. Blood flukes infect the human body by penetrating human skin when humans swim in contaminated water. Other types of parasitic flatworms usually enter the human body when individuals eat encysted plant or animal intermediate hosts. 3. The symptoms of taeniasis can include loss of appetite, nausea, vomiting, and diarrhea, but often there are no symptoms until the person has been infected for a long time. Symptoms such as malnutrition and possibly intestinal blockage eventually develop. 4. The pork tapeworm can cause cysticercosis, a disease where the larval form of the tapeworm forms cysts in various tissue. This is especially serious if cysts form in the brain. Beef tapeworms cannot cause cysticercosis.

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1.15 Roundworms - Advanced

• Describe the structural features of roundworms that distinguish them from flatworms.

When most people picture a worm, do they picture a roundworm? Actually, they do not. Whereas flatworms are flat, roundworms obviously appear round. With over 80,000 species, there are plenty of different types of roundworms. But these are still not the earthworms most people picture when they think of worms.

Characteristics of Roundworms (Nematoda)

Roundworms make up the phylum Nematoda. This is one of the most abundant animal phyla, with greater than 80,000 known species (although not all of them have been classified). They range in size from less than one millimeter to 7 meters. Roundworms are found nearly everywhere, including aquatic and terrestrial environments as well as parasitically on or within a variety of plants and animals. In the next several sections we will review the anatomy of roundworms, highlighting the features specific to this phylum.

Anatomy of Roundworms

As the name implies, roundworms are cylindrical in shape. Like flatworms, roundworms have excretory, nervous, and reproductive systems. Another similarity to flatworms is that they lack an enclosed circulatory and respiratory system. The two most prominent features that distinguish roundworms from flatworms are the presence of a pseudocoelom, or partially developed body cavity, and a complete digestive tract with two openings, a mouth and an anus. A pseudocoelom differs from a true coelom in that the cavity is lined with mesoderm only on the ectodermal side. The presence of a body cavity allows circulatory fluid to flow freely throughout the body of the organism and facilitates the exchange of material between cells (including the release of waste). A complete digestive tract with

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separate openings for the mouth and anus allows the animals to simultaneously feed, digest, and eliminate waste. The main differences between flatworms and roundworms are summarized in the Comparison Table.

TABLE 1.7: A Comparison of Flatworms and Roundworms

Flatworms (Platyhelminthes) Roundworms (Nematoda) Flat body Round body Primitive gut (1 opening) Complete digestive tract (2 openings) Acoelomate (no body cavity) Pseudocoelem (partial body cavity)

Roundworms have a thick substance called a cuticle on the surface of their bodies that is secreted by the outer epidermal cells. The cuticle is fairly rigid and limits the volume of the worm. This allows the build up of hydrostatic pressure from fluid accumulated in the worm and contributes to what is called a hydrostatic skeleton. The force of the hydrostatic pressure allows the worm to maintain its cylindrical shape. This hydrostatic force along with contractions from the muscles lining the pseudocoelom allows the worm to move along solid surfaces. As is the case for flatworms, there are both free-living and parasitic species of roundworms. The nervous system of nematodes, like that of flatworms, is simple and has a concentrated nerve center in the anterior, or head, region. Nematodes also have sensory organs connected to their nervous system. They generally have either one, termed the amphid, or two: an anterior amphid and a posterior phasmid. An example of a roundworm, the soybean cyst nematode, is shown in the Figure 1.38.

FIGURE 1.38 Scanning electron micrograph (micro- scopic photograph) of a soybean cyst ne- matode magnified 1000x.

Vocabulary

• amphid: A circular depression situated laterally at the anterior end of nematodes and believed to be chemore- ceptors.

• coelom: A fluid filled cavity formed within the mesoderm; it forms between the digestive cavity and the body wall.

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• cuticle: A thick organic layer surrounding the outer surface of nematodes and arthropods; a waxy waterproof covering over the aerial surfaces of a plant.

• hydrostatic skeleton: A structure consisting of a fluid-filled cavity surrounded by muscles; it is used to change an organism’s shape and produce movement.

• phasmid: One of a pair of circular depressions situated laterally at the posterior end of nematodes and believed to be chemoreceptors.

• pseudocoelom: A partially developed, fluid-filled body cavity that is lined with mesoderm only on the ectodermal side.

Summary

• The two most prominent features that distinguish roundworms from flatworms are the presence of a pseudo- coelom, or partially developed body cavity, and a complete digestive tract with two openings, a mouth and an anus. • Roundworms have a thick substance called a cuticle on the surface of their body that is secreted by the outer epidermal cells. • There are both free-living and parasitic species of roundworms.

Practice

Use this resource to answer the questions that follow.

• Introduction to the Nematoda at http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html .

1. How prevalent are species of Nematoda? 2. What groups are nematodes most closely related to? 3. What is unique about a nematodes nerve and muscle cells? 4. How are nutrients distributed within a nematode? 5. What is cryptobiosis? 6. Where can you find nematodes?

Practice Answers

1. If scientists are correct in their estimated number of species, roundworms are the second most diverse group of animals behind arthropods. 2. Nematodes are most closely related to arthropods and priapulids. 3. Whereas most other animals have nerves that branch out to the muscles, nematodes have muscle cells that branch toward the nerves. 4. Nutrients are distributed in the pseudocoelom, where contents are regulated by an excretory canal. 5. Cryptobiosis is the ability to suspend life process completely when conditions are unfavorable and return to life when favorable conditions return. Nematodes share this feature with rotifers and tardigrades. 6. Nematodes can be found in most habitats: extreme cold, hotsprings, on or in most animals and plants, in soils and sendiment, and other types of environments.

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Review

1. What are the main differences between roundworms and flatworms? 2. What differentiates a pseudocoelom from a true coelom? 3. Why is a body cavity so important as an evolutionary development? 4. How is the roundworm’s digestive system superior to the digestive system of flatworms? 5. What allows roundworms to maintain their shape and move?

Review Answers

1. The two most prominent features that distinguish roundworms from flatworms are the presence of a pseudo- coelom, or partially developed body cavity, and a complete digestive tract with two openings, a mouth and an anus. 2. A pseudocoelom differs from a true coelom in that the cavity is lined with mesoderm only on the ectodermal side. 3. The presence of a body cavity allows circulatory fluid to flow freely throughout the body of the organism and facilitates the exchange of material between cells (including the release of waste). 4. Roundworms have complete digestive tracts with separate openings for the mouth and anus, allowing round- worms to simultaneously feed, digest, and eliminate waste. 5. The cuticle of roundworms allows for hydrostatic pressure to build up and contributes to a hydrostatic skeleton that maintains shape and allows for movement.

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1.16 Roundworm Classification - Advanced

• Understand the main subdivision of nematodes. • Learn about how a certain species of roundworm acts as a model organism.

What can we learn from the roundworm? In fact, quite a bit. Caenorhabditis elegans, a free-living, transparent nematode (roundworm) about 1 mm in length, is a model organism. This worm has been repeatedly used for scientific research, such as genetic and developmental studies, and has provided lots of important information.

Classification of Roundworms (Nematoda)

Roundworm Classes

The phylum Nematoda is divided into two classes, Adenophorea and Secernentea. The main basis for this sub- division is the absence (Adenophorea) or presence (Secernentea) of the phasmid sensory organ described in the Roundworms: Characteristics (Advanced) concept. Most Adenophorea are non-parasitic. Free-living nematodes generally feed on bacteria, fungi, and protozoans. Secernentea are almost all parasitic and primarily terrestrial. Their hosts include plants, invertebrates, and vertebrates. Some parasitic species have a fairly simple life cycle with only one host, but other species have more complex life cycles resembling those of the trematodes. The number of parasitic nematode species is very large. Almost all vertebrate species can be preyed upon by at least one parasitic nematode. Despite this sizable negative impact on the population, there are also positive roles played by roundworms. For example, free-living nematodes that consume bacteria and decaying material in soil contribute to the breakdown of organic matter and play an important role in the carbon cycle. Another positive role of a particular nematode species is described in the next section.

Roundworms as Model Organisms in Biology

In the Flatworms: Diseases (Advanced) and Roundworm: Diseases (Advanced) concepts, we will consider the negative impacts that both parasitic flatworms and parasitic roundworms have on the human population. However,

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there is one species of free-living roundworm that has had a significant positive impact on human health: the species C. elegans, shown in the Figure 1.39.

FIGURE 1.39 The free-living nematodes C. elegans.

C. elegans, a tiny nematode about 1 millimeter in length, is what is known as a model organism.. Model organisms are organisms that have been selected by the scientific community, based on fundamental similarities they have with other organisms (particularly humans), for extensive study. The hope is that these studies will lead to advances in our understanding of disease and possibly treatment. Studies first began on C.elegans in the 1970s, and a great deal of information has been gained since then. The entire genome of this species was sequenced to completion in 1998, and this revealed that 36% of the worm’s genes have relatives in the human genome, including many human genes known to be involved in disease. One very exciting area of research in C.elegans focuses on the mechanisms that control the lifespan of an organism. By engineering mutations in the C.elegans genome, several genes have been found that, when mutated, lead to a significant increase in the lifespan of the worm. What is most fascinating about this research is that many of these genes have closely related genes in the human genome. Another example of the beneficial work that can be done using such a simple organism is a recent research study finding that C.elegans have a variety of behavioral responses to nicotine that are similar to those of humans, including withdrawal symptoms and the ability to develop tolerance to the drug. It was also shown that mutations in certain worm genes could eliminate those behavioral responses to nicotine and that inserting a human version of those genes into the worm could rescue the nicotine-response defects. This work could have important implications on the study of nicotine addiction in humans. Currently, C.elegans are being sent into space for studies on the long-term effects of the spaceflight environment on processes such as development, aging, and disease. Clearly these tiny worms are having a large impact on the science of human health.

Vocabulary

• model organism: A non-human species that is extensively studied to understand particular biological phe- nomena.

Summary

• The phylum Nematoda is divided into two classes: Adenophorea and Secernentea. • C. elegans, a tiny nematode about 1 millimeter in length, is a model organism.

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Practice

Use this resource to answer the questions that follow.

• The Nematodes at http://cal.vet.upenn.edu/projects/merial/nematodes/nems_2.htm .

1. How are species named? 2. What are some common suffixes that stand for taxonomic groups? 3. Which nematodes are bursate nematodes? What characterizes them?

Practice Answers

1. Species are given binomial, Latin names based on the system devised by Linnaeus in 1753. 2. The suffix -ida is given to the order, -oidea for superfamily, -idae for family, and -inae for subfamily. 3. Nematodes in the order Strongylida are bursate nematodes because each male has a pronounced copulatory bursa at the tail (posterior) end.

Review

1. What is the main difference between Adenophorea and Secernentea? 2. Which nematodes are usually free-living? Which are usually parasitic? 3. What positive roles do some nematodes play in the environment? 4. Give a few examples of how research into C. elegans might lead to benefits for mankind.

Review Answers

1. The main basis for the subdivision of the phylum Nematoda is the absence (Adenophorea) or presence (Secernentea) of the phasmid sensory organ. 2. Most Adenophorea are non-parasitic. Free-living nematodes generally feed on bacteria, fungi, and protozoans. Secernentea are almost all parasitic and primarily terrestrial. 3. Free-living nematodes that consume bacteria and decaying material in soil contribute to the breakdown of organic matter and play an important role in the carbon cycle. 4. Scientists have studied C. elegans in relation to aging, nicotine addiction, and spaceflight in an effort to further treatment from side-effects.

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1.17 Roundworm Diseases - Advanced

• Explain several types of parasitic worm infections and the impact of those infections on human health.

What’s round, looks like a worm, and can cause disease? Whatever you do, do not eat a roundworm, like those shown here. Some of these worms like to live in a human environment. Some can cause diseases. Though most can be treated, having worms is probably not pleasant.

Parasitic Roundworms

There is a large number of parasitic roundworm species, including pinworms, hookworms, and whipworms. The most common types of human roundworm infections are summarized in the Roundworm Infections Table, and we will highlight distinct features of several of these infections in this section. In general, the lifecycle of parasitic roundworms is simpler than those of flukes and tapeworms - it usually involves a single host. Most roundworm infections can be treated with various medications that target the parasite.

TABLE 1.8: Common Human Roundworm Infections

Disease Worm type Worm species Host entry method Infected tissue Enterobiasis Pinworm E. vermicularis Ingestion of eggs Intestine Ascariasis A. lumbricoides Ingestion of eggs Intestine Ancylostomiasis Hookworm Skin Intestine

A. duodenale

N. americanus

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TABLE 1.8: (continued)

Disease Worm type Worm species Host entry method Infected tissue Trichuriasis Whipworm T. trichiura Ingestion of eggs Intestine Strongyloidiasis - S. stercoralis Skin Intestine Trichinosis - T. spiralis Ingestion Skeletal muscle of encysted intermediate host

Enterobiasis, the disease caused by pinworm infections, is the most common roundworm infection in the United States. It is estimated that in some regions of the country one-third of children are infected. Pinworms are tiny worms (roughly 1 cm in length) that have a simple life cycle involving only one host. Humans become infected when they ingest pinworm eggs. The eggs hatch inside the host’s digestive tract and develop into adults. The adults migrate to the colon and lay eggs in the rectal region. Despite the high prevalence, enterobiasis is not a dangerous disease. The primary symptom is itching in the rectal region. Ascariasis is a disease caused by the roundworm Ascaris lumbricoides. It occurs worldwide, and it is believed to infect up to 25% of the world’s population - roughly 1.5 billion people! Infection occurs when eggs are consumed and hatch in the intestine. Larvae penetrate the intestinal wall and migrate to the lungs. In the lungs they enter the air passageways and migrate to the throat where they are re-swallowed. The larvae mature into adulthood in the intestine and attach to the intestinal wall. This complicated migration pathway is also shared by hookworms and the roundworm species Strongyloides stercoralis. Early infections usually do not have any symptoms, although the migration from the intestine to the lungs sometimes leads to tissue damage. When the infection has progressed, symptoms include malnutrition and bowel obstruction. Interestingly, A. lumbricoides feeds on digested material in the host’s intestine, similar to a tapeworm. Ancylostomiasis can be caused by infection with either of two hookworm species: Ancylostoma duodenale or Necator americanus. Hookworm eggs hatch outside of a host and develop into larvae that infect humans by penetrating the skin. Once inside the body, they travel a route similar to A. lumbricoides, arriving at the intestine through the lungs. Hookworm infections are widespread and symptoms may include diarrhea, nausea, abdominal pain, and anemia. Strongyloidiasis is caused by the roundworm species Strongyloides stercoralis. This species is very unusual in that it can alternate between a free-living and a parasitic life cycle. Following hatching outside of a host, S. stercoralis eggs develop into an early larval stage. These larvae then have two life cycle options. They can continue to develop through several more larval stages and into reproductive adults outside of a host (free-living cycle), or they can adopt an intermediate, infective larval form that must enter a host in order to reproduce (parasitic cycle). The second unusual feature of S. stercoralis is that infective larvae that have reached the intestines mature into adult females that are able to reproduce using a type of asexual reproduction called parthenogenesis (the eggs develop into adults without being fertilized by sperm). The eggs hatch and form larvae which either exit the host through the feces or penetrate the intestinal wall and repeat the infective migration cycle. If the larvae penetrate the intestinal wall and repeatedly infect the host, it is called an autoinfection and can lead to a more severe and persistent form of the disease. Unless autoinfection occurs, symptoms of strongyloidiasis are generally mild and can include abdominal pain and diarrhea. All of the common parasitic roundworms have parasitic life cycles involving only one host with the exception of Trichinella spiralis. T.spiralis causes trichinosis and has a more complex life cycle involving intermediate hosts. An additional difference between T. spiralis and the roundworm infections discussed above is the site of infection. Most roundworms infect the intestines, but T. spiralis forms an infection in the skeletal muscle. Ingested T. spiralis larval cysts develop into adulthood in the intestine. The adults reproduce in the intestine to generate larvae that can penetrate the intestinal wall and migrate to the skeletal muscles where they form cysts. Symptoms are often mild but include muscle aches, fever, and swelling. More severe infections can lead to breathing and heart problems. Generally, trichinosis can only be treated with medications that target the worms early during infection. Once the cysts have formed, treatment is primarily directed at alleviating symptoms, for example, steroids to counteract

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FIGURE 1.40 Hookworm Parasite. Hookworms like this one are common human parasites.

inflammation caused by migration and cysts. The infection usually subsides over time. T. spiralis can be passed from pigs to humans through under-cooked pork. For a video examining a case of roundworm infection, see Deadly Roundworm - Monsters Inside Me at http://www.y outube.com/watch?v=Z9RJHkzQtXc (3:15).

MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/139355

Vocabulary

• ancylostomiasis: A disease caused by a hookworm infection (Ancylostoma duodenale or Necator ameri- canus); symptoms include diarrhea, nausea, abdominal pain, and anemia.

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• ascariasis: A disease caused by an Ascaris lumbricoides infection; symptoms may include tissue damage, malnutrition, and bowel obstruction.

• autoinfection: An infection caused by a disease agent that is already present in the body.

• enterobiasis: A disease caused by a pinworm infection; symptoms include itching in the rectal region.

• parthenogenesis: A form of asexual reproduction where growth and development of embryos occur without fertilization.

• strongyloidiasis: A disease caused by Strongyloides stercoralis; symptoms are generally mild and can include abdominal pain and diarrhea.

• trichinosis: A disease caused by T. spirals; symptoms include muscle ache, fever, swelling, and possibly breathing and heart problems.

Summary

• There is a large number of parasitic roundworm species, including pinworms, hookworms, and whipworms. • Most roundworm infections are not deadly and can be treated with various medications that target the parasite. • In general, the lifecycle of parasitic roundworms is simpler than those of flukes and tapeworms - it usually involves a single host.

Practice

Use this resource to answer the questions that follow.

• Roundworms at http://umm.edu/health/medical/altmed/condition/roundworms .

1. What is the main cause of roundworm infection? 2. What is the leading cause of blindness worldwide? 3. List some ways with which to prevent infection by roundworms. 4. How are roundworm infections usually treated?

Practice Answers

1. The main cause of roundworm infection is poor sanitation and hygiene. People eat or drink things that are contaminated with roundworm eggs. 2. River blindness, a disease caused by the roundworm Onchocerca volvulus, is the leading cause of blindness worldwide and is contracted through day-biting flies. 3. Roundworm infections can be prevented by using proper hygiene, using bug spray, staying away from mosquito/fly- infested areas, cooking meat thoroughly, checking your pets, and preventing children from coming into contact with pet feces. 4. Anti-parasitic drugs are available that kill roundworms. Medication varies depending on the type of infection. Surgery may be necessary.

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Review

1. What is the most common roundworm infection in the United States? What disease does it cause? 2. Describe the complicated migration pathway of Ascaris lumbricoides. 3. How do hookworms infect humans? 4. What are some distinguishing features of S. stercoralis? 5. What roundworm forms an infection in skeletal muscle?

Review Answers

1. The most common roundworm infections in the United States are pinworm infections, which cause enterobi- asis. The primary symptom is itching in the rectal region. 2. The roundworm Ascaris lumbricoides infects the human body when eggs are consumed and hatch in the intestine. Larvae penetrate the intestinal wall and migrate to the lungs. They then migrate to the throat and are re-swallowed, returning back to the intestines. 3. Hookworm eggs hatch outside of a host and develop into larvae that infect humans by penetrating the skin. 4. S. stercoralis can develop into free-living adults or adopt an intermediate, infective larval form that must enter a host in order to reproduce. The infective larvae of S. stercoralis can reproduce using parthenogenesis and repeat the infective migration cycle through autoinfection. 5. The roundworm T. spiralis forms an infection in the skeletal muscle of humans. Larvae penetrate the intestinal wall and migrate to the skeletal muscles where they form cysts.

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1.18 Mollusks - Advanced

• Describe the characteristics and the different classes of mollusks.

Fish or squid? Neither. This is a mollusk, a cuttlefish to be specific. What is a mollusk? Well, to start, mollusks are aquatic species that are not fish. There are over 100,000 different mollusks, so there are bound to be some interesting looking organisms, like this one. Most people would not consider a simple worm to be a highly evolved organism. But in fact, that is exactly what the millions of worm species on the planet today are. This is evidenced by the fact that worms have evolved and retained a worm-like shape and structure over the course of millions of years of evolution. You learned in the Evolution concepts that evolution does not always lead to more complex structures. Sponges and worms are two examples of a highly successful body form that is well adapted to life on Earth. In this set of concepts and the next, we will be discussing the invertebrate phyla mollusks and annelids. Annelids are segmented worms, so why aren’t they grouped together with the flatworms and roundworms? Because annelids are actually more closely related to species within the mollusk phylum than they are to their worm cousins in the flatworm and roundworm phyla. Although they look quite different on the outside, mollusks and annelids share a more recent common ancestor than roundworms and annelids. In this set of concepts and the next, you will learn about the features of annelids and mollusks, and you will come to understand how their internal organization reflects their common ancestry.

Characteristics of Mollusks

The phylum Mollusca is a large and diverse group of highly successful animals found in many different habitats throughout the world. Next to arthropods, mollusks are probably the most familiar of all of the invertebrates because of their abundance in shallow marine areas. If you have ever been to a beach on the ocean, then you have likely encountered members of the phylum Mollusca such as snails and clams. If you have not been near the ocean, but

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have eaten seafood, then it is also very likely that you have seen mollusks since they make up a large fraction of the seafood consumed by humans. The word mollusk means soft-bodied, but when we think of many of the commonly known mollusks, such as clams or snails, it is the hard outer shell that is the most obvious feature. Within the shell lies the soft-bodied animal. In the first section of this lesson we will consider the characteristics of mollusks and how they are classified. Subsequent sections will address the structural features, body plan, and ecology of mollusks.

FIGURE 1.41 This figure shows some of the more com- mon and familiar mollusks.

Based on the number of described species, mollusks make up the second largest invertebrate phylum, with over 100,000 known species in existence today and many thousands more species that are now extinct. This phylum includes such varied organisms as snails, clams, and octopi. They range in size from nearly microscopic organisms to the giant and colossal squid that can be over 40 feet long. Examples of the extreme size differences in the phylum Mollusca are shown in the Figure 1.42.

FIGURE 1.42 (a) A small slug. (b) The body of a giant squid found in 1954.

Because of the hard outer shell that is present on most mollusks, they are well-preserved and have a long and abundant fossil record. Their fossil record begins about 600 million years ago and reveals alternating periods of large extinctions and major diversifications of species. The most significant evolutionary change seen in the phylum Mollusca is the increasing complexity of organ systems. This increasing complexity continues throughout the evolution of higher invertebrates. Mollusks have also evolved several structural features that are unique to the species of this phylum and do not persist in higher invertebrates. These will be considered in a later section. Mollusks are triploblasts meaning that they develop from three basic embryonic germ layers: an endoderm, a mesoderm, and an ectoderm. Most mollusk species have a distinct head region, a muscular foot, and a hard shell on the top, or dorsal, side that contains the internal organs. A defining feature of the phylum that is present on most mollusks is a dorsal layer of tissue called the mantle that lies between the body and the shell. Like the more primitive roundworms, mollusks have a complete digestive system and a nervous system. In addition, they have evolved an open circulatory system with a heart, a tubular excretory system, and a respiratory system. The Figure 1.43 shows the common snail, a classic example of a mollusk. Mollusks are generally considered a phylum of bilaterally symmetrical animals, although there are also many asymmetrical species. Bilateral symmetry means that an organism can only be divided into two equal halves if it is

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FIGURE 1.43 A common snail. Note the distinct head region, the hard shell, and the muscular foot below the shell.

cut from top to bottom along the middle of the anterior-posterior (front-back) axis. Asymmetric organisms cannot be divided into two equal halves along any axis. The major bilaterally symmetrical phyla include the following:

• Flatworms (Platyhelminthes). • Roundworms (Nematoda). • Mollusca. • Annelida. • Echinodermata. • Chordata.

These phyla are divided into two groups based on their pattern of embryonic development: the protostomes and the deuterostomes. Flatworms, roundworms, mollusks, and annelids are all protostomes. As you learned in the Invertebrates concepts, these are the major features that distinguish protostomes from deuterostomes:

1. The orientation of the first embryonic cell divisions. 2. The origins of the mouth and anus. 3. Formation of the mesoderm.

During the first several embryonic cell divisions, protostomes exhibit spiral, determinant cleavage, while deuteros- tomes undergo radial, indeterminant cleavage. The blastopore formed during gastrulation ultimately becomes the mouth of the protostome. In deuterostomes, the blastopore becomes the anus, and a separate opening for the mouth develops later. In protostomes, the mesoderm and coelom are formed by the differentiation and splitting of cells that lie between the endoderm and the ectoderm in the developing gastrula. Deuterostomes form mesodermal tissue and a coelom by the invagination and differentiation of part of the endoderm. Along with their close relatives the annelids, mollusks are the first true coelomates. Coelomates are animals with a fluid-filled body cavity called the coelom that is completely lined with mesodermal tissue. The coelom serves a number of functions within an animal’s body. It provides space and attachment sites within the mesoderm for internal organs, and the fluid in the cavity cushions these organs to protect them from shock when the animal is moving. In mollusks, however, the coelom is often reduced to a small cavity surrounding the heart. Annelids generally have a well-developed coelom. Another major difference between mollusks and annelids is that annelids have segmented bodies and mollusks do not.

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KQED: The Fierce Humboldt Squid

The Humboldt squid is a large, predatory invertebrate found in the waters of the Pacific Ocean. A mysterious sea creature up to 7 feet long, with 10 arms, a sharp beak, and a ravenous appetite, packs of fierce Humboldt Squid attack nearly everything they see, from fish to scuba divers. Traveling in groups of 1,000 or more and swimming at speeds of more than 15 miles an hour, these animals hunt and feed together, and use jet propulsion to shoot out of the water to escape predators. Humboldt squid live at depths of between 600 and about 2,000 feet, coming to the surface at night to feed. They live for approximately two years and spend much of their short life in the ocean’s oxygen- minimum zone, where very little life exists. Because they live at such depths, little is known about these mysterious sea creatures. The Humboldt squid usually lives in the waters of the Humboldt Current, ranging from the southern tip of South America north to California, but in recent years, this squid has been found as far north as Alaska. Marine biologists are working to discover why they have headed north from their traditional homes off South America. See http://www.kqed.org/quest/television/the-fierce-humboldt-squid (10:02) for additional information.

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KQED: Cool Critters: Dwarf Cuttlefish

What’s the coolest critter in the ocean under 4 inches long? The Dwarf Cuttlefish! Cuttlefish are marine animals that belong to the class Cephalopoda. Despite their name, cuttlefish are not fish but mollusks. Recent studies indicate that cuttlefish are among the most intelligent invertebrates, with one of the largest brain-to-body size ratios of all invertebrates. Cuttlefish have an internal shell called the cuttlebone used for buoyancy control and eight arms and two tentacles furnished with suckers, with which they secure their prey. For more information on the cuttlefish, see http://www.kqed.org/quest/television/cool-critters-dwarf-cuttlefish (1:59).

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Vocabulary

• bilateral symmetry: A body plan of an organism with a distinct head and a distinct tail region; a cut along the middle of the anterior-posterior axis divides the animal into two equal halves.

• coelom: A fluid filled cavity formed within the mesoderm; it forms between the digestive cavity and the body wall.

• cuttlebone: A hard, brittle internal structure found in all members of cuttlefish which is used for buoyancy control.

• deuterostome: An animal in which the first opening formed during development (the blastopore) becomes the anus.

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• mantle: A distinguishing characteristic of mollusks; it is the dorsal layer of tissue that lies between the body and the shell; it secretes layers of calcium carbonate that form the shell and is also a muscular foot that can be used for locomotion in some species.

• protostome: An animal in which the first opening formed during development (the blastopore) becomes the mouth.

• triploblast: An animal with three germ layers: an endoderm, a mesoderm, and an ectoderm.

Summary

• Mollusks are soft-bodied coelomates that generally have a head, a shell containing most of their internal organs, and a muscular foot. • The most significant evolutionary change seen in the phylum Mollusca is the increasing complexity of organ systems. • Flatworms, roundworms, mollusks, and annelids are all protostomes.

Practice

Use this resource to answer the questions that follow.

• The Mollusca at http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php .

1. What environments do mollusks occupy? 2. What have humans traditionally done with mollusks? 3. What is the radula?

Practice Answers

1. Mollusks occupy a wide range of habitats including hot vents, mountain tops, and cold seeps of the deep sea. 2. Several mollusks have been used as food. Native Americans, for instance, consumed large quantities of abalone. Shells of some mollusks have also been used as jewelry. 3. The radula is a ribbon of teeth surrounded by a muscular structure. It is located in the buccal cavity and is generally used for feeding.

Review

1. Why do mollusks have better preserved fossils than other early organisms? 2. What organ systems did mollusks develop that were not present in roundworms? 3. Mollusks are the first true coelomates. What role does the coelom serve? 4. What stands out about the little cuttlefish?

Review Answers

1. The hard outer shell present on most mollusks made them well-preserved in the fossil record. Their fossil record begins about 600 million years ago and reveals alternating periods of large extinctions and major diversifications of species.

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2. Like the more primitive roundworms, mollusks have a complete digestive system and a nervous system. In addition, they have evolved an open circulatory system with a heart, a tubular excretory system, and a respiratory system. 3. The coelom provides space and attachment sites within the mesoderm for internal organs, and the fluid in the cavity cushions these organs to protect them from shock. In mollusks, however, the coelom is often reduced to a small cavity surrounding the heart. 4. Recent studies indicate that cuttlefish are among the most intelligent invertebrates, with one of the largest brain-to-body size ratios of all invertebrates.

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1.19 Mollusk Classification - Advanced

• Distinguish between the seven classes of mollusks and their different characteristics.

Blue Ringed Octopus, California Two-Spot Octopus, Caribbean Reef Octopus, East Pacific Red Octopus, Seven-Arm Octopus, North Pacific Giant Octopus, or another type? Whatever type of octopus it is (and this is a Giant Octopus), it is a mollusk. As mollusks include animals ranging from clams and oysters, to slugs and snails, to octopi and squids, there must be ways to separate the organisms into groups (or classes).

Classification of Mollusks

Mollusks that exist today are divided into seven different classes, and these are listed in the Mollusk Classes Table.

TABLE 1.9: Mollusk Classes

Class Common Names of Members Gastropoda Snails, slugs Bivalvia Clams, oysters, scallops, mussels Cephalopoda Nautilus, octopus, squid, cuttlefish Scaphopoda Tusk shells Polyplacophora Chitons Aplacophora No common name Monoplacophora No common name

In the following sections we will discuss the major characteristics of each mollusk class.

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Gastropoda

Gastropods make up the largest class of mollusks, and they include both snails and slugs. They make up more than 80% of all living mollusk species and are the only mollusk class that has terrestrial species. The terrestrial species include slugs and land snails. Gastropods, such as snails, have a single shell that is often coiled. Examples of the variety of shell patterns in the gastropod class are shown in the Figure 1.44. In contrast to snails, most slug species have lost or reduced their shells during the course of evolution.

FIGURE 1.44 Shells from various snail species.

Gastropods are generally asymmetrical although they evolved from a bilaterally symmetrical mollusk ancestor. The asymmetry of gastropods is achieved through a process called torsion that occurs as the animal is developing into adulthood. During torsion, the top, or dorsal, part of the body twists relative to the bottom, or ventral, region. This results in the anus being oriented just above the mouth, as shown in the Figure 1.45. Gastropods have a fairly well- developed head with tentacles and primitive eyes capable of sensing light. They are well known for their beautiful shell structures.

FIGURE 1.45 This diagram illustrates the process of torsion that takes place during gastropod development. The drawing on the left shows the configuration of the mouth (M) and the anus (A) on opposite sides of the animal prior to torsion. The figure on the right shows that the mouth and the anus are on the same side of the organism following torsion and that the digestive tract is twisted.

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Bivalvia

Species within the class Bivalvia include a number of organisms that are important components of the human diet, including clams, oysters, and scallops. There are approximately 8000 species of bivalves, and two examples are shown in the Figure 1.46.

FIGURE 1.46 Members of the class Bivalvia. (a) A species of sea scallop. Notice the rows of blue eyes along the edge of the mantle. (b) A giant clam.

As the name implies, bivalves have two half-shells called valves that are connected by a dorsal hinge region. Similar to the gastropods, there are a few species of bivalves that have completely lost their shells through evolution. Unlike gastropods, most bivalves are bilaterally symmetrical, and they do not have a defined head region. Despite not having a head, they often have primitive eyes located on other regions of their bodies. Bivalves are generally filter feeders, and, as a result, they lack specialized feeding structures present in most other mollusks that will be discussed later. Filter feeders obtain their food by pulling water through their bodies and trapping small organisms and nutrients contained in the water. Bivalves have strong muscles called adductor muscles that attach to the inner side of each valve. The adductors allow them to close the valves very tightly to keep predators out. Some oyster species propel themselves through the water by rapidly opening and closing their valves when escaping from predators. However, most bivalves are generally either sedentary or move very slowly using their foot. Although the majority of mollusks inhabit marine environments, both Gastropoda and Bivalvia include many freshwater species.

Cephalopoda

The class Cephalopoda is a remarkable group of mollusks comprising roughly 800 living species that includes the largest known invertebrate: the colossal squid. The largest colossal squid ever captured was 10 meters long (almost the length of a school bus) and weighed over 1000 pounds. This amazing creature was caught off the coast of Antarctica in 2007. In addition to squid, the class Cephalopoda also includes octopi, cuttlefish, and nautili. These mollusks are unusual in a number of ways, including both their highly developed nervous system with a complex brain that allows them to learn and remember information and eyes that are capable of forming images. As a result of this complexity, cephalopods are considered the most intelligent invertebrates. They are the only mollusks with a closed circulatory system, and some are capable of very rapid movement. The shell is generally reduced in cephalopods. Species of squid and cuttlefish have a reduced internal shell that is more of a support rod, while octopi have lost the shell entirely. Nautili still retain an external shell. A fascinating feature of octopus, squid, and cuttlefish species is that they are able to change color to camouflage with their surroundings or to communicate with each other. They use specialized pigment cells called chromatophores to induce these color changes. Several examples of cephalopods are shown in the Figure 1.47.

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FIGURE 1.47 Members of the class Cephalopoda. (a) Nautilus Species (b) Cuttlefish (c) East Pacific Red Octopus (d) Squid

FIGURE 1.48 Use this figure to compare and contrast gastropods, bivalves, and cephalopods.

Scaphopoda

The mollusks of the class Scaphopoda are referred to as the tusk shells because their shells are shaped like elephant tusks, although they are much smaller. Most species are about two centimeters long, and they have a more primitive circulatory system than most mollusks. Tusk shells lack both a heart and blood vessels. They feed using over 100 tentacles that emerge from the wide end of the shell near the foot. These tentacles are called captacula, and they have a sticky surface that traps microscopic food particles and delivers them to the mouth. The Figure 1.49 shows some examples of the tusk-shaped shells.

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FIGURE 1.49 Tusk-shaped shells of the class Scaphopoda.

Polyplacophora, Aplacophora, and Monoplacophora

Polyplacophora are marine mollusks commonly known as chitons. The word Polyplacophora means “many plates,” and this describes the shell structure of these species. Chiton shells are made up of eight distinct but overlapping plates surrounded by a circle of muscular tissue called a girdle. There are roughly 900 chiton species alive today. A beautiful example of a chiton species is shown in the Figure 1.50.

FIGURE 1.50 A species of chiton in the class Polypla- cophora. Notice the eight overlapping shell plates.

Monoplacophora is the smallest mollusk class, with only about 11 species. For a long time it was believed that the species of this class had been extinct for over 350 million years. This was found to be incorrect with the exciting discovery of a living Monoplacophora species off the coast of Costa Rica in 1952. Species of Monoplacophora resemble chitons, but they have only one flat shell. The Aplacophora class is made up of approximately 300 species of small worm-like animals that lack a shell. Instead, they have an outer skeleton that is composed of tiny spicules made of calcium.

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Vocabulary

• adductor muscles: Strong muscles that attach to the inner side of each valve of a bivalve.

• captacula: Tentacles of Scaphopoda that have a sticky surface; they trap microscopic food particles and deliver them to the mouth.

• chromatophores: Specialized pigment cells that induce color changes in octopus, squid, and cuttlefish species; this allows them to camouflage with their surroundings or to communicate with each other.

• girdle: A circle of muscular tissue surrounding chiton shells.

• torsion: The process that occurs as gastropods are developing into adulthood; the top, or dorsal, part of the body twists relative to the bottom, or ventral, region.

Summary

• Mollusks are subdivided into seven classes with most species found in three of those classes: Gastropoda, Bivalvia, and Cephalopoda. • Gastropods make up the largest class of mollusks, and they include both snails and slugs. • Species within the class Bivalvia include a number of organisms that are important components of the human diet, including clams, oysters, and scallops. • The class Cephalopoda is characterized by mollusks with highly a developed nervous systems. • The mollusks of the class Scaphopoda are referred to as the tusk shells because their shells are shaped like elephant tusks. • The remaining classes (Polyplacophora, Aplacophora, and Monoplacophora) have fewer species.

Practice

Use this resource to answer the questions that follow.

• Snails and Their Relatives at http://www.oceanicresearch.org/education/wonders/mollusk.html .

1. What are the three body regions found in most mollusks? 2. Which portion of the mollusk is responsible for secreting the shell? 3. What do chitons feed off of? What makes their radula so effective? 4. Pearls are beautiful and often employed in jewelry. Which animals are responsible for pearls and how are they made? 5. How are the eyes of cephalopods an example of convergent evolution?

Practice Answers

1. Mollusks have a head, a visceral mass, and a foot. The visceral mass contains the internal organs. 2. The mantle of a mollusk is responsible for secreting the shell if the mollusk has one. 3. Chitons usually forage for food, usually algae, which they scrape off with their radulae. Chitons use magnetite to harden the teeth of their radulae. 4. Although most pearls that we think of come from oysters, most bivalves can produce pearls, as well as some snails. A pearl begins as a small irritant that becomes stuck in the mantle of a mollusk. The mollusk coats the irritant with the same material as the inner lining of its shell, and it eventually becomes a large pearl. 5. The eyes of cephalopods are very similar in function to mammalian eyes even though they evolved from different ancestors. Cephalopods have extremely good eyesight.

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Review

1. What class contains the majority of all mollusks? 2. How do bivalves protect themselves from predators? 3. What distinguishes the class Cephalopoda from other groups of mollusks? 4. How do tusk shells feed? 5. Which class of mollusks was thought to be extinct?

Review Answers

1. Gastropods make up more than 80% of all living mollusk species. 2. Bivalves use adductor muscles to close their valves tightly and keep out predators. Some species also propel themselves away from predators by opening and closing their valves. 3. Cephalopods have both highly developed nervous systems with a complex brain that allows them to learn and remember and eyes that are capable of forming images. They are considered the most intelligent invertebrates. They are also the only mollusks with a closed circulatory system. 4. Tusk shells feed using over 100 captaculas that emerge from the wide end of the shell near the foot and trap microscopic food particles. 5. For a long time it was believed that the Monoplacophora class had been extinct for over 350 million years. This was found to be incorrect with the exciting discovery of a living Monoplacophora species off the coast of Costa Rica in 1952.

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1.20 Mollusk Structure and Function - Ad- vanced

• Identify the structural features of mollusks, and describe how they function. • Understand the important evolutionary developments that occurred in the mollusks.

What is the function of a slug? Just looking at the slug, it may be difficult to discern any function. But slugs, like all animals, occupy a specific niche and do play a vital role in their ecosystem. Interestingly, this powerful slug, or specifically the Banana Slug Ariolimax dolichophallus, is the mascot of the University of California, Santa Cruz. In February 2008, ESPN Sports named the UCSC Banana Slug as one of the ten best nicknames in college basketball.

Structure and Function in Mollusks

Superficially, it is difficult to see how a shelled, marine organism such as a clam or a snail could be related to a human being. But in fact, mollusks are important invertebrate ancestors to the more recently evolved vertebrates, including humans. This becomes more clear when we examine the details of the internal organs and tissues of mollusks, and consider how they fit into the step-wise evolution of the complex organ systems that make up higher invertebrates and vertebrates. For example, mollusks are the first animals to have evolved organ systems for respiration and circulation. Those systems are essential components of higher organisms since they allow our bodies to more efficiently exchange gases with the environment. There are also two unusual organs that are found exclusively in mollusks: the mantle and the radula. These organs do not persist in higher organisms, but they were important adaptations to the environments inhabited by mollusks. Although they are unique to mollusks, some species have lost these features during the course of evolution. The Figure 1.51 illustrates the basic mollusk body plan and how these organs and the systems discussed in this section are organized. In this section we will first examine the structure and function of the mantle and the radula, then we will consider the details of several other mollusk organ systems. Additional concepts will address variations of the basic mollusk body plan shown in the Figure 1.51.

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FIGURE 1.51 The basic mollusk body plan showing the relative positions of the head, foot, and shell. The following organ systems are also shown: circulatory and respiratory systems, digestive system, nervous sys- tem, excretory system, and reproductive system.

Mantle and Radula

The mantle is a soft tissue layer that is formed from folds of the dorsal body wall. It lies beneath the shell where it covers the body of the animal. The mantle has several critical functions. The outer cells of the mantle secrete layers of calcium carbonate that form the shell. A second important role of the mantle is in the formation of a cavity called the mantle cavity. The mantle cavity is formed between the mantle tissue and the body of the animal. This cavity serves as a water pumping station for aquatic mollusks. It contains gills used for respiration and exit pores for the digestive, excretory, and reproductive systems. Water is pumped into the mantle cavity to allow the gills to absorb oxygen. Waste eliminated through the anus and excretory pores is released into the mantle cavity and pumped out of the animal into the aquatic environment. The mantle cavity is also where gametes are released to be dispersed by the out-flowing water. Filter feeders, such as bivalves, also use the intake water of the mantle cavity to obtain food. In cephalopods, the mantle cavity has been adapted for use in locomotion. Water is expelled from the cavity through an organ called the siphon with great jet-propulsive force. This force allows some cephalopods to move with rapid speed. The radula is a specialized muscular feeding organ that contains teeth made of a carbohydrate (chitin) substance. It is located in front of the mouth in the head region of all mollusk classes except Bivalvia. This makes sense since bivalves are filter feeders that lack a distinct head region. Some mollusks use the radula to scrape food, such as algae, off of rocks and into the mouth, while other predatory species use it to drill holes into the shells of their prey. The Figure 1.52 shows a diagram of the radula.

FIGURE 1.52 A diagram showing the detailed structure of the radula. The blue regions indicate food being scraped into the mouth by the gray teeth of the radula. The letters correspond to the following regions: r = radula, e = esophagus, m = mouth, mu = muscles of the radula.

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Respiratory and Circulatory Systems

Circulatory systems use blood to transport both oxygen from the respiratory system to organs and tissues within the animal and carbon dioxide waste from these tissues back to the respiratory system. These systems are far more efficient than the simple diffusion between tissues and the environment that occurs in simpler invertebrates such as roundworms. The mollusk circulatory system uses a heart to pump blood through the organism, and the respiratory system of aquatic mollusks centers around their gills. Mollusk gills are called ctenidia, and they are made up of a series of thin filaments of tissue that resemble the teeth of a comb. These filaments absorb oxygen from water and transfer it to the blood stream. They also receive carbon dioxide from the blood and release it into the surrounding water. Terrestrial mollusk species have primitive lungs that absorb oxygen directly from the air around them. As we discussed earlier, the gills are located in the water-filled mantle cavity. All mollusks except those in the class Cephalopoda have an open circulatory system. In an open circulatory system, blood is not contained entirely in enclosed blood vessels. The heart pumps blood through blood vessels that lead from the gills into body cavities called hemocoels. A hemocoel is a blood-filled body cavity that is distinct from the fluid-filled coelom. Once the blood is released into the hemocoels, it is no longer contained in blood vessels, and it can diffuse freely to the various tissues of the body in order to deliver oxygen and receive carbon dioxide. The blood then re-enters vessels to be returned to the gills where carbon dioxide is exported to the water in the mantle cavity. A schematic of an open circulatory system is shown in the Figure 1.53. This system of gas exchange is not adequate for the fast moving cephalopods. They have evolved a closed circulatory system that is not dependent on diffusion for blood to reach tissues throughout the body.

FIGURE 1.53 A schematic of an open circulatory sys- tem showing blood flow from the gills through blood vessels (in red) and the heart into the hemocoel. Black arrows show the flow of blood.

Excretory System

The excretory system of mollusks is made up of tubular organs called nephridia that filter waste from internal body fluids. Waste generated by cells within the organism is dumped into the coelom, the fluid-filled internal body cavity. The nephridia are small tubes that open into the coelom. They have tiny cilia, hair-like extensions, that surround the tube openings and cause fluid to flow from the coelom into the nephridia tubules. Once the fluid enters the nephridia, non-waste molecules, such as sugars and water, can be reabsorbed into the animal’s body. What remains in the tubes is concentrated waste that is then excreted out of the nephridia exit pores in the mantle cavity.

Squid Skin with a Mind of Its Own

When you cut the nerves from a squid brain to the skin, something unexpected happens with the tiny pouches of colored pigment, called chromatophores. Find out more about this phenomenon at http://youtu.be/mlzxxD9A07E? list=PLzMhsCgGKd1hoofiKuifwy6qRXZs7NG6a .

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Vocabulary

• closed circulatory system: A circulatory system in which the blood is enclosed at all times within vessels.

• ctenidia: Mollusk gills made up of a series of thin filaments of tissue; these gills resemble the teeth of a comb.

• gills: Respiratory organs found in many aquatic organisms; they extract dissolved oxygen from water and excrete carbon dioxide.

• hemocoel: A blood-filled body cavity that is distinct from the fluid-filled coelom.

• mantle: A distinguishing characteristic of mollusks; it is a dorsal layer of tissue that lies between the body and the shell which secretes layers of calcium carbonate that form the shell and is also a muscular foot that can be used for locomotion in some species.

• mantle cavity: Formed between the mantle tissue and the body of a mollusk and serves as a water pumping station for aquatic mollusks.

• nephridia: Tubular organs that filter waste from internal body fluids.

• open circulatory system: A circulatory system in which blood is not contained entirely in enclosed blood vessels; the heart pumps blood through blood vessels that lead from the respiratory organs into the hemocoel.

• radula: A specialized feeding structure located within the mouth which contains teeth made of chitin that are used to chew or scrape food; this is a unique feature of mollusks.

Summary

• Most mollusk have two organs that are unique to this phylum: a specialized feeding organ called a radula and a dorsal layer of tissue called a mantle. • Mollusks are the first animals to have evolved organ systems for respiration and circulation. • All mollusks except those in the class Cephalopoda have an open circulatory system. • The excretory system of mollusks is made up of tubular organs called nephridia that filter waste from internal body fluids.

Practice

Use this resource to answer the questions that follow.

• Mollusca at http://eol.org/pages/2195/details .

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1. Describe how different species of mollusks have adapted their radulae. 2. Which species of mollusks have a muscular foot? What is its function? 3. What is the shell of mollusks made of? 4. How developed is the typical mollusk’s nervous system?

Practice Answers

1. Some gastropods and cephalopods have adapted their radulae to be drill-like, allowing them to bore holes in the shell of prey, sometimes with the aid of acids. Cone snails have adapted their radulae to be slung out like harpoons and inject toxins into prey. The radulae in bivalves are highly reduced or lost because they are filter feeders. 2. Most mollusks other than aplacophorans have a muscular foot. Depending on the species, the muscular foot is used for locomotion, clinging, burrowing, swimming, or modified into tentacles, as in the case of the octopus. 3. The shell of most mollusks is usually produced in layers of calcium carbonate, in calcite or aragonite form. 4. Mollusks have a well developed nervous system, normally consisting of a dorsal ganglion, a ring of nerves around the esophagus, and two pairs of lateral nerve cords.

Review

1. Which two organs are found exclusively in mollusks and in no other phylum of animals? 2. What important functions does the mantle of a mollusk serve? 3. Cephalopods developed a different role for the mantle cavity. What is the special adaptation that cephalopods have for the mantle cavity? 4. Describe how an open circulatory system works. How does this differ from a closed circulatory system? 5. How does the excretory system of mollusks function?

Review Answers

1. The mantle and the radula are two organs found exclusively in mollusks, although there are some species that have lost these features during the course of evolution. 2. The outer cells of the mantle have the important role of secreting the shell of the mollusk. The mantle also forms the mantle cavity which pumps water necessary for the respiratory, excretory, and reproductive systems. 3. In cephalopods, the mantle cavity has been adapted for use in locomotion. Water is expelled from the cavity through an organ called the siphon with great jet-propulsive force. 4. In an open circulatory system, blood pumped from the heart travels through blood vessels that lead into hemocoels. Blood diffuses freely to various tissues of the body, then re-enters vessels that return to the respiratory organs. This differs from a closed circulatory system, where blood is contained entirely in enclosed blood vessels. 5. Waste from cells are dumped into the coelom. Cilia that surround the nephridia cause fluid to flow from the coelom into the nephridia tubules, where any remaining nutrients are reabsorbed. The concentrated waste is excreted out into the mantle cavity.

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1.21 Mollusk Nervous System and Reproduc- tion - Advanced

• Explain the mollusk nervous system and mollusk reproduction.

What makes a mollusk open or close? In this bivalve, it is not the brain (there is no brain). But there is a nervous system, and the nervous system reacts to stimuli.

Nervous System

The nervous system of mollusks varies greatly depending on the particular class. In general, it is more complex than those of roundworms or flatworms. Bivalves have a simple nervous system with usually three sets of ganglia connected by nerve fibers. Ganglia are clusters of nerve cells that form simple nerve centers distinct from the brain. Bivalves do not have brains. Since they do not have a head region, their tentacles and eyes are often located along the mantle edge. Gastropods have a more complex nervous system with six sets of ganglia. They have tentacles containing sensory organs located on their head. These sensory organs range form simple light sensing tissues called ocelli, to more complex eyes that have a lens (for example, in scallop species). Cephalopods have the most complicated nervous system found in the invertebrates. Octopi, squids, and cuttlefish are considered the most intelligent of all invertebrates. They are capable of some forms of learning, and some species exhibit elaborate social interactions. This is due to their large, well-developed brains and their sophisticated sensory organs. For example, they have large complex eyes that are capable of forming images. It is quite amazing that this level of complexity within the nervous system evolved independently in this phylum through a very different path than that of vertebrates.

Locomotion

Essentially all mollusks have a muscular foot that is primarily used for locomotion but may be adapted for different purposes in different species. Although gastropods are somewhat famous for their slowness (shown by the phrase “at a snail’s pace”), they are fairly active, and the foot is critical for their movement. The gastropod foot is generally

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large and positioned along the ventral, or bottom, surface of the animal. It is adapted for slow crawling along a solid surface. Bivalves are primarily sedentary, but they also possess a foot. The bivalve foot is wedge-shaped, and several species use it to burrow into the sea floor in order to hide from predators. The bivalve foot is shown in the Figure 1.54. Compare this with the gastropod foot.

FIGURE 1.54 A clam (bivalve) with its wedge-shaped foot protruding from the valves.

Cephalopods are active predators that can move very quickly, but they do not generally use their foot for movement. Instead, they contract muscles surrounding the mantle cavity to thrust water out of the cavity. This jet-propulsion mechanism is responsible for their rapid movement. The cephalopods do have a foot, but it has been drastically modified to form a number of arms and tentacles surrounding the mouth in the head region. The arms and tentacles are primarily used to capture prey, but they can also be used for an awkward walking motion in species that live on the bottom of the ocean. Arms can be distinguished from tentacles in that they often have rows of powerful suckers along their length, whereas tentacles usually have suckers only at their tips. The suckers can be used to grip prey or objects.

Reproduction

Like many invertebrates, the mollusk life cycle includes one or more juvenile or larval stages that are very different from the adult form of the animal. Both mollusks and annelids develop through a larval stage called a trochophore larva. Trochophore larvae are characterized by having a band of cilia that wraps around the body. The presence of trochophore larvae in both mollusks and annelids is consistent with molecular phylogenetic studies that indicate they share a close evolutionary relationship. Some mollusks, particularly gastropods, also have a second larval stage that is not present in any other phylum, called veliger. An example of a veliger larva is shown in the Figure 1.55. It is during the veliger larval stage that gastropods undergo the process of torsion described in an earlier concept. Mollusks reproduce sexually, and most species have separate sexes. Sexual reproduction is achieved by the formation and fusion of gametes: sperm and eggs. Some species are hermaphrodites meaning that individuals are capable of forming both sperm and eggs. Fertilization can be either internal or external depending on the class and species. Internal fertilization takes place when the male transfers sperm into the body of the female through mating. During external fertilization, the female lays eggs, and they are fertilized by the male sperm outside of the female’s body. For a video on the complex mating ritual of the leopard slug, see Hermaphrodite Leopard Slugs Mating at http://w ww.youtube.com/watch?v=CnnIk-6wR00 (3:29).

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FIGURE 1.55 A sketch of a veliger larva. Notice the many hair-like cilia located around the body.

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Vocabulary

• ganglia (plural, ganglion): Clusters of nerve cells that form simple nerve centers distinct from a brain.

• ocelli: Simple light sensing tissues.

• trochophore larva: Juvenile stages that are very different from the adult form; these larvae have a band of cilia that wraps around the body and are characteristic of mollusks and annelids.

• veliger: A second larval stage present in some mollusks, particularly gastropods.

Summary

• The nervous system of mollusks varies greatly depending on the particular class, but it is generally more complex than those of roundworms or flatworms. • Cephalopods have the most complicated nervous system found in invertebrates.

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• Essentially all mollusks have a muscular foot that is primarily used for locomotion but may be adapted for different purposes in different species. • The mollusk life cycle includes one or more juvenile, larval stages. • Mollusks reproduce sexually, and most species have separate sexes.

Practice

Use this resource to answer the questions that follow.

• Gastropod: Reproduction and Life Cycles at http://www.britannica.com/EBchecked/topic/226777/gastrop od/35711/Reproduction-and-life-cycles .

1. How long does the veliger stage last in gastropods? 2. What do mollusks in the veliger stage feed on? 3. Give an example of an evolutionary trend in gastropod reproduction. 4. What are spermatophores?

Practice Answers

1. The veliger stage may persist for weeks or even months before gastropods develop into their adult stage. 2. The veligers feed on diatoms and other small plankton collected by ciliary currents of the velum. 3. There has been an evolutionary trend toward internal fertilization within gastropods, as well as hermaphroditism. Internal fertilization was definitely a necessity for gastropods that live on land. 4. Spermatophores are horny, calcerous sperm bundles with elaborate, ornamented exteriors. They are used to distinguish different species and inject sperm.

Review

1. Compare the nervous system of bivalves with that of cephalopods, and explain how the differences correspond to their different lifestyles. 2. Explain the importance of the trochophore larval stage of mollusks and annelids. 3. How have cephalopods adapted their muscular foot? 4. What allows cephalopods to travel fast through the water and hunt down prey?

Review Answers

1. The nervous system of bivalves is more simple, with usually three sets of ganglia connected by nerve fibers and no brain. Tentacles and eyes are located along the mantle edge. Cephalopods have a more developed nervous systems with well-developed brains and complex eyes. Since bivalves are mostly filter feeders, they do not require the complex nervous system of cephalopods, which need to hunt down prey. 2. Mollusks and annelids both develop through a trochophore larval stage. This supports the theory that these two phyla share a close evolutionary relationship. 3. Cephalopods modified their muscular foot to form a number of arms and tentacles surrounding the mouth region. Arms can be distinguished from tentacles in that they often have rows of powerful suckers along their length, whereas tentacles usually have suckers only at their tips. 4. Cephalopods contract muscles surrounding their mantle cavity to thrust water out, using this jet-propulsion system to hunt down prey.

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1.22 Mollusk Body Plans - Advanced

• Describe the basic mollusk body plan and variations that are found within different classes of the phylum.

Does this body have a plan? Believe it or not, it does. A snail is actually a complex organism. In addition to having the obvious shell, there is a foot that allows for movement and a mass of organs under that shell.

Mollusk Body Plans

The mollusk body can generally be divided into three regions: the head, the foot, and a cluster of internal organs called the visceral mass. The visceral mass includes many of the organs mentioned in the previous concepts such as the stomach, the heart, the nephridia, and the gonads. The mantle covers the visceral mass and is itself covered by the shell. The mantle cavity is usually found in the posterior end of the animal and is formed between the mantle and the visceral mass. There are several variations on this basic body plan, and these variations, along with the structure of the shell, are the primary bases for the separation of mollusks into distinct classes. The Figure 1.56 shows the variations on the basic body plan for each class. In this concept we will consider some of the details of the gastropod, bivalve, and cephalopod body plan.

Gastropod Body Plan

The word gastropod means “stomach-foot,” and this is appropriate for the species in Gastropoda because their foot is situated close to the stomach on their ventral surface. The visceral mass forms a hump above, or dorsal to, the foot. The mantle covers the visceral mass, and the shell, if it is present, surrounds both the mantle and the enclosed

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FIGURE 1.56 Variations on the basic mollusk body plan found in each class. M = mouth, A = anus, F = foot, Dark (bold) line = shell.

visceral mass. There is a distinct head with tentacles. Some gastropod species are able to withdraw their entire bodies into their shells and seal off the opening with a structure called the operculum. This ability serves to protect them from predators and from drying out, during low tides for example. The organization of the gastropod body is very similar to the basic body plan of mollusks. The Figure 1.57 illustrates the details of the internal organization of a terrestrial snail.

FIGURE 1.57 A diagram showing details of the body of a terrestrial snail. Notice that the snail possesses both male and fe- male reproductive organs. This reflects the fact that most terrestrial snails are hermaphrodites. Numbers correspond to the following organs: 1. shell 2. liver 3. lung 4. anus 5. respiratory pore 6. eye 7. tentacle 8. cerebral ganglia 9. salivary duct 10. mouth 11. digestive tract 12. digestive gland 13. genital pore 14. penis 15. vagina 16. mucous gland 17. and 18. reproductive organs 19. foot 20. stomach 21. nephridia 22. mantle 23. heart 24. reproductive organ.

Bivalve Body Plan

Two of the most prominent features of the bivalve body plan are the lack of a head region and the half shells connected by a dorsal hinge that surround the body. The foot is often smaller in bivalves than in gastropods, and it is usually located more to the front, or anterior, end of the body. The shell halves, or valves, are located on the lateral sides of the animal rather than on the dorsal, or top, side, which is occupied by a single shell in gastropods. In order to accommodate large volumes of water for both respiration and filter feeding, the mantle cavity is generally larger than in other classes. The organization of this body plan is shown in the Figure 1.58.

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FIGURE 1.58 The anatomy of a bivalve positioned with the anterior, or front, region facing down and the bottom, or ventral, region facing out. The following regions are labeled in this drawing: 1. and 2. muscle attach- ments inside the shell 3. and 4. visceral mass 5. and 6. siphons 7. foot 8. valves 9. hinge 10. mantle.

Cephalopod Body Plan

In cephalopods, the foot is modified into muscular arms that extend from the head. The word cephalopod means “head-foot,” reflecting the connection between the two regions. The cephalopod mouth is equipped with a hard beak resembling that of a parrot which is used to tear apart prey. The body plan of cephalopods also differs in the relative position of the visceral mass. The visceral mass is located in a more posterior (back end) position as opposed to on the dorsal, or top, side. The head lies between the visceral mass and the arms. This is shown in the Figure 1.59. In the absence of an external shell, the mantle of cuttlefish, octopi, and squids plays a greater role in protecting the contents of the visceral mass.

FIGURE 1.59 The body plan of a cephalopod.

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Vocabulary

• mantle: A distinguishing characteristic of mollusks; it is the dorsal layer of tissue that lies between the body and the shell which secretes layers of calcium carbonate that form the shell and it is also a muscular foot that can be used for locomotion in some species.

• mantle cavity: Formed between the mantle tissue and the body of a mollusk which serves as a water pumping station for aquatic mollusks.

• nephridia: Tubular organs that filter waste from internal body fluids.

• operculum: A structure found in gastropods that covers the shell opening; it is a gill cover and acts as a protective barrier.

• visceral mass: A cluster of internal organs within mollusks.

Summary

• The body plan of a mollusk usually consists of a head region, a muscular foot, and a visceral mass of internal organs that is often contained within a dorsal shell. Each class possesses some variation on this basic plan. • The organization of the gastropod body is very similar to the basic body plan of mollusks. • Two of the most prominent features of the bivalve body plan are the lack of a head region and the two half shells connected by a dorsal hinge that surround the body. • The cephalopods have a body plan where the foot is modified into arms, and the visceral mass is located in a more posterior position.

Practice

Use these resources to answer the questions that follow.

• The Gastropoda at http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/gastropoda.php . • The Bivalvia at http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/bivalvia.php . • The Cephalopoda at http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/cephalopoda.php .

1. How are the shells of gastropods usually coiled? 2. What do bivalves use to feed? 3. How are cephalopods subdivided based on their shells?

Practice Answers

1. The shell is typically coiled dextrally; the axis of coiling is around a central columella to which a large retractor muscle is attached. 2. In most bivalves, the mantle cavity contains a pair of very large gills that are used to capture food particles as well as for respiration. 3. Clades without an external shell are called endocochleate and include the coleoids: squids, cuttlefish, and octopuses. The presence of an external shell in Nautiloids, Ammonoids, and orthoconic cephalopods is conisdered a plesiomorphic state.

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Review

1. What is the function of the operculum? 2. Which mollusks usually have the largest mantle cavities? 3. What are two prominent features found in cephalopods that deviate from the typical mollusk body plan?

Review Answers

1. Some gastropod species uses the operculum to seal off the opening of their shells in order to protect themselves from predators or from drying out. 2. Bivalves usually have mantle cavities that are larger because they require large volumes of water for both respiration and filter feeding. 3. The cephalopods have a body plan where the foot is modified into arms, and the visceral mass is located in a more posterior position.

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1.23 Mollusk Ecology - Advanced

• Identify the regions of the world inhabited by mollusks, and explain the importance of mollusks to humans.

Add some garlic and butter, sauté and what do you get? Lunch? Maybe not to you. But to many people all over the world, snails are delicious. And of course, other mollusks are consumed by people as a staple of their diet.

Mollusk Ecology

Mollusks can be found in pretty much any terrestrial or aquatic environment: both shallow and deep regions of the ocean, fresh water, and most land areas. However, the vast majority of species are found in the ocean. The class Gastropoda includes terrestrial species, and both Gastropoda and Bivalvia contain freshwater species. Cephalopods are strictly marine animals.

Gastropod Ecology

Gastropod species have a variety of different feeding styles. Some species are seaweed-eating herbivores, while others are predatory carnivores. There are even a few species of gastropods that are internal parasites. Most aquatic gastropods do not float or swim so they usually live and crawl around on solid areas of the ocean such as rocks, coral reefs, and mud. Recall that the foot is adapted for slow movements along a solid surface. Land-dwelling gastropods have found success in a wide range of ecological habitats, including high mountain elevations, deserts, rainforests, and polar regions. Although marine gastropods are found in all oceans, from the tropics to the poles and at all depths, the most concentrated regions of gastropod populations are the shallow tidal regions along the coasts.

Bivalve Ecology

Adult bivalves are generally sedentary. They feed on small organisms, such as plankton, as well as non-living organic matter that is filtered from their aquatic environments (filter feeding). Some bivalves attach themselves

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to solid substrates, such as rocks or corals, but many species burrow into areas of soft sediment. Bivalves are successful inhabitants of most marine environments, ranging from shallow coastal regions to deep within the ocean. As mentioned above, there are also many bivalve species that live in freshwater.

Cephalopod Ecology

As exclusively marine inhabitants, cephalopods can be found swimming through the open ocean, or in the case of the octopus, residing on the ocean floor. There are some species, particularly squids, (see http://news.nationalgeograph ic.com/news/2007/02/070222-squid-pictures.html?source=G1902&kwid=ContentNetwork|787369405 ) that live near the shoreline. Cephalopods are carnivorous predators or scavengers that eat other invertebrates and fish. In addition to the ability to change color, cephalopods have the ability to create a hazy “cloud” around them when they feel threatened. This is due to an internal sac of ink contained within their digestive system. If they find themselves in a bind, they can release the ink to obscure the view of their predators while they escape. The Figure 1.60 shows a cloud of ink released by an alarmed octopus.

FIGURE 1.60 A startled octopus releasing a cloud of ink into the open ocean.

Where’s the Octopus?

When marine biologist Roger Hanlon captured the first scene in this video he started screaming. Hanlon, a senior scientist at the Marine Biological Laboratory in Woods Hole, studies camouflage in cephalopods: squids, cuttlefish, and octopuses. They are masters of optical illusion. The video at http://www.sciencefriday.com/videos/watch/10397 (4:37) shows some of Hanlon’s top video picks of sea creatures going in and out of hiding.

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How Smart is an Octopus? also explores this concept: http://www.pbs.org/wgbh/nova/nature/cephalapod-intellige nce.html (9:58).

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Importance to Humans

Mollusks and humans have long had a close and mostly productive relationship, at least from the human perspective. Species within each of the major classes, Gastropoda, Bivalvia, and Cephalopoda, are major sources of human food. Mollusks can also be used as indicators of environmental health. Species of bivalves are often used as gauges to detect pollution levels in aquatic environments. As they filter feed, they accumulate any toxins or heavy metals that are present in high concentrations in the water. Culturally, mollusks have played an important role in the jewelry and art worlds. When a sharp piece of sediment or debris becomes stuck inside of an oyster, the mantle secretes the same material it normally uses to generate inner layers of the shell in order to smooth over the rough edges of the sediment and prevent it from tearing up the inner tissues of the animal. In this way, oysters produce highly valued pearls. Mollusks have also made significant contributions to our understanding of the process of evolution. The rich mollusk fossil record, preserved so well due to their hard shells, has made them an important focus of paleontology research. Despite the numerous positive roles played by mollusks in human society, they have also had some negative impacts. Certain species of terrestrial slugs are harmful to agricultural crops. Mollusks are also intermediate hosts for many parasitic flatworm species. As part of the complex life cycle of these parasites, an intermediate host is required for several stages of their development. Most mollusks pose no threat to humans as predators, but a few octopi have poison that could kill someone if they bit them. Some people are highly allergic to shellfish, and, in this way, mollusks can also be deadly to humans.

Vocabulary

• camouflage: A common adaptation in predator and prey species that involves disguise.

• parasite: The species that benefits in a parasitic relationship.

Summary

• Mollusks are primarily marine organisms, however, gastropods and bivalves include many freshwater species, and gastropods also include terrestrial species. Overall, mollusk species can be found almost anywhere on earth.

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• Cephalopods are exclusively marine inhabitants. • Gastropods and bivalves can be found in marine and freshwater environments, although gastropods can also be found on land. • Many species have important relationships with humans (for example, as food).

Practice

Use this resource to answer the questions that follow.

• Gastropod: Ecology and Habitats at http://www.britannica.com/EBchecked/topic/226777/gastropod/35712 /Ecology-and-habitats .

1. Which part of the oceans do gastropods inhabit in large quantities? 2. Describe the characteristic known as homing. 3. What are some causes for the diversity of mollusks? 4. What obstacles did gastropods face when they migrated onto land?

Practice Answers

1. Although gastropods inhabit almost all parts of the ocean, they appear in the greatest numbers below the tidal zones, where food is more abundant. 2. Homing is when numerous species have a tendency to settle into a certain area and radiate out. Individuals return to the settled area for rest or when stressed. Limpets exhibit this behavior. 3. Specialization in food, temperature, and salinity may have all factored into the diversity of mollusks. Temper- ature and salinity, in particular, impact breeding. 4. First, the veliger stage was suppressed once gastropods migrated to land because the environment was not suitable for the larval stage. Dispersion of eggs and fertilization were also obstacles because fertilization had to occur internally and offspring could only colonize by being transported by a parent.

Review

1. Which major group of mollusks inhabits the most diverse habitats? 2. What kind of environments do bivalves inhabit? 3. Name some mechanisms with which cephalopods escape predators. 4. Why are some mollusks great indicators of environmental health? 5. Name some negative impacts that mollusks have on humans.

Review Answers

1. Gastropods inhabit the widest range of habitats. Aquatic gastropods inhabit rocks and coral reefs; they live in marine as well as freshwater environments. There are also land-dwelling gastropods that inhabit high mountain elevations, deserts, rainforests, and polar regions. 2. Bivalves inhabit most marine environments, ranging from shallow coastal regions to deep within the ocean. They also live in freshwater. 3. Cephalopods can change color using their chromatophores and camouflage in with their environment. They also have an internal sac of ink contained within their digestive system which they release in order to obscure the view of predators as they escape. 4. Since bivalves filter feed, they accumulate any toxins that are present in the water and act as gauges to detect pollution levels.

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5. Some mollusks are harmful to agricultural crops. Mollusks are also common intermediate hosts for a variety of parasites and commonly pass those parasites on to humans.

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1.24 Annelids - Advanced

• Describe the major characteristics of annelids.

How can this be an animal? This is a worm. And even though it is blue, this is a Christmas tree worm. These "Christmas tree" structures are actually specialized mouth appendages. Each spiral is composed of feather-like tentacles which are heavily ciliated. These appendages trap prey and transport the food straight towards the worm’s mouth. And these worms are annelids.

Characteristics of Annelids

The phylum Annelida is made up of the segmented worms. These worms have more complex anatomies and behavior than the flatworms and roundworms of earlier phyla. The annelids that most of us are familiar with are the earthworms that live in the soil of our backyards and parks. However, most of the species in the phylum Annelida actually live in the ocean. In contrast to the dull-looking, brown earthworms that we know so well, marine annelids are often beautifully colored and sometimes oddly shaped, as shown in the Figure 1.61. Despite the boring outward appearance of earthworms, they are actually extremely important organisms in the human ecosystem. They are responsible for the rich soils that we require to grow our food, and we often take for granted their diligent work in the ground beneath us. In the Annelids concepts we will describe the characteristics of segmented worms and consider how they are classified into different groups. We will also discuss their anatomical structures and how they function. Finally, we will consider the many ecological habitats that are occupied by annelids. There are roughly 15,000 species of annelids, including marine, freshwater, and terrestrial species. They range in size from less than one millimeter (the size of a comma on this page) to over three meters (about the distance from the floor to the ceiling). Annelids are characterized by having a segmented body, and most species have tiny hair-like bristles on their outer surface called setae or chaetae. In this section we will first examine two major characteristics

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FIGURE 1.61 A colorful marine annelid species.

that arose with the evolution of annelids: segmentation and a well-developed body cavity. We will then consider the features that distinguish the different classes that make up the phylum Annelida.

Segmentation

The most obvious of the distinguishing features that that arose in the annelid phylum is their segmented body. The annelid body is made up of a number of repeating units, or segments, called metameres. The word annelid means “little ring” and from the outside, the segments of the worm’s body look like a series of rings stacked together, as shown in the Figure 1.62.

FIGURE 1.62 A redworm of the species Eisenia fetida. Notice the segmented body plan.

Segmentation is a condition that seems to have evolved independently several times during invertebrate evolution. It was originally thought that the segmented bodies of arthropods and annelids made them very close relatives with a fairly recent common ancestor. Further analysis using modern techniques that allow scientists to compare the genetic sequences of organisms have shown that this is incorrect. In fact, the annelids are much more closely related to the non-segmented mollusks that we discussed in earlier concepts. This means that segmentation is an example of convergent evolution. Convergent evolution occurs when the same trait evolves independently in two different group of organisms. This often results from selective pressure on the organisms to adapt to a similar environment or way of life. What are the evolutionary advantages of segmentation? For one thing, it allows the animal to move more efficiently. This is because localized muscle contractions involving only certain segments can occur. Secondly, segmentation

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allows the animal to form specialized regions that are distinct or somewhat separated from the rest of the organism. This also contributes to a more efficiently functioning organism. An example of the specialization of segments will be discussed in the Annelids: Reproduction (Advanced) concept.

Coelom

An important evolutionary development in annelids, that is not obvious when you see the animal from the outside, is the presence of a well-developed coelom, or body cavity. Along with mollusks, annelids are among the first animals to have a coelom. A coelom is a fluid-filled body cavity that is lined with mesodermal tissue. The coelom provides a space within the organism for internal organs to attach and develop. The fluid within the cavity acts as a type of shock absorber to prevent damage to internal organs when the organism moves around. The Figure 1.63 shows the difference between a coelomate, an organism with a true coelom, and an acoelomate, an organism that lacks a coelom (such as a flatworm).

FIGURE 1.63 Cross-sections of an acoelomate and a coelomate. In an acoelomate, the space between the ectoderm and the digestive tract (made of endodermal tissue) is filled with mesodermal tissue, essentially leav- ing no room for internal organ systems. The mesodermal tissue of a coelomate, in contrast, divides to form a space, or cavity, between the digestive tract and the ectoderm. Internal organs are usually attached to the mesodermal tissue lining the cavity.

Another important role for the coelom of invertebrates is to provide an internal support system. This is called a hydrostatic skeleton. The pressure of the fluid inside of the cavity acts as a counterforce for muscle contractions, allowing the animal to move efficiently without the help of an internal bony skeleton. It also allows the animal to maintain its shape. As discussed in the Mollusks concepts, mollusks have a reduced coelom that only encompasses the heart. It was not until the evolution of the well-developed coelom, seen in most annelids, that the full potential of the this cavity was realized.

Vocabulary

• chaetae: Hair-like bristles made of chitin, usually found on the outer surface of annelids.

• coelom: A fluid filled cavity formed within the mesoderm; it forms between the digestive cavity and the body wall.

• convergent evolution: Evolution whereby distantly related species each independently develops the same trait over time.

• hydrostatic skeleton: A structure consisting of a fluid-filled cavity, the coelom, surrounded by muscles; it is used to change an organism’s shape and produce movement.

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• metameres: A series of similar body segments into which some animals are divided longitudinally.

• setae: Stiff hair-like bristles found on organisms.

Summary

• Annelids are characterized by having a segmented body, and most species have tiny, hair-like bristles. • The annelid body is made up of a number of repeating units, or segments, called metameres. • Segmentation is a condition that seems to have evolved independently several times during invertebrate evolution. • Along with mollusks, annelids are among the first animals to have a coelom.

Practice

Use this resource to answer the questions that follow.

• Annelida at http://tolweb.org/Annelida .

1. What are the three body regions of the typical annelid? 2. Describe the structure of each segment in an annelid. 3. When is the earliest appearance of annelids in the fossil record?

Practice Answers

1. Annelids usually have a head, a series of repeating segments, and a terminal post-segmental region called the pygidium. 2. Each segment is limited by septa dividing it from neighboring segments and structures such as the excretory, locomotory, and respiratory organs are usually repeated in each segment. 3. Annelids first appear in the fossil record during the Cambrian period. The majority of polychaete lineages appeared by the end of the Carboniferous period.

Review

1. How many species of annelids are there? What characterizes annelids? 2. How is segmentation an example of convergent evolution? 3. What kind of evolutionary advantages are there to segmentation? 4. How is the coelom different between mollusks and annelids?

Review Answers

1. There are about 15,000 species of annelids, and they are characterized by their segmented bodies (most also have chaetae). 2. Segmentation evolved in both annelids and arthropods even though they are not close relatives with a fairly common ancestor. This means that segmentation evolved separately in two different groups of invertebrates. 3. Segmentation allows animals to move more efficiently by allowing localized muscle contractions to occur. Segmentation also means that an animal can form specialized regions that are distinct from the rest of the body. 4. Mollusks have a reduced coelom that only encompasses the heart, whereas annelids use the coelom for a variety of internal organs.

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1.25 Annelid Classification - Advanced

• Describe the major differences between the annelid classes.

How do you tell one segmented worm annelid from another? With over 22,000 segmented annelids, there is some controversy about their classification. But one thing is certain. They are all worms.

Classification of Annelids

There is currently a lot of controversy among scientists about exactly how to subdivide and classify many species within the annelid phylum. For the purposes of this concept, we will consider two main classes of annelids: Polychaetae and Clitellata. The Clitellata are characterized by the presence of a specialized reproductive organ called the clitellum. This organ will be discussed in the Annelids: Reproduction (Advanced) concept. The class Clitellata is further divided into three subclasses: Oligochaetae, Hirudinea, and a small subclass, Branchiobdella. In this section we will consider the major characteristics of the class Polychaetae and the Clitellata subclasses Oligochaetae and Hirudinea.

Polychaetes

Polychaetae is the largest class of annelids. It includes over 10,000 species of mostly marine and highly diverse segmented worms. The word Polychaetae means “many bristled.” Indeed, polychaetes have many bristles, or setae. The bristles are made up of a carbohydrate substance called chitin. They are stiff hair-like projections on the outer surface of the worms that help them adhere to surfaces and to swim. They can use the adhesion of the setae to keep one part of the body in place while moving another part. Polychaetes are distinguished from other annelids by having pairs of appendages called parapodia attached to the outside of each segment. Parapodia are shaped like paddles, and they are used for both locomotion and respiration. The Figure 1.64 shows a polychaete species with prominent parapodia, that look like small legs, all along the sides of the animal. Most polychaetes do not have gills, and respiration occurs through the surface of the body, particularly through the parapodia. This is similar to respiration in roundworms and flatworms. The parapodia are filled with tiny blood

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FIGURE 1.64 A polychaete species. Notice the para- podia attached on both sides of each segment.

vessels that can absorb oxygen through the surface of these appendages. The parapodia are also where the many bristles of polychaetes are located. Polychaetes are divided into two groups based on their lifestyle: sedentary burrowers and active predators. This grouping does not reflect evolutionary differences between species of the two groups, but it is useful for describing the members of this class. The differences in the anatomy and habitat of these sedentary and predatory polychaetes will be examined in later sections of this lesson.

Oligochaetes

The subclass Oligochaetae includes the well-known earthworms. Unlike polychaetes, oligochaetes do not have parapodia. The word oligochaete means “few bristled,” and that is an accurate description of the members of this class. As shown in the Figure 1.65, the pairs of bristles on the outside of the oligochaete body are almost impossible to see with the naked eye. Compare this with the polychaete in the Figure 1.64. Earthworm species usually have four pairs of small bristles on each segment. There are over 3,000 species of oligochaetes, which makes up about one third of the annelid phylum. They range in size from a few centimeters up to three meters. Although the number of species of oligochaetes is much lower than the number of polychaete species, the actual population of oligochaetes is much greater. This is primarily due to the incredibly large population of these worms in some regions of the Earth’s soil. It is estimated that in certain areas of the world there are up to 40,000 oligochaetes per square meter of soil! These organisms are clearly a very successful group.

Hirudinea

The worms that make up the subclass Hirudinea are the species that we know as leeches. Leeches are close relatives of oligochaetes. Like oligochaetes, leeches lack parapodia, but, unlike oligochaetes, they also lack bristles. The smooth, segmented body of a leech is shown in the Figure 1.66. There are roughly 500 species of leeches. What first comes to mind when we think of leeches is their blood-sucking ability. Many leeches are blood-sucking parasites, but there are also many species that are invertebrate predators. These leeches often eat their prey whole. Leeches have two structures called suckers, one located on the anterior,

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FIGURE 1.65 Oligochaetes, like polychaetes, have bris- tles on each segment, but oligocheate bristles are so small that they can hardly be seen at all.

FIGURE 1.66 A leech species of the subclass Hirudinea. Notice the lack of bristles on the smooth surface of the worm.

or front, region and one located on the posterior, or tail end, that are used for locomotion. Leeches with the suckers visible are shown in the Figure 1.67. For blood-sucking and consuming prey, leeches use a tubular feeding organ called a proboscis. The proboscis is also seen in predatory species of the other classes and subclasses of annelids. One interesting feature of leeches is that they all have exactly 34 body segments. Another characteristic that is unique to leeches within the annelid phylum is that they generally have a reduced coelom that is mostly filled with tissue.

Vocabulary

• branchiobdella: A subclass of the Clitellata, which is composed of small worms that live, attached by suckers, on the gills or surface of crayfish.

• chitin: The tough carbohydrate (polysaccharide) that makes up the cell walls of fungi and the exoskeletons of

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FIGURE 1.67 A jar containing several leech specimens. Notice the structure of the suckers that are visible on the ends of several of the worms.

insects, crustaceans, and other arthropods.

• clitellata: A class of annelids characterized by the clitellum, a reproductive collar.

• clitellum: A specialized gland-like structure that secretes a cocoon during reproduction, encasing the eggs.

• hirudinea: A subclass of the Clitellata, which is composed of leeches with no bristles.

• oligochaetae: A subclass of the Clitellata, which is composed of worms with few bristles, including earth- worms.

• parapodia: Fleshy, paired appendages of polychete annelids that function in locomotion and breathing.

• polychaetae: A class of annelids characterized by their bristles.

• proboscis: An elongated sucking mouth part that is typically tubular and flexible.

• sucker: An organ or other structure adapted for sucking nourishment or for clinging to objects by suction; it is also a shoot produced from the base of a plant (roots or trunk) which can become a new individual.

Summary

• There is currently a lot of controversy among scientists about exactly how to subdivide and classify many species within the annelid phylum. • Polychaetae is the largest class of annelids and are characterized by their bristles. • The subclass Oligochaetae includes the well-known earthworms and have fewer bristles than Polychaetae. • The worms that make up the subclass Hirudinea are the species that we know as leeches, and they lack bristles.

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Practice

Use this resource to answer the questions that follow.

• Polychaetous Annelids (or Polychaetes) FAQ at http://www.nhm.org/site/research-collections/polychaetou s-annelids/faqs .

1. How long have polychaetes been around? 2. Where can you find polychaetes? 3. What do polychaetes eat?

Practice Answers

1. Polychaetes have been found in fossils dating back to the mid-Cambrian period. 2. Polychaetes inhabit all sorts of marine habitats, from the sands of beaches to the deepest depths of the ocean. There are also a few freshwater species. 3. Some polychaetes are carnivorous, while others actively consume sand or mud, stripping off bacteria and other nutrients.

Review

1. Which class of annelids have the most bristles? What are the bristles made of? 2. What functions do the setae serve? 3. Which group of annelids has more species: polychaetes or oligochaetes? What about the actual population? 4. What do annelids in the class Hirudinea feed on? 5. What are some unique features only found in leeches?

Review Answers

1. Polychaetes are the annelids defined by their many bristles. The bristles are made of chitin. 2. Setae, the bristles, are used to help annelids adhere to surfaces and to swim. They allow the annelid to keep one part of their body in place while moving another part. 3. Polychaetes contain over 10,000 species and is the largest class of annelids, but oligochaetes outnumber polychaetes in pure population. 4. Leeches are mainly blood-sucking parasites, but there are also many species that are invertebrate predators that often eat their prey whole. 5. Leeches have exactly 34 body segments and generally have a reduced coelom that is mostly filled with tissue.

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1.26 Annelid Structure and Function - Advanced

• Describe the structural features of annelids and how they contribute to the function of the organism.

Do these have structure and function? Like all animals, these Christmas tree worms must maintain homeostasis to stay alive. They must also obtain energy and reproduce. And they are made of cells. So yes, there are structures with functions that are characteristic of these animals.

Structure and Function in Annelids

The overall structure of annelids does not vary nearly as much as the structure of mollusks, which we examined in a previous concept. All annelids have a worm-shaped, segmented body. As we saw in the discussion of annelid classes in the previous concept, one main variable in annelid anatomy is the number and organization of bristles and appendages protruding from the body. There is also a lot of variation between species in the number and form of feeding tentacles on the animal. Some of the parapodia and tentacles can be extremely elaborate and cause the animals to look quite unlike worms. One example of this is seen in feather dusters, a family of polychaetes shown in the Figure 1.68. However, beneath the feathery-looking protrusions of this animal, is a cylindrical worm-shaped body whose internal anatomy is very similar to that of a plain looking earthworm.

Overall Body Organization

Like flatworms and roundworms, annelids are bilaterally symmetrical triploblasts. Recall that bilateral symmetry means that they can only be divided into two equal halves by a cut along the middle of the anterior-posterior, or front- back, axis. Triploblasts develop from three embryonic cell layers: the ectoderm, which forms the outer surface of the animal and the nervous system, the mesoderm, which forms the middle layers of the organism, including muscle and circulatory tissues, and the endoderm, which forms the gut. Similar to mollusks, annelids are protostomes. Recall that protostomes exhibit spiral, determinant cleavage in their early embryonic cell divisions, and the blastopore ultimately forms the mouth.

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FIGURE 1.68 A polychaete commonly called the feather duster. These animals have elaborate feeding tentacles that emerge from the head and look like feathers.

The body of an annelid is divided into three main regions: the prostomium, the trunk, and the pygidium. These are shown in the Figure 1.69.

FIGURE 1.69 The three main regions of the annelid body.

The prostomium is the segment at the anterior, or front, end of the worm. The mouth usually lies just behind this segment. The region of the body surrounding the mouth is called the peristomium. Peristomium means “around the mouth,” and this region makes up the back section of the head. The trunk includes the many segments that make up the bulk of the worm body and is located between the prostomium and the pygidium. The pygidium is the posterior, or back, end of the worm and houses the anus. Annelids grow by adding segments to the region just in front of, or anterior to, the pygidium. Segmented worms are also capable of regrowing regions of the worm that are broken off. This process is called regeneration, and the ability to regenerate depends on how much of the worm and what part of the body has been lost. The coelom, mentioned earlier, runs the length of the worm and is separated into segments by thin layers of tissue called septa (singular septum). Organs that span these compartments, such as the digestive tract and major blood vessels, pass through the septa. A cross section of an annelid body segment, showing the organization of various internal organs, is shown in the Figure 1.70. These organ systems will be discussed in the next several sections. Annelids commonly secrete a thick protective substance called the cuticle that covers the outer body wall and helps to keep the worm moist. The cuticle is made up of proteins and lipids.

Head Structure

There is a lot of variation in the head region among annelids. In general, species that have a sedentary or non- predatory, burrowing existence have simple heads that cannot be distinguished from their back ends unless you look very carefully. They do not usually have complex sensory organs because they really have no need for them. In contrast, active predatory annelids (see http://rmbr.nus.edu.sg/polychaete/head.html ) can have fairly complex head regions with eyes, tentacles, and sensory projections called palps and antennae. Predatory polychaetes also have

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FIGURE 1.70 A cross-section of an annelid body (poly- chaete). This diagram shows the arrange- ment of organs within each segment. It also highlights the detailed architecture of the parapodium, which is divided into a number of regions: the parapod, notopod, neuropod, and several sensory projec- tions called cirri (singular cirrus). The space between the intestine and the mus- cles of the body wall is the coelom, or body cavity.

jaws with teeth made up of a carbohydrate substance called chitin. A diagram of a predatory polychaete head is shown in the Figure 1.71.

FIGURE 1.71 A diagram of the head of a predatory polychaete showing the antennae, palps, jaws, tentacles, and eyes.

Exceptions to this general trend of simpler head structures for sedentary worms are found in a number of sedentary, marine polychaete species that have long, feathery feeding tentacles extending from their heads, as discussed earlier. While the worm’s body usually remains burrowed or enclosed in a tube-shaped structure that will be discussed in a later section, these tentacles emerge from the burrow and spread out into the water to trap food particles.

Closed Circulatory System

Unlike most mollusks, annelids have a closed circulatory system. In a closed circulatory system, blood is always contained within blood vessels. The annelid circulatory system includes two major blood vessels that run the length of the worm: the dorsal blood vessel along the top and a ventral blood vessel along the bottom. The ventral vessel transports blood from the head to the tail region, and the dorsal vessel transports blood from the tail to the head. These two vessels are connected to each other within each segment by two smaller blood vessels. There are also many small capillaries that extend into the skin of the worm, and this is where gas exchange occurs in most species. Annelids do not have a central, well-developed heart, and usually the muscular dorsal blood vessel functions to pump blood through the circulatory system. In some species there may be a number of muscular blood vessels that

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function as blood-pumping hearts. Earthworms have five of these muscular, heart-like vessels located in the front region of the body. A diagram of a dissected annelid showing part of the closed circulatory system is shown in the Figure 1.72.

FIGURE 1.72 A diagram of the internal anatomy of an annelid. This diagram shows the orga- nization of the closed circulatory system, including the muscular heart-like vessels and the dorsal blood vessel. Notice the smaller blood vessels branching from the dorsal blood vessel. In a closed circula- tory system, blood is transported to each tissue inside of tiny blood vessels. It never flows freely within body cavities of the an- imal. Several components of the annelid nervous system are also depicted in this diagram, including the central ganglion in the head region, the ventral nerve cord, and the many segmental ganglia.

Excretion

The annelid excretory system is made up of long tubular organs called nephridia. Many species have a pair of nephridia in each segment. The nephridia each have an opening called a nephrostome that takes in body fluids from the coelom, and an exit pore in the body wall called the nephridiopore releases waste from the worm. As body fluids travel through the nephridia, both nutrients that are useful to the organism and water are reabsorbed, leaving behind concentrated waste fluid that is excreted through the nephridiopore. The nephridia are lined with short, hair-like projections called cilia that beat repeatedly to maintain the flow of fluid through the organ.

Digestion

The digestive system of annelids shows a lot of variation depending on the feeding habits of the species. All species have a complete digestive tract that begins with a mouth and ends with an anal opening. The digestive system is generally organized in the following order:

• Mouth. • Pharynx. • Esophagus. • Stomach. • Intestine. • Anus.

The mouth has a fairly simple structure in species that consume decaying matter or filter feed, whereas predators and blood-sucking parasites have modifications such as jaws with teeth and a tubular sucking organ, mentioned earlier, called a proboscis. On a side note, the proboscis is also the term used for the trunk of elephants, so there is quite a large range of proboscis structures. The pharynx is the compartment that connects the mouth to the esophagus. The

128 www.ck12.org Chapter 1. Invertebrates - Advanced digestive tract also has modifications depending on the diet of the species. Earthworms, for example, are deposit feeders. That means that they eat soil and sand, and their digestive systems extract organic matter, such as leaves and plant materials, from the sediment. In order to handle this diet, earthworms have a muscular digestive organ called a gizzard that breaks up the soil so that the organic matter can be extracted. An interesting modification of the digestive system of Hirudinea that helps them to successfully obtain nutrition by blood-sucking is the make up of their saliva. Leech saliva contains both painkillers and anti-clotting agents. The painkillers prevent prey from noticing that there is a leech feeding on them, and the anti-clotting factors are important to keep the blood flowing out of the wound created from the bite.

Nervous System

Annelids have a central brain located in the head region. The brain is connected to a ventral nerve cord that runs through the segments along the bottom side of the body. Several components of the annelid nervous system are shown in the Figure 1.72. Within each segment, there is a ganglion (plural ganglia) that is a concentrated region of nerve tissue located on the ventral nerve cord. Nerve fibers extend into each segment from their ganglion. There is also a pair of ganglia located near the pharynx called the sub-pharyngeal ganglia. These ganglia connect to the brain using special nerve fibers called the circum-pharyngeal connectives. Along with their centralized nervous system, annelids have a number of different sensory organs that help them to determine what is going on in their environments. These organs include the eyes, palps, and antennae located in the head region, which we discussed earlier. Many species also have sensory organs that can detect chemicals present in the environment. These are called chemosensory organs. The nervous system of active predatory annelid species often has a larger brain and more sensory organs than sedentary, burrowing species. This makes sense because the lifestyle of predatory species requires more detailed sensory input from the environment and greater nervous system function for complex behavioral responses to that input.

Locomotion

Non-sedentary annelids have several means of locomotion. Aquatic polychaetes can use their paddle-shaped para- podia to swim. Longitudinal muscles that run along the body wall from the front to the back are surrounded by a sheath of circular muscles. Annelids crawl or burrow by combining the actions of these muscles and the bristles, or setae, on the outside of the body. The organism first extends forward and then anchors the front end of the body to a solid surface using the bristles. With the front of the body immobilized, the worm contracts muscles to bring the rear end closer to the front end. The front end is then released and extended forward again while the rear end becomes anchored so that it does not slip backward as the body extends forward.

Vocabulary

• antennae: A pair of long, thin sensory appendages on the heads of insects, crustaceans, some annelids, and some other arthropods.

• bilateral symmetry: A body plan of an organism with a distinct head and a distinct tail region; a cut along the middle of the anterior-posterior axis divides the animal into two equal halves.

• blastopore: The opening of an embryo’s central cavity in the early stage of development.

• chemosensory organs: Sensory receptor organs that transduce chemical signals into action potentials.

• closed circulatory system: A circulatory system in which the blood is enclosed at all times within vessels.

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• cuticle: A thick organic layer surrounding the outer surface of nematodes and arthropods; it is also a waxy waterproof covering over the aerial surfaces of a plant.

• ganglia (plural, ganglion): A cluster of nerve cells that form simple nerve centers distinct from a brain.

• nephridia: Tubular organs that filter waste from internal body fluids.

• palp: An elongated, often segmented appendage usually found near the mouth in invertebrate organisms. The functions of the palp include sensation, locomotion, and feeding.

• peristomium: The area or parts around the mouth in certain invertebrates.

• protostome: An animal in which the first opening formed during development (the blastopore) becomes the mouth.

• regeneration: The regrowth of lost or destroyed parts or organs.

• septa (singular, septum): Thin partitions or membranes that divide two cavities or soft masses of tissue in an organism; they are also internal walls that divide the hyphae of most fungi.

• triploblast: An animal with three germ layers: an endoderm, a mesoderm, and an ectoderm.

Summary

• The overall structure of annelids does not vary very much compared to other phyla. • All annelids have a worm-shaped, segmented body, but one main difference among them is the number and organization of bristles and appendages. • Annelids are bilaterally symmetrical, triploblasts, and protostomes. • Annelids commonly secrete a thick protective substance called the cuticle that covers the outer body wall and helps to keep the worm moist. • There is a lot of variation in the head region among annelids depending on whether they are sedentary, burrowing, or predatory. • Unlike most mollusks, annelids have a closed circulatory system. • The annelid excretory system is made up of long tubular organs called nephridia. • The digestive system of annelids shows a lot of variation depending on the feeding habits of the species.

Practice

Use this resource to answer the questions that follow.

• Characteristics of Annelida: Plesiomorphies and Other Features at http://tolweb.org/articles/?article_id=5 7 .

1. What are some advantages that annelids gain by having septa separate each segment? 2. How is the cuticle of annelids different from the cuticle found in arthropods and some nematodes? 3. What are the six major kinds of sensory structures found in annelids?

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Practice Answers

1. The septa ensure that if the worm is damaged, only the damaged segments lose their coelomic content. Locomotion can still occur, and the damage to the worm is minimized. 2. Annelids have an external cuticle that is never shed or molted, whereas arthropods and nematodes must shed their cuticle. 3. The six major sensory structures are palps, antennae, eyes, statocysts, nuchal organs, and lateral organs.

Review

1. What are the three major sections of an annelid? 2. If you look at different annelids, there is a lot of variation in head structure. Why might this be? 3. Many kids know that you can split a worm in half and it still might live. How does the circulatory system of annelids allow for a worm to regenerate? 4. Can you recall which other phylum has an excretory system made up of nephridia? 5. What allows earthworms to extract organic matter from soil? 6. Describe the nervous system of a basic annelid.

Review Answers

1. The three main sections of the annelid are the prostomium, the trunk, and the pygidium. The prostomium is the segment at the anterior of the worm, where the mouth is located. The pygidium is the posterior and houses the anus. The trunk lies in between. 2. The large variation in the head region is due to the different life-styles of annelids. Some are sedentary or non-predatory and have less use for complex sensory organs, while others need palps and antennae in order to find prey. 3. Annelids do not have a central heart. Some species have a number of muscular blood vessels that function as blood-pumping hearts. Earthworms, for example, have five of these muscular, heart-like vessels, so the circulatory system is able to function even if the worm is split. 4. Mollusks also have an excretory system consisting of nephridia, which supports the fact that these two phyla are closely related. 5. Earthworms have a muscular digestive organ called a gizzard that breaks up the soil so that the organic matter can be extracted. 6. Annelids have a central brain located in the head region that is connected to a ventral nerve cord that runs through the segments along the bottom side of the body. Within each of its segments is a ganglion. Many annelids also have chemosensory organs.

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1.27 Annelid Reproduction - Advanced

• Describe both sexual and asexual reproduction in annelids.

Dig a hole and what might you find? Earthworms. And where there is one worm, there are usually lots of worms. So reproduction is obviously not an issue with worms.

Reproduction

Reproduction in annelids is a fairly complex topic. Polychaete and oligochaete species can reproduce both sexually and asexually, while leeches can only reproduce sexually. Asexual reproduction does not involve the formation of gametes (eggs and sperm), and it usually occurs either by budding or fission. During budding, the worm forms a small protrusion, or bud, that slowly develops into a new organism. When the bud has developed to a point where it can function independently of the parent, it breaks off. Fission differs from budding in that the parent simply splits into two pieces, and then each piece reforms the missing components to form a whole organism. The Figure 1.73 shows a diagram of an annelid in the process of budding. For sexual reproduction involving the formation and fusion of gametes, polychaete species normally have separate sexes, while most oligochaetes are hermaphrodites. In hermaphroditic species, each individual has both male and female sex organs. Sexual reproduction is quite different between polychaetes and oligochaetes. In order to reproduce sexually, marine polychaetes must undergo a major physical transformation. This transforma- tion varies from species to species, but it generally involves the conversion of several of their posterior segments into specialized reproductive segments containing ovaries or testes that produce gametes. At the same time, organs such as the digestive tract degenerate, and the worm develops enhanced appendages for swimming to the ocean surface. Polychaetes time this reproductive transformation precisely so that many individuals participate at the same time. After they have transformed, they all swim to the surface of the ocean and release their gametes to undergo external fertilization. Following this reproductive burst, the organisms die. The externally fertilized zygotes go on to develop into a specialized larval form called a trochophore larva, which was discussed in a previous concept on mollusks.

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FIGURE 1.73 An annelid undergoing asexual reproduc- tion by budding. Notice the new organism forming from the differentiation of seg- ments at the tail end of the animal.

FIGURE 1.74 A polychaete worm in the reproductive epitoky form. Notice the well-developed appendages for swimming.

This form of reproduction is called epitoky, and there are a number of variations seen in different polychaete species. A polychaete in the epitoky reproductive form is shown in the Figure 1.74. Sexual reproduction in oligochaetes and Hirudinea is drastically different from the process of epitoky. Although

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oligochaetes and Hirudinea are hermaphrodites, they are not able to use their own sperm to self-fertilize their eggs. The hermaphrodites must fertilize each other’s eggs through mating. Mating and fertilization are actually separate events in these subclasses. This is possible because the worms have a sac-like organ called a spermatheca that holds onto the sperm after it has been delivered by the mating partner until the worm is ready to use it to fertilize the eggs. The Figure 1.75 shows two earthworms mating.

FIGURE 1.75 Two earthworms mating. In order to mate, the worms line up so that the clitellum of each worm is near the gamete-producing, sexual organs of the other worm. The worms then exchange sperm. The clitella in this picture can be seen as the slightly swollen, pinkish regions on the anterior half of the worm bodies.

In order for oligochaetes to fertilize their eggs, they use the clitellum to secrete a cocoon. The clitellum, mentioned earlier, is a specialized gland-like structure that can be seen from the outside of the animal as a thickened region of segments. The clitellum of each worm can be clearly seen receiving sperm from reproductive segments of its mating partner in the Figure 1.75. Once the cocoon is ready, they deposit their eggs and sperm, stored from the mating process, into the cocoon where fertilization takes place. In this way, oligochaetes have external fertilization. Leeches carry out a slight variation on this process by using the sperm to fertilize the eggs while they are still within the body. Once they are fertilized, the eggs are then deposited into the cocoon. Therefore, leeches have internal fertilization. Once the fertilized eggs are safely in the cocoon, the worm usually separates from it. The embryos develop and emerge from the cocoon looking like small adults that will continue to grow to full-size. There are no larval stages in the oligochaete and Hirudinea subclasses.

Vocabulary

• budding: A form of asexual reproduction in which a new organism develops from an outgrowth, or bud, on another one; the bud may stay attached or break free from the parent.

• clitellum: A specialized gland-like structure that secretes a cocoon during reproduction, encasing the eggs.

• epitoky: A mode of reproduction unique to polychaete worms in which the worm undergoes a partial or entire transition into a pelagic, sexually reproductive form, known as an epitoke.

• fission: Aexual reproduction in which a parent separates into two or more individuals of about equal size.

• hermaphrodite: An animal that can produce both eggs and sperm.

• spermatheca: A receptacle in the reproductive tracts of certain invertebrates in which spermatozoa are received and stored until needed to fertilize the ova.

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• trochophore larva: Juvenile stages that are very different from the adult form; they have a band of cilia that wrap around the body and are characteristic of mollusks and annelids.

Summary

• Polychaete and oligochaete species can reproduce both sexually and asexually, while leeches can only repro- duce sexually. • For sexual reproduction involving the formation and fusion of gametes, polychaete species normally have separate sexes, while most oligochaetes are hermaphrodites. • In order to reproduce sexually, marine polychaetes must undergo a major physical transformation known as epitoky.

Practice

Use this resource to answer the questions that follow.

• Life Cycle of a Polychaete at http://www.ehow.com/about_5344364_life-cycle-polychaete.html .

1. How do female polychaetes release their eggs? 2. Describe what happens during epitoky.

Practice Answers

1. Some females will release eggs into the surrounding water through a small opening in her body, whereas others rupture their bodies, quickly resulting in death. 2. Some worms develop an epitoke that buds off of the atoke or anterior region. These epitokes reach the surface and burst open, releasing thousands of eggs and sperm into the water.

Review

1. Describe the differences between two forms of asexual reproduction: budding and fission. 2. What happens during epitoky in polychaetes? 3. Which annelids are usually hermaphrodites? 4. How is mating and fertilization separate in oligochaetes and Hirudinea? 5. Which worms fertilize internally and which fertilize externally?

Review Answers

1. Budding is when a small protrusion on a mature organism, called a bud, slowly develops into a new organism. Fission differs from budding in that the parent simply splits into two pieces, and then each piece reforms the missing components to form a whole organism. 2. Polychaetes usually undergo epitoky, a process where posterior segments start producing gametes, organs in the digestive tract degenerate, and the worm develops enhanced appendages for swimming. After this transformation, they swim to the surface and release their gametes to undergo external fertilization. 3. Oligochaetes and Hirudinea are usually hermaphrodites, although they cannot self-fertilize. 4. Mating and fertilization are separated because sperm is stored in the spermatheca until the worm is ready to fertilize the eggs. 5. Oligochaetes usually fertilize externally in the cocoon excreted by the clitellum. Leeches, on the other hand, have internal fertilization and then deposit the fertilized eggs in the cocoon.

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1.28 Annelid Ecology - Advanced

• Describe the different habitats in which annelids live. • Examine the roles annelids play in the ecosystem and their effect on humans.

If earthworms live in the dirt, where do worms like this feather duster worm live? Many worms, like this feather duster, are actually part of underwater ecosystems. Shown here is a feather duster worm over a sponge on a coral reef in the Caribbean Sea.

Annelid Ecology

Annelids can be found in almost all parts of the Earth, from high mountain areas to deep oceanic regions. Poly- chaetes, oligochaetes, and Hirudinea all include marine, freshwater, and terrestrial species as well as parasitic species. However, the relative abundance in each of these habitats is different for each class or subclass. Polychaetes are primarily marine organisms that either live a sedentary, burrowed existence or are active predators that move

136 www.ck12.org Chapter 1. Invertebrates - Advanced by crawling on the ocean floor or swimming. The diet of polychaete species shows large variation. Some species are active hunters, while others are scavengers. Among the burrowing species, there are species of deposit feeders and filter feeders. A number of annelid species, including many polychaetes, engage in commensal symbiotic relationships ( commensalism). A commensal relationship between two organisms is beneficial to one member but provides neither benefit nor harm to the other partner. An example of this is seen in the polychaete species that live on the outside of echinoderms. The echinoderm is unaffected, but the polychaete benefits from the stray food particles that can be found around the mouth region. Oligochaetae is mostly made up of freshwater and terrestrial species. They are usually deposit feeders that eat soil and sediment as they burrow through the ground, but some species are predatory. In general, predatory oligochaetes are usually smaller than their soil-consuming relatives. Some species of oligochaetes are vulnerable to infection by internal parasites, including by species of other worm phyla such as flatworms and nematodes. The leeches of the subclass Hirudinea are essentially all carnivores. Most species are predators that attack and eat whole organisms, often other invertebrates. The remaining species are external parasites that feed off the blood of their hosts.

Life in a Tube

Throughout the Annelid concepts, we have described both sedentary polychaetes and oligochaetes, such as earth- worms, as burrowers. The burrowing of oligochaetes is very different from that of sedentary polychaetes because oligochaetes are not sedentary. Oligochaetes form burrows that are long and continuously growing as they tunnel through the soil. Sedentary polychaetes do not crawl through the sand or soil, they simply dig a hole and live in it. In these holes, the worm often forms a tube that is just slightly larger than the worm itself. For this reason, most sedentary polychaetes are called tube-dwellers. Some species will remain in this tube throughout their lives. The tube can be formed from a variety of materials such as substances secreted by the worm, sand and debris, or a mix of these. In order to eat, they either extend feeding tentacles out of the tube to trap food particles or just eat the sediment they are burrowed in. However, not all tube-dwellers form their tubes burrowed in a hole. Some attach themselves to a rock or some other solid support within the ocean and then secrete a tube in which to live. One such species is shown in the Figure 1.76.

FIGURE 1.76 This highly unusual species of poly- chaete, Riftia pachyptila, shows an ex- ample of the tubular structure formed by a tube-dweller. The white tubes formed by these worms are made of chitin, and they look like bandages wrapped around their bodies. The velvety-looking, reddish- brown extensions are vascular organs called plumes. These particular tube- dwellers are found deep in the Pacific ocean near hydrothermal vents. They attach themselves to a solid substrate, as shown here.

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Importance to Humans

As Charles Darwin pointed out well over 100 years ago, annelids are enormously important to the Earth’s ecosys- tems. Without earthworms, it is likely that the earth’s soil would not be capable of sustaining the growth of human food and the food of many other plant-eating species. How is it that these small animals are so important for plant growth? Earthworms essentially form the organic compounds that make soil suitable for vegetation. They do this by digesting dead plant matter and microorganisms, many of which feed on dead animal carcasses, and converting them into simpler molecules that can be absorbed and used by living plants. Earthworm feces are referred to as worm casts, and they are extremely rich in plant nutrients like nitrogen and phosphates. The amount of organic material that is processed by earthworms is so large that it is difficult to imagine. If you live in a temperate climate, then almost all of the soil in your environment has passed through the body of an earthworm. It is also estimated that most of the molecules in our bodies were at one time inside the body of an earthworm. Thinking about this really makes you look at the humble earthworm in a different light. Another way that earthworms contribute to soil health is through their burrowing actions described in the previous section. The extensive tunnel networks produced by earthworm burrowing allow water and oxygen to penetrate the soil more efficiently. Earthworms also form the bottom of the food chain for a number of organisms. They are a source of food for many birds, mammals, and other invertebrates. Other annelids, such as the lugworms of the class Polychaetae, contribute to human food sources as well. We use these species as bait to catch fish. Although they probably are not organisms that you think about very often, annelids are undoubtedly essential to the existence of human life and the lives of many other species.

Vocabulary

• commensalism: A symbiotic relationship in which one species benefits while the other species is not affected.

• tube-dwellers: Sedentary polychaetes that form tubes around their bodies.

• worm cast: A convoluted mass of soil, mud, or sand excreted by an earthworm after digestion; it is rich in nitrogen and phosphates.

Summary

• Annelids can be found in almost all parts of the Earth, from high mountain areas to deep oceanic regions. • Polychaetes are primarily marine organisms that either live a sedentary, burrowed existence or are active predators that move by crawling on the ocean floor or swimming. • Oligochaetae is mostly made up of freshwater and terrestrial species. • The leeches of the subclass Hirudinea are essentially all carnivores. • Without earthworms, it is likely that the Earth’s soil would not be capable of sustaining the growth of human food and the food of many other plant-eating species.

Practice

Use this resource to answer the questions that follow.

• Earthworms’ role in the ecosystem at http://www.sciencelearn.org.nz/Science-Stories/Earthworms/Earthw orms-role-in-the-ecosystem .

1. Imagine that there is an area with a low population of earthworms. What nutrients would you expect to find in lower quantities in the soil?

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2. How does the burrowing of earthworms help the soil? 3. How are earthworms particularly helpful for humans?

Practice Answers

1. You would expect there to be less phosphorus and nitrogen for plants. Scientists have recorded 5 fold increases in nitrogen from earthworm casts. Without sufficient earthworms, plants won’t get enough nitrogen or phosphorus. 2. When earthworms burrow into soil, they open up pores so that water and soluble nutrients can reach down to plant roots. It also improves soil aeration and enhances plant root penetration. 3. Earthworms increase pastoral productivity, leading to greater food supplies for humans. They also help restore ecosystems after mining.

Review

1. What environments do polychaetes occupy? What forms the diet of polychaete species? 2. Give an example of a commensal symbiotic relationship. 3. Which subclass of annelids is composed of purely carnivorous worms? 4. What are the tubes of tube-dwellers composed of? 5. Briefly describe why earthworms are so important to humans.

Review Answers

1. Polychaetes are primarily marine organisms that either live on the ocean floor or swim. Some are scavengers and hunters, while others are deposit or filter feeders. 2. Some polychaete species live on the outside of echinoderms and steal stray food particles found around the mouth region. The polychaetes gain a food source while not directly harming the echinoderm. 3. The leeches of the subclass Hirudinea are essentially all carnivores. Most species are predators that attack and eat whole organisms, often other invertebrates. The remaining species are external parasites that feed off the blood of their hosts. 4. The tube of tube-dwellers can be formed from a variety of materials such as substances secreted by the worm, sand and debris, or a mix of these. 5. Earthworms are important to humans because they break down dead plant matter and microorganisms into simpler molecules that can be absorbed and used by living plants. Almost all the soil in temperate climates has passed through the body of an earthworm. Their extensive tunnel networks also help water and oxygen penetrate the soil more efficiently. Without earthworms, human agriculture would be greatly reduced.

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1.29 Arthropods - Advanced

• Give a brief overview of the characteristics that define arthropods and their ecology.

What has more species than any other animal phylum? Arthropods are not only the largest phylum of invertebrates. They are by far the largest phylum of the animal kingdom. Roughly 80 percent of all animal species living on Earth today are arthropods. Obviously, arthropods have been extremely successful. What accounts for their success?

Characteristics of Arthropods

Arthropods are literally everywhere around us. It is almost impossible to live on Earth and not interact with members of the arthropod phylum. Based on the number of species, they are by far the largest phylum in the animal kingdom. To put the importance of arthropods in perspective, roughly 80% of all species living on the planet today are members of the phylum Arthropoda. In this respect, arthropods have been wildly successful organisms. Arthropods include insects, spiders, lobsters, centipedes, and many, many other diverse species. You have very likely eaten arthropods, you share your home with arthropods, and you have probably been bothered by pesky arthropods on many occasions. What are the characteristics that unite the diverse species found in the arthropod phylum? How are the organ systems of arthropods organized, and how do they function? What are the details of their growth and development? How did arthropods evolve, and what do we know about their ancestors? In this concept we will take a closer look at the members of this vast phylum in order to answer these questions. The more than one million diverse species of arthropods that have been described range in size from less than one millimeter (smaller than a comma on this page) to four meters (about the length of a car). The smallest arthropods include the dust mites that may live in your house and feed on flakes of skin that have fallen off your body. The largest arthropod is the Japanese spider crab that measures almost four meters in length and is found on the ocean floor near Japan. Both of these examples are shown in the Figure 1.77 along with a number of other members of the phylum.

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FIGURE 1.77 Arthropod Diversity. Dust mites are among the smallest of arthropods. Japanese spider crabs are the largest. Besides size, what other differences among arthropods do you see in these photos?

The word arthropod means “jointed foot,” and arthropods are so named because of the jointed appendages that protrude from their body segments. The Figure 1.78 shows the jointed appendage of a lobster. The most prominent characteristics of arthropods are a segmented body with jointed appendages on at least one segment and a hard exoskeleton. The structural details of these features will be discussed in more detail below. Arthropods are bilaterally symmetrical protostomes. In previous concepts you learned that bilaterally symmetrical animals can only be divided into two equal halves by a cut down the middle of the anterior-posterior axis, or the head-to-tail axis. You also learned that protostomes, such as mollusks and annelids, all share similar patterns of early embryonic development. Another feature that organisms within these phyla share is that they are all triploblasts, meaning that they develop from three basic tissue layers, or germ layers: an endoderm, an ectoderm, and a mesoderm. The habitats of arthropods include almost any location on Earth. Arthropods have successfully inhabited marine, freshwater, terrestrial, and aerial environments. On land and in water, arthropods can be found from the tropics to polar regions and from great sea depths to high mountain elevations. The methods of feeding in arthropods are also highly varied. They include free-living predators, scavengers, filter feeders, and even some parasites. Parasitic arthropods include fleas, lice, and the bedbug shown in the Figure 1.79. There are some species that are primarily sessile, such as the barnacles shown earlier in the Figure 1.77, and some arthropods enter into symbiotic relationships with other organisms, including bacteria that may live in their gut. Throughout human history, arthropods have played a prominent role. They have contributed to human life in many positive ways, including as a source of food, but they have also been and continue to be the cause of much human

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FIGURE 1.78 The jointed appendages of a squat lob- ster. The labels show the names of each segment of the appendage.

FIGURE 1.79 The common bedbug. Bedbugs are about the size of apple seeds, and they live in human environments, for example, inside homes. They feed on human blood, a behavior known as hematophagy.

suffering. In the past, species of arthropods have caused devastating plagues and famines. Today, species of mosquitoes rapidly spread diseases like malaria, leading to the death of over one million people each year. The relationships between humans and arthropods, specifically insects, will be discussed in greater detail in later lessons. It is difficult to describe a typical arthropod body plan beyond the presence of segments, jointed appendages, and a tough exoskeleton. The body of a butterfly, for example, is quite different from that of a crab. However, there are some common characteristics among arthropod structures, and these will be discussed in the next concept.

Vocabulary

• exoskeleton: A non-bony skeleton that forms on the outside of the body of some invertebrates and provides protection and support.

• jointed appendage: An appendage with points in the exoskeleton at which movable parts join, as along the leg of an arthropod.

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• protostome: An animal in which the first opening formed during development (the blastopore) becomes the mouth.

• triploblast: An animal with three germ layers: an endoderm, a mesoderm, and an ectoderm.

Summary

• Arthropods are bilaterally symmetrical protostomes. • Characteristics that define arthropods include a segmented body, jointed appendages, and a hard exoskeleton.

Practice

Use this resource to answer the questions that follow.

• Arthropods at http://www.biology-online.org/10/6_arthropods.htm .

1. How many species of arthropods are there? 2. What advanced evolutionary adaptations did arthropods develop? 3. What taxon of arthropods were some of the first species to occupy dry land?

Practice Answers

1. Scientists estimate that there are over two million species of arthropods. 2. Arthropods developed more advanced receptors such as eyes and other various chemoreceptors that allowed arthropods to be much more sensitive to their environment. 3. Arachnids were one of the first taxon to transition from life in the sea to life on dry land.

Review

1. How large is the phylum Arthropoda? 2. What are some characteristics that are common to most arthropods? 3. Where can you find arthropods?

Review Answers

1. Arthropods make up the largest phylum in the animal kingdom and include roughly 80% of all species living on the planet today. 2. Most arthropods have a segmented body, jointed appendages, and a hard exoskeleton. 3. Arthropods can be found in almost any habitat on Earth, ranging from marine, freshwater, terrestrial, and aerial environments.

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1.30 Arthropod Structure and Function - Ad- vanced

• Examine the characteristic segments of the arthropod body. • Discuss how an arthropod’s circulatory, nervous, and excretory systems work.

Why a segmented body? The body structure of this beetle is easily distinguishable. The three segments each have their own function and have led to the success of these organisms.

Structure and Function in Arthropods

The great success of the arthropods is likely owed to the very features that make them unique, namely their segmented bodies, jointed appendages, and tough exoskeletons. In this section we will examine these characteristics in more detail. Then we will analyze the internal organ systems of arthropods, including their digestive system, circulatory system, respiratory system, and nervous system.

Segments and Exoskeleton

The general body plan of arthropods is divided into three main regions:

• Head. • Thorax. • Abdomen.

These regions are called tagmata, and they are formed by groups of body segments that are either fused together or linked by joint tissue. The Figure 1.80 shows the overall body plan of arthropods.

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FIGURE 1.80 Arthropod Body Plan. Notice the head, thorax, and abdomen segments of each organism.

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(left) Arthropod Head. Arthropods have evolved a variety of specialized appendages and other structures on their heads. (right) Generalized body plan of an arthropod. This diagram shows the three major tagmata of an arthropod: head, thorax, and abdomen. Notice that the tagmata consist of groups of segments and that the abdomen is divided into two subregions: the pleosome and the urosome. The specific name of each specialized appendage is indicated. The last region of the body is called the telson, and it is not considered a true segment. It forms the tail of the animal. The earliest arthropods have a set of appendages or limbs on each segment. More recently evolved species, such as the bedbug, have a reduced number of appendages, and these appendages are often highly modified to carry out distinct functions. Some of these functions include the following:

• Feeding (mouthparts). • Sensing the environment (antennae). • Walking (legs). • Swimming (paddle-like limbs). • Defending against predators and capturing prey (claws or fangs).

As mentioned earlier, the appendages of arthropods are distinct from those of annelids in that they are jointed. In some species, the appendages are also branched, a condition called biramous. Unbranched appendages are called uniramous. Jointed appendages allow the animal much greater flexibility and range of movement. Imagine walking without bending your knees. It would be possible but very awkward and inefficient. Diagrams of uniramous and biramous arthropod limbs are shown in the Figure 1.81. The arthropod exoskeleton is impermeable to water, a feature that is key for survival on land. It consists of a multilayer cuticle composed of a complex carbohydrate called chitin, as well as proteins and lipids. The structure of the cuticle/exoskeleton is shown in the Figure below. There are many advantages to having a hard outer layer:

• Protection. • Water retention. • Structural support (particularly on land). • Counterforce for attachment and contraction of muscles.

However, there are also some limitations imposed by the exoskeleton. For one thing, it can severely limit the flexibility of the animal due to its rigid nature. Arthropods have partially alleviated this problem by having the

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FIGURE 1.81 Uniramous versus biramous arthropod appendages. Uniramous appendages consist of sequential segments that do not branch. Biramous appendages have segments that branch off of each other.

FIGURE 1.82 Structure of the cuticle/exoskeleton of arthropods. The cuticle components are labeled as follows: 1. Epicuticle 2. Exo- cuticle 3. Endocuticle 4. Epidermal cell layer 5. Basement membrane 6. Bristle- forming cell 7. Bristle socket 8. Bristle 9. Dermal gland 10. Dermal gland duct opening.

exoskeleton form as individual plates on each body segment or individual cylinders on each segment of the ap- pendages. The plates are called sclerites, and they are connected by flexible tissue to allow the animal to bend during movement. Many species also have an additional exoskeletal component called a carapace. This is a shield- like plate of exoskeleton that often forms over the thorax and head region. An example of a species with a carapace is shown in the Figure 1.83.

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FIGURE 1.83 A species of crab. The shield-like cover- ing on the top of this crab is the carapace, a modified plate of exoskeleton.

Movement

Arthropods move using their appendages as legs on land and as paddles in aquatic environments. They have striated and smooth muscles, similar to those of vertebrates, that connect to the exoskeleton for support. Winged insects are also able to move by flying. Flight will be discussed in more detail in the Insects: Flight (Advanced) concept.

Respiration and Circulation

Similar to annelids, arthropods are coelomates with a true coelom, or body cavity. However, in arthropods the coelom is reduced to a small compartment surrounding the reproductive and excretory organs. Like the mollusks you learned about in the Mollusks chapters, arthropods have an open circulatory system that consists of a dorsal blood vessel and large body cavities called hemocoels. These blood-filled regions are the primary body cavities in arthropods. In an open circulatory system, the blood is only transported part of the way through the body within blood vessels. These vessels empty into the hemocoels, and blood must diffuse from there to reach tissues throughout the body. This is less efficient than a closed circulatory system in which blood is transported to all tissues of the body within blood vessels. In arthropods, the dorsal blood vessel runs along the dorsal, or top, interior of the body, and it consists of two major regions. The abdominal region is highly muscular and is considered the heart. It consists of a number of chambers that pump hemolymph (arthropod blood) collected from the hemocoel in the abdomen toward the head region. The hemolymph enters the heart through ostia, or pores, located on each chamber. The portion of the dorsal blood vessel that goes through the thorax is not muscular, and it is called the aorta. Hemolymph emerges from the aorta in the head region, and from there it re-enters the hemocoels and diffuses throughout the body to reach all tissues. The Figure 1.84 illustrates the internal anatomy of an arthropod with the dorsal circulatory system shown in red. The respiratory system of arthropods includes several different adaptations depending on the subgroup and habitat. Aquatic arthropods respire using gills that absorb oxygen from water. Some terrestrial arthropods, such as spiders and scorpions, have book lungs to breath gaseous oxygen from the air. Book lungs are made up of stacks of folded tissue that have a large surface area for absorbing oxygen. Most other terrestrial arthropods have evolved an elaborate system of trachea that take in air and deliver it throughout many regions of the body. There are essentially two types of excretory systems in arthropods:

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FIGURE 1.84 Internal anatomy of an arthropod. The digestive system is shown in green, the circulatory system is shown in red, and the nervous system is shown in blue.

FIGURE 1.85 How Terrestrial Arthropods Breathe Air. Terrestrial arthropods have respiratory structures that let them breathe air: dia- grams of the trachea and book lungs are shown here.

• Malphigian tubules. • Coxal glands.

Crustaceans and some arachnids have an excretory system that uses coxal glands (also called green glands, antennal

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glands, or maxillary glands). Coxal glands consist of a sac that collects waste from the hemocoel, a tubule that facilitates the reabsorbtion of useful nutrients and the concentration of waste, and excretory pores. The pores are located at the base of the appendages. In contrast, most terrestrial species use Malphigian tubules for excretion. A diagram of a Malphigian tubule is shown in the Figure below. The waste collected in these tubules is not excreted directly out of the organism. Instead, it is delivered to the posterior end of the digestive tract to be eliminated through the anus. The evolution of Malphigian tubules was important for terrestrial arthropods to be able to efficiently regulate water and salt balance within their bodies.

FIGURE 1.86 A diagram of a Malphigian tubule. This diagram illustrates how the tubule empties into the hindgut of the digestive tract.

Nervous System

The nervous system of arthropods is made up of two nerve cords running along the ventral, or bottom, interior surface of the organism. A series of ganglia are located on the nerve cords, typically one pair for each body segment. Arthropods are cephalized organisms meaning that they have a central brain that is located in a distinct head region. The head region also houses the mouth and some sensory organs. Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia. It controls the mouthparts, the salivary glands, and certain muscles in the head. The nervous system of a typical arthropod is shown in blue in the diagram above ( Figure 1.84). Many arthropods have well-developed sensory organs, including both simple and compound eyes for vision and antennae that are used to “smell” or detect chemicals in the environment. Compound eyes are made up of many individual photoreceptor units that each form a portion of an image of the world around the organism. If you have ever looked through a kaleidoscope, then you have an idea of what the images seen with compound eyes may look like. The Figure 1.87 shows the compound eyes of a dragonfly. Arthropods also often have hair-like bristles on the surface of their abdomens that allow them to sense touch. This is important because it would be difficult for nerves

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on the body wall to sense touch through the rigid exoskeleton.

FIGURE 1.87 The compound eyes of a dragonfly.

Underwater Spiders

In the ponds of northern Europe lives a tiny brown spider that spends its entire life underwater. But just like land spiders, it needs oxygen to breathe. So, how does this spider breath? Does it use book lungs? No. In fact, aquatic spiders, known as "diving bell spiders," have gills. Every so often, the spider leaves its underwater web to visit the surface and bring back a bubble of air that sticks to its hairy abdomen. It deposits the bubble into a little silk air tank. This "diving bell" is a gill that sucks oxygen from the water, allowing the spider to stay underwater for up to 24 hours. See http://news.sciencemag.org/sciencenow/2011/06/spiders.html?ref=hp for additional information and additional pictures. http://www.youtube.com/watch?v=GidrcvjoeKE (2:15) shows these spiders in action.

MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/460

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FIGURE 1.88 A pair of diving bell (water) spiders.

Vocabulary

• biramous: An appendage that divides to form two branches, found mainly in arthropods.

• book lungs: Respiratory organs found in some arachnids, such as scorpions and spiders, consisting of several parallel membranous folds arranged like the pages in a book.

• carapace: A hard, bony or chitinous outer covering such as the fused dorsal plates of a turtle or the portion of the exoskeleton covering the head and thorax of a crustacean.

• chitin: The tough carbohydrate (polysaccharide) that makes up the cell walls of fungi and the exoskeletons of insects, crustaceans, and other arthropods.

• compound eye: An eye consisting of an array of numerous small photoreceptor units that each form a portion of an image, as found in insects and crustaceans.

• coxal glands: Glands with ducts opening in the coxal region of arthropods and, in some forms, (such as in spiders) functioning as excretory organs.

• cuticle: A thick organic layer surrounding the outer surface of nematodes and arthropods; it is also a waxy waterproof covering over the aerial surfaces of a plant.

• hemocoel: A blood-filled body cavity that is distinct from the fluid-filled coelom.

• hemolymph: The circulatory fluid of certain invertebrates, analogous to blood in arthropods.

• Malphigian tubules: Excretory organs of insects and many other arthropods; they are narrow tubules opening into the anterior part of the hindgut.

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• sclerite: A chitinous or calcareous plate, spicule, or similar part of an invertebrate, especially one of the hard outer plates forming part of the exoskeleton of an arthropod.

• tagmata: A distinct section of an arthropod consisting of two or more adjoining segments.

• trachea: A long tube that leads down to the chest; each of a number of fine chitinous tubes in the body of an insect, conveying air directly to the tissues. It is also known as the windpipe.

• uniramous: An appendage that does not branch but consists of a single branch.

Summary

• The general body plan of arthropods is divided into three main regions: the head, thorax, and abdomen. • Some arthropods have modified appendages that have functions such as feeding, sensing the environment, walking, swimming, defending against predators, and capturing prey. • Jointed appendages allowed arthropods to have much greater flexibility and range of movement. • Advantages of having a hard outer layer are protection, water retention, structural support (particularly on land), and counterforce for attachment and contraction of muscles. • Like mollusks, arthropods have an open circulatory system that consists of a dorsal blood vessel and hemo- coels. • Arthropods have two types of excretory systems: Malphigian tubes and coxal glands. • The nervous system of arthropods is made up of two nerve cords running along the ventral, or bottom, interior surface of the organism.

Practice

Use this resource to answer the questions that follow.

• Arthropod: Digestive System and Feeding at http://www.britannica.com/EBchecked/topic/36943/arthropo d/42365/Digestive-system-and-feeding .

1. Which types of feeding modes do arthropods exhibit? 2. What part of the arthropod’s digestive system handles absorption of digested food? 3. Why is the tracheal system found in arthropods efficient? 4. What makes the blood of some crustaceans blue?

Practice Answers

1. Arthropods include carnivores, herbivores, detritus feeders, filter feeders, and parasites. 2. The midgut region is primarily responsible for enzyme production and absorption of digested food. 3. There are several reasons why arthropods evolved their tracheal system. The small external openings reduce water loss, the chitinous lining prevents collapse, and the small size of arthropods are ideally suited for the short length of the tubules. Tracheal tubes eliminates the need for moving gases in and out by active ventilation. 4. The blood of large crustaceans contains hemocyanin, a blue oxygen-carrying molecule. Some insects and small crustaceans have blood containing hemoglobin.

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Review

1. List the three tagmata of arthropods. 2. What evolutionary advantages did arthropods gain from jointed appendages? 3. What is one limitation of a hard exoskeleton? How have arthropods dealt with this problem? 4. How is the coelom of an arthropod different from the coelom of a mollusk? 5. What kind of a circulatory system do arthropods have? 6. What are the hair-like bristles found on some arthropods used for?

Review Answers

1. The general body plan of arthropods is divided into three main regions, or tagmata: the head, thorax, and abdomen. 2. Jointed appendages allowed arthropods to have much greater flexibility and range of movement. 3. An exoskeleton limits flexibility due to its rigid nature. Arthropods have partially alleviated this problem by having the exoskeleton form as individual plates on each body segment or individual cylinders on each segment of the appendages. 4. In arthropods, the coelom is reduced to a small compartment surrounding the reproductive and excretory organs. In most mollusks, the coelom only surrounds the heart. 5. Like mollusks, arthropods have an open circulatory system that consists of a dorsal blood vessel and hemo- coels. 6. Arthropods often have hair-like bristles on the surface of their abdomens that allow them to sense touch. This is important because it would be difficult for nerves on the body wall to sense touch through the rigid exoskeleton.

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1.31 Arthropod Growth and Development - Ad- vanced

• Describe how arthropods grow and develop. • Understand how arthropods reproduce and mature into adults.

What’s going on here? Here we see the metamorphosis of a cicada. This is the last molt, as the cicada turns into an adult insect.

Growth and Development in Arthropods

Some aspects of arthropod growth resemble those of annelids, but there are several important distinctions. In this section we will examine those similarities and differences.

Growth and Molting

Arthropods grow by forming new segments near the tail, or posterior, end. This is similar to the growth of annelids. Mollusks and arthropods both have tough chitinous exoskeletons. The mollusk exoskeleton, or shell, also contains high levels of calcium carbonate minerals, making it more rigid. Unlike mollusk shells, the exoskeleton of arthropods does not grow along with the rest of the animal. As the body underneath the exoskeleton grows, the animal begins

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to outgrow its tough exterior. In order to cope with this situation, arthropods undergo a process of shedding their exoskeleton and forming a new one periodically throughout their lifetime. This process, shown in the Figure 1.89, is called molting, or ecdysis, and is regulated by special hormones that coordinate molting events with stages of growth.

FIGURE 1.89 A crab undergoing the process of ecdysis, or molting. During molting, a new cuticle begins to form beneath the old cuticle. As this proceeds, the old cuticle detaches and eventually is completely separated from the organism.

For a video of a scorpion molting, visit http://www.youtube.com/watch?v=4pIuXqxEwlI (1:21)

MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/139365

Reproduction and Development

Arthropods reproduce by sexual reproduction, which involves the generation and fusion of gametes. Most arthropods are either male or female, and they undergo internal fertilization. Once the egg has been fertilized, the female usually lays the egg, and it continues developing outside of the mother’s body. During most of their life cycle, arthropods have the segmented bodies described earlier. Early on in arthropod development, however, some species exist in intermediate stages that are very different from the adult form. These are called larval stages. None of the arthropod larval stages include the trochophore larva that is characteristic of mollusk and annelid development. You are probably most familiar with this process in caterpillars that form a cocoon in which they undergo a drastic change in form called metamorphosis, ultimately emerging as a moth or butterfly. Development involving larval stages is called indirect development. There are also many species, such as spiders, that undergo direct development,

156 www.ck12.org Chapter 1. Invertebrates - Advanced in which the young hatch looking essentially like a smaller version of the adult. These species do not go through larval stages or metamorphosis. For a video covering the full life cycle of a monarch butterfly, visit http://www.youtube.com/watch?v=7AUeM8Mba Ik (4:33).

MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/139366

Vocabulary

• ecdysis: The shedding of an outer integument or layer of skin, as seen in insects, crustaceans, and snakes; it is also known as molting.

• exoskeleton: A non-bony skeleton that forms on the outside of the body of some invertebrates and provides protection and support.

• larval stages: The early developing stages of various animals in which the organism differs from its adult form.

• metamorphosis: The process of transformation from an immature form to an adult form in distinct stages.

• trochophore larva: Juvenile stages that are very different from the adult form; the larva has a band of cilia that wraps around the body and is characteristic of mollusks and annelids.

Summary

• Arthropods grow by forming new segments near the tail, or posterior, end. • The exoskeleton of arthropods does not grow along with the rest of the animal. • Arthropods reproduce by sexual reproduction, which involves the generation and fusion of gametes.

Practice

Use this resource to answer the questions that follow.

• Caterpillar Metamorphosis: The Magic Within the Chrysalis at http://science.howstuffworks.com/zoo logy/insects-arachnids/caterpillar3.htm .

1. How do monarch caterpillars attach themselves to the underside of branches? 2. True or false: all caterpillars use very similar methods to secure themselves when they undergo metamorpho- sis. 3. What happens within the chrysalis during the metamorphosis of a caterpillar? 4. Can butterflies remember what happened during their caterpillar phase?

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Practice Answers

1. Monarch caterpillars first make a silk pad on the underside of a branch, then they embed what’s called a cremaster firmly in the silk, securing themselves. 2. False. Caterpillars use a variety of ways to support themselves as they pupate. Some make a silk sling, while others support themselves with a silk hammock. 3. Inside the chrysalis, the caterpillar’s body breaks down into imaginal cells, which are undifferentiated cells that are very similar to stem cells. These imaginal cells assemble into the new body form that emerges from the chrysalis. 4. Scientists have proven that butterflies remember specific smells that they encountered as caterpillars, although this learning must occur in the last instars of the caterpillars’ life.

Review

1. How do arthropods usually grow in length? 2. How is the exoskeleton of an arthropod different from the shell of a mollusk? 3. How do arthropods reproduce?

Review Answers

1. Arthropods grow by forming new segments near the tail, or posterior, end. 2. Unlike the shells of mollusks, the exoskeleton of arthropods does not grow along with the rest of the animal, so it must undergo ecdysis. 3. Arthropods reproduce through sexual reproduction and internal fertilization. The female usually lays the eggs after fertilization.

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1.32 Arthropod Evolution - Advanced

• Understand how the large variety of arthropod species evolved from a common ancestor. • Learn about the current scientific discussion on the placement of arthropods on the phylogenetic tree.

Why would such a creature evolve? Arthropods originated well over 500 million years ago. The trilobites, shown in this fossil from the Devonian Period (about 419 to 358 mya), were some of the earliest arthropods. The external skeleton, segmented body, and jointed appendages are clearly visible and were important evolutionary steps. Many similar trilobites were 1-3 feet long (300mm to over 700 mm). Imagine if they were still around today.

Evolution of Arthropods

Although arthropods have a hard exoskeleton, it is not as mineralized and tough as the mollusk shell, and the fossil record of arthropods is not as complete as you might expect for an animal with an exoskeleton. However, there are still many useful arthropod fossils, particularly of the trilobites, as we will learn later in this section. Even with this considerable record, establishing the precise evolutionary history of the phylum has been challenging. The study of arthropod evolution is an ongoing process, and theories are modified regularly as more information is obtained through molecular comparisons between species. This information will ultimately help to clarify the exact relationships between living species and their ancestors. For this lesson, we will consider one of the current hypotheses on arthropod evolution.

Fossil Record and the Trilobites

Arthropod fossils date back to the Cambrian period (over 500 million years ago). The most extensive group of arthropod fossils is the trilobites, a subphylum of marine arthropods that is entirely extinct. Trilobite fossils, like the one shown in the Figure 1.90, are abundant throughout the Cambrian period (from about 540 million years ago to about 490 million years ago). Trilobites are the oldest known arthropods. Following the Cambrian period, their numbers slowly declined until about 250 million years ago when they were entirely lost from the fossil record due

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to extinction. This large (over 17,000 species) and well-preserved evolutionary record has contributed greatly to the study of animal and plant fossils, a field known as paleontology. Close examination of trilobite fossils has helped scientists learn about rates of speciation within the Cambrian period and has also contributed to our understanding of the age of different sediment layers buried beneath the surface of the Earth.

FIGURE 1.90 A fossil of a trilobite dated to the Cam- brian period.

The common ancestor of all arthropods likely had a basic segmented body plan like the one shown in the Figure 1.91. This Figure also shows how variations on the body plan for different arthropod subphyla, which we will examine in the next two lessons, likely evolved from this common ancestor.

FIGURE 1.91 The likely ancestral arthropod body plan and evolutionary modifications.

You will notice from the figure that the ancestral arthropod had an identical pair of appendages on each segment, and the segments were nearly identical, repeating units. This is true of arthropods existing today that resemble this early ancestor. During arthropod evolution, one variation on the body plan that arose involved the loss or modification of many of these appendages to carry out different functions. For example, you will read in the Arthropods: Arachnids (Advanced) concept about how spiders have modified appendages near the head region into sharp fangs that can be used to pierce and pulverize prey. Another variation is the fusion or grouping of several segments to form distinct body regions. The order and extent of this grouping varies greatly within the phylum. There are two small phyla with species that exist today that are very closely related to arthropods by molecular analysis. These are the Onychophora and the Tardigrada. The Comparison of Features Table shows a list comparing the major characteristics of these two phyla with Arthropoda. These three groups are thought to share a more recent common ancestor with each other than with any other phyla and may suggest some information about their common ancestor. For example, the

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fact that all three phyla exhibit molting suggests that their common ancestor also shed its cuticle by the process of molting.

TABLE 1.10: Comparison of Features Between Phyla

Arthropoda Onychophora Tardigrada Segmented Partially segmented Segmented Rigid exoskeleton Soft outer cuticle Rigid exoskeleton Jointed appendages Unjointed appendages Unjointed appendages Molting Molting Molting

Ecdysozoa and Arthropods

Earlier evolutionary groupings of invertebrate phyla considered flatworms to be close relatives of roundworms and annelids to be closely related to arthropods. These groupings were based on the fact that both flatworms and roundworms lack a true coelom (although you may recall from the Roundworms concept that roundworms have an incomplete, or "pseudo," coelom) and that both annelids and arthropods have segmented bodies. The increasing amount of data gathered from molecular phylogenetic studies, which compare the genetic make up of different organisms to determine how closely they are related, has called these groupings into question. It now seems likely that segmentation evolved independently in several different groups of invertebrates, a process known as convergent evolution. These studies are not yet fully resolved, but one theory that seems to be emerging is that annelids and mollusks shared a more recent common ancestor than annelids and arthropods, while arthropods and roundworms of the nematode phylum share a more recent common ancestor than either roundworms and flatworms or arthropods and annelids. If this seems confusing, that is because these studies are currently incomplete, and there are major debates going on right now between scientists about how to categorize these complex relationships. The Figure 1.92 compares phylogenetic trees that show both older groupings and a more current version.

FIGURE 1.92 Phylogenetic trees showing the relation- ships of several of the bilateral inverte- brate phyla. (a) This tree illustrates our outdated understanding of the relation- ships between the bilateral phyla. (b) A more modern phylogenetic tree of these phyla indicates a close relationship be- tween annelids and mollusks as well as between arthropods and nematodes.

The process of ecdysis, described earlier in the Arthropods: Growth and Development (Advanced) concept, is one aspect of growth and development in arthropods that may reflect their close evolutionary relationship with the phylum Nematoda. Both arthropods and roundworms undergo ecdysis. Although the cuticle of roundworms and the cuticle/exoskeleton of arthropods are structurally quite different, both must be shed periodically in order for the animal to continue growing. In this classification, arthropods and nematodes are called ecdysozoa to reflect the shared characteristic of molting. Future molecular studies should reveal whether the more recent groupings of these phyla are sound.

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First Land Animals

The earliest animals found in the fossil record were all aquatic organisms. Most people think that the first animals to transition from aquatic life to life on land were the amphibians. This is probably because they provide such an obvious example of the water to land transition; they have the ability to both breath air and extract oxygen from water. However, it was actually the arthropods that were the first animals to adapt to life on land. This required arthropods to evolve structures that would both support their bodies outside of buoyant aquatic environments and efficiently retain water on dry land. The hard, water-impermeable exoskeleton was an important adaptation that served both of these purposes. The ability to obtain oxygen from air was also essential. The evolution of the tracheal system in insects helped alleviate the problem of obtaining oxygen.

Vocabulary

• ecdysis: The shedding of an outer integument or layer of skin, as seen in insects, crustaceans, and snakes; it is also known as molting.

• ecdysozoa: A group of protostome animals composed of arthropods, nematodes, and several other small phyla; this group includes all animals that shed their exoskeleton.

• paleontology: The study of fossils.

• trilobite: A small ocean invertebrate that became abundant during the Cambrian Period; it is the oldest known arthropod, which is now extinct and known only from numerous fossils.

Summary

• The study of arthropod evolution is an ongoing process, and theories are modified regularly as more informa- tion is obtained through molecular comparisons between species. • Arthropod fossils date back to the Cambrian period (over 500 million years ago). • The ancestral arthropod likely had an identical pair of appendages on each segment, and the segments were nearly identical, repeating units. • During arthropod evolution, one variation on the body plan that arose involved the loss or modification of many appendages to carry out different functions.

Practice

Use this resource to answer the questions that follow.

• Arthropods: Evolution and Diversity at http://www.cals.ncsu.edu/course/ent425/text02/arthropods.html .

1. How are onychophorans related to arthropods? 2. What major controversy remains regarding the evolutionary pathway of arthropods? 3. How does embryonic development differ between different arthropods?

Practice Answers

1. Previously, it was suggested that onychophorans were a possible evolutionary link between annelids and arthropods. Today, scientists believe that Onychophora and Arthropoda represent sister groups.

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2. Some scientists argue that arthropods diverged from a single primitive ancestor, while others argue that the features associated with arthropods evolved independently several times from different ancestors. 3. Insects and myriapods have a head that develops as its own region, whereas crustaceans and other arthropods have their head and thorax develop together as a single body region: the cephalothorax.

Review

1. How old are arthropod fossils? Are they preserved as well as mollusks? 2. What group of extinct arthropods has an extensive fossil record? What did scientists learn from this group of arthropods? 3. What do scientists believe the ancestral arthropod looked like? 4. What occurred during the evolution of arthropods that led to so much variation? 5. Which two small phyla of animals are very closely related to arthropods?

Review Answers

1. Arthropod fossils date back to the Cambrian period (over 500 million years ago), but arthropods are usually not as well preserved as mollusks. The shells of mollusks are usually tougher, so mollusks have a more complete fossil record. 2. The most extensive group of arthropod fossils is the trilobites, a subphylum of marine arthropods that is entirely extinct. Close examination of trilobite fossils has helped scientists learn about rates of speciation within the Cambrian period. 3. The ancestral arthropod likely had an identical pair of appendages on each segment, and the segments were nearly identical, repeating units. 4. During arthropod evolution, one variation on the body plan that arose involved the loss or modification of many appendages to carry out different functions. Another variation was the fusion or grouping of several segments to form distinct body regions. 5. There are two small phyla with species that exist today that are very closely related to arthropods by molecular analysis: the Onychophora and the Tardigrada.

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1.33 Arthropod Classification - Advanced

• Understand how the various arthropods are subdivided within the phylum. • Explore myriapods, chelicerates, and hexapods.

How would you classify this arthropod? This arthropod is a preying mantis. This mantis mimics flowers. Its coloration is an example of aggressive mimicry, a form of camouflage in which a predator’s colors and patterns lure prey. It has numerous common names such as the Small Devil’s Flower Mantis. Though a preying mantis may look scary, most of the 2,400 species are less than 6 inches (15 cm) long, and some are even kept as pets.

Overview of Arthropod Classification

Previously you learned that there are over one million arthropod species. How are the organisms within this diverse group related to each other? How are they subdivided into smaller groups based on shared characteristics and evolutionary history? As you will read, these questions are not always straightforward. Arthropods can be grouped into several subphyla, with each of these subphyla then divided into different classes. In this lesson we will examine how arthropods are divided into these subphyla, and then we will consider the details of one well-known subphylum: Crustacea. We will also take a look at one of the arthropod classes that contains many feared and reviled species: the arachnids. The sheer number of arthropod species (over one million and growing!) presents a substantial challenge for scientists trying to subdivide them into distinct groups based on their evolutionary relatedness. Just to give you an idea of some of the conflicts that exist in the field of arthropod systematics, some scientists have suggested (based on molecular data) that some species within the group Crustacea are actually more closely related to insects (in the group Hexapoda) than they are to other crustacean species. This is currently a controversial topic, and in this lesson we will consider the established, traditional groupings of arthropods. In this classification, arthropods are divided into five subphyla:

1. Trilobitomorpha (Trilobites).

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2. Chelicerata. 3. Crustacea. 4. Myriapoda. 5. Hexapoda.

The Subphyla and Classes Table lists the classes within each of these subphyla and shows an example of a typical member of each subphylum.

TABLE 1.11: Subphyla and Classes of Arthropods

Subphylum Class Examples Common members Trilobitomorpha Trilobites (extinct)

Extinct

Myriapoda

Chilopoda Centipedes

Diplopoda Millipedes

Pauropoda

Symphyla

Chelicerata

Arachnids Spiders

Xiphosura Scorpions

Pycnogonida Mites

Ticks

Horseshoe crabs

Sea spiders

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TABLE 1.11: (continued)

Subphylum Class Examples Common members Crustacea

Remipedia Lobsters

Cephalocarida Crabs

Branchiopoda Shrimp

Maxillopoda Barnacles

Malacostraca Krill

Hexapoda

Collembola Ants

Diplura Flies

Protura Grasshoppers

Insecta Beetles

Butterflies

Moths

Bees

Springtails

How are each of these groups related to each other? The Figure 1.93 shows a traditional phylogenetic tree illustrating the relationships between the arthropod subphyla. Recently, studies comparing the molecular make up of each of these groups have called this tree into question. One important outcome of these studies may be that myriapods are more closely related to Chelicerata, and crustaceans are more closely related to hexapods than previously thought. Further analysis of these findings will be necessary to say for certain. In the Arthropods: Evolution (Advanced) concept, it was mentioned that the trilobites are a group of extinct marine arthropods that left a rich and informative fossil record. As mentioned earlier, we will be looking very closely at crustaceans, arachnids, and insects in both later sections of this lesson and the next lesson. In the remainder of this section, let’s take a look at the myriapods, chelicerates (other than arachnids), and hexapods (other than insects).

Myriapods

The name myriapod means “many feet,” and this subphylum does indeed include centipedes, millipedes, and other “many feet” species. There is, however, a lot of variation in leg number in this subphylum, and even millipedes can range from having less than ten legs up to 750 legs. Most millipedes actually have between 80 and 400 legs. In total

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FIGURE 1.93 A phylogenetic tree showing the relation- ships between different arthropod sub- phyla.

there are approximately 13,000 species of myriapods that are divided into four classes:

• Chilopoda (centipedes). • Diplopoda (millipedes). • Pauropoda. • Symphyla.

Myriapods are all terrestrial animals and are nearly all herbivores (plant-eaters). The exceptions are the predatory centipedes. Members of this class are characterized by having a modified pair of appendages called forcipules on their heads that function as poison claws. These are shown in the Figure 1.94. Their prey ranges from other invertebrates to vertebrates, such as mice and frogs, and can vary widely depending on the size of the centipede species. As plant-eating organisms, the other three classes of myriapods play a critical role in many ecosystems by breaking down dead plant matter for use by other organisms.

FIGURE 1.94 A centipede head showing the modified appendages that function as poison claws called forcipules. This adaptation is crit- ical to the predatory lifestyle that distin- guishes centipedes from other classes of myriapods.

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The body of myriapods, as shown in the Figure 1.95, is divided into two major regions: the head and the trunk. The head possesses a single pair of antennae for sensing the environment and simple eyes.

FIGURE 1.95 The general body plan of a myriapod. Notice the similarities to many annelids: the segments are nearly identical, and each has a pair of appendages. Unlike annelids, the myriapod appendages are jointed.

There are also several sets of modified appendages found on the head that function as mouthparts. These include the mandible and the maxillae. The mandible and maxillae are features shared by myriapods, crustaceans, and hexapods. The mandible is a jaw-like structure that is used to grasp, bite, and even chew food. The maxillae are usually used for manipulating food in the mouth and swallowing. The Figure 1.96 shows the mandible of a crustacean. The myriapod mandible and maxillae are actually more similar to those of insects. You will learn more about the features of insect mouthparts in the next lesson on insects.

FIGURE 1.96 A close-up view of the mouth region of a crustacean. The various mouthparts that can be seen in this photograph include 1- the ventral groove, 2 –the labrum (upper lip), and 3 –the mandible. The mandible functions essentially like a jaw for chewing food.

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The trunk is made up of nearly-identical, repeating segments with a pair of appendages located on each one, as shown in the Figure 1.95. Millipedes are an exception to this, as they have two pairs of appendages on each segment. This occurs because each segment of a millipede is actually two segments fused together. The two smaller classes of Myriapoda, Pauropoda and Symphyla, have only a few hundred species each. The Symphyla and Pauropoda essentially look like small centipedes. They are usually only a few millimeters long (about the size of the letter “l” on this page), while centipedes and millipedes range from a few millimeters to 30 centimeters (slightly longer than a sheet of paper) in length. Myriapods reproduce by sexual reproduction. Like most arthropods, they exist as separate sexes, and fertilization is internal. However, sperm is not transferred directly from males to females by copulation. Instead, the male deposits sperm into a packet and leaves it on or near the female. Development in myriapods is defined as direct development. In the last lesson you learned that in direct development the immature organisms look like smaller versions of the adult, and there are no larval stages or metamorphosis. Millipedes were likely the very first animals to live on land. So far, the first land-dwelling organism to be found in the fossil record is a species of millipede called Pneumodesmus newmani that lived on Earth 428 million years ago. A fossil of this species is shown in the Figure 1.97. It would be 50 million more years before the first vertebrate would join these early land arthropods.

FIGURE 1.97 A fossilized region of the first known land animal, the myriapod species Pneumod- esmus newmani. This fossil was discov- ered in Scotland by an amateur paleon- tologist who earned a living by driving a bus.

Chelicerata

Chelicerata is a fairly large subphylum with about 70,000 species (mostly predatory) including spiders, scorpions, mites, and ticks. Their defining characteristic is a pair of specialized appendages called chelicerae. Chelicerae are pointed mouthparts used to grasp and immobilize prey. Some have a sharp fang connected to venom glands that allow the animal to poison its prey, and others are strong enough to pulverize the bodies of prey. This is important because chelicerates lack a mandible and maxillae for chewing food. A spider with prominent chelicerae is shown in the Figure 1.98. The chelicerate body shown in the Figure 1.99 is organized into two main regions: the cephalothorax and the abdomen. The cephalothorax is made up of eight segments, including the head region, and is often covered by a carapace. The abdomen is made up of 12 segments with no appendages and is followed by the tail, or telson.

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FIGURE 1.98 A spider with prominent blue-green che- licerae. The chelicerae are appendages modified as pointed mouthparts that are used to pierce and grasp food. They often release poisonous venom and can be used for defense.

FIGURE 1.99 The basic body plan of a chelicerate. Notice that all of the appendages are attached to the cephalothorax. The che- licerae are not shown in this drawing.

The head contains the eyes, mouth, chelicerae, and a pair of appendages behind the chelicerae called pedipalps. Pedipalps often have extensions that function as feeding appendages, but they can also be used for locomotion, sensing the environment, and reproduction. The posterior cephalothorax segments have four sets of appendages modified as walking legs. The Figure 1.100 shows the head region of a horseshoe crab with the chelicerae, pedipalps, and walking legs labeled. Unlike other arthropods, chelicerates do not possess antennae.

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FIGURE 1.100 The appendages located on the cephalothorax of a member of the Chelicerata subphylum, the horseshoe crab. Ch = chelicerae, PdP = pedipalps, Pd = walking legs.

There are three existing classes within the phylum Chelicerata:

• Arachnida - spiders, scorpions, mites, ticks. • Xiphosura - horseshoe crabs (marine). • Pycnogonida - sea spiders.

The arachnids include the vast majority of species within the Chelicerata, and you will learn about them in a later section of this lesson. The Xiphosura consist of the horseshoe crabs, like the one shown above in the Figure 1.100. They are marine animals that are actually not crabs at all (if they were, they would be classified in the subphylum Crustacea). Probably the most fascinating thing about horseshoe crabs is that they are considered living fossils. This term means that they look almost exactly the same as species that only exist as fossils, and there are very few living species that are closely related to them. Living fossils may represent a small group of animals that survived a major extinction event and then did not continue to diversify after this event. While the horseshoe crab has all of the features characteristic of Chelicerata (chelicerae, no antennae or mandible), they do not look much like other modern arachnids. Instead, they look almost exactly the same as fossils of horseshoe crab species that existed over 400 million years ago. Members of the class Picnogonida, or sea spiders, are also marine animals. They are unusual for arthropods in that they do not have a respiratory system. They exchange gases by diffusion through the body wall. A sea spider is shown in the Figure 1.101.

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FIGURE 1.101 A sea spider of the class Picnogonida in the subphylum Chelicerata. These are some of the few marine species of Che- licerata that are still in existence today.

Hexapoda

The subphylum Hexapoda is enormous, with approximately one million species alive today. Hexapods are char- acterized as having six legs or six appendages that are dedicated to walking. Almost all of these species, with the exception of around 1,000, are members of the class Insecta, which will be discussed in the next lesson. The other three classes of hexapoda are Collembola, Protura, and Diplura. They are all small classes of soil-dwelling animals on the order of a few millimeters in size. Protura and Diplura generally do not have eyes or antennae. The class Collembola includes the springtails. Springtails are a small group of hexapods in terms of species number (about 6,000), but they are incredibly large in numbers of individuals. So what are they, and have you seen them? Springtails, like the one shown in the Figure 1.102, generally live in soil and leaf litter where they are important contributors to the breakdown of plant matter and the generation of soil. You have probably encountered them but may not have noticed them due to their small size (a few millimeters). They are more widely distributed across the planet than any other group of hexapods. In some regions there may be as many as 100,000 individuals in one cubic meter of topsoil.

Vocabulary

• antennae: A pair of long, thin sensory appendages on the heads of insects, crustaceans, some annelids, and some other arthropods.

• cephalothorax: The fused head and thorax of spiders and other chelicerate arthropods; it is usually covered by a carapace.

• chelicerae: Either of the first pair of fanglike appendages near the mouth of an arachnid, such as a spider, often modified for grasping and piercing.

• forcipules: A characteristic of centipedes; they are modifications of the first pair of legs, forming a pincer-like appendage that is always found just behind the head.

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FIGURE 1.102 A species of springtail. These tiny ani- mals are found in enormous numbers all over the planet.

• herbivores: Organisms that consume only producers such as plants or algae; they form a necessary link between producers and other consumers.

• living fossil: A living species (or clade) of organism which appears to be the same as a species only known from fossils; they have no close living relatives.

• mandible: A jaw-like structure that is used to grasp, bite, and even chew food.

• maxillae: One of the paired appendages immediately behind the mandibles of arthropods, which is used for manipulating food in the mouth and swallowing.

• pedipalps: A pair of appendages near the mouth of a spider or other arachnid that is modified for various reproductive, predatory, locomotory, or sensory functions.

Summary

• Arthropods can be grouped into several subphyla, with each of these subphyla then divided into different classes. • Arthropods are traditionally divided into 5 subphyla: Trilobitomorpha (Trilobites), Chelicerata, Crustacea, Myriapoda, and Hexapoda. • Myriapoda is divided into four classes: Chilopoda (centipedes), Diplopoda (millipedes), Pauropoda, and Symphyla. • Millipedes were likely the very first animals to live on land. • Chelicerata is a fairly large subphylum, with about 70,000 species (mostly predatory) including spiders, scorpions, mites, and ticks. • The chelicerate body is organized into two main regions: the cephalothorax and the abdomen. • There are three existing classes within the phylum Chelicerata: Arachnida (spiders, scorpions, mites, ticks), Xiphosura (horseshoe crabs), and Pycnogonida (sea spiders). • The subphylum Hexapoda is enormous, with approximately one million species alive today. Hexapods are characterized as having six legs or six appendages that are dedicated to walking.

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Practice

Use this resource to answer the questions that follow.

• What is an arthropod? at http://evolution.berkeley.edu/evolibrary/article/_0/arthropods_01 .

1. There are five types of modern arthropods. List them. 2. List the five arthropod characteristics. 3. Which of the following animals have bilateral symmetry: scorpion, moth, onychophoran, mouse, or millipede? 4. Which of the following animals have a segmented body plan: scorpion, moth, onychophoran, mouse, or millipede? 5. Which of the following animals have a hard exoskeleton: scorpion, moth, onychophoran, mouse, or millipede? 6. Which of the following animals have jointed appendages: scorpion, moth, onychophoran, mouse, or milli- pede? 7. Which of the following animals have many pairs of limbs: scorpion, moth, onychophoran, mouse, or milli- pede? 8. Which of the following animals are arthropods: scorpion, moth, onychophoran, mouse, or millipede?

Practice Answers

1. Modern arthropods include insects, spiders, centipedes, shrimp, and crayfish. 2. Bilateral symmetry, segmented body, hard exoskeleton, jointed legs, and many pairs of limbs. 3. All five have bilateral symmetry. 4. The scorpion, moth, and millipede all have segmented bodies. 5. The scorpion, moth, and millipede all have hard exoskeletons. 6. The scorpion, moth, mouse, and millipede have jointed appendages. 7. The scorpion, moth, onychophoran, and millipede all have many pairs of limbs. 8. The scorpion, moth, and millipede are arthropods.

Review

1. The arthropods comprise a very large phylum. How is it traditionally divided? 2. How has recent scientific research changed our conception of the relationships between the different subphyla of arthropods? 3. What kind of environments do myriapods inhabit? What niches do they fill in their environment? 4. How do myriapods reproduce? 5. Which arthropods were likely to be the first animals to live on land? 6. How is the chelicerate body divided?

Review Answers

1. Arthropods are traditionally divided into 5 subphyla: Trilobitomorpha (Trilobites), Chelicerata, Crustacea, Myriapoda, and Hexapoda. These 5 subphyla are further divided into classes. 2. Recent scientific studies suggest that myriapods are more closely related to Chelicerata, and crustaceans are more closely related to hexapods than previously thought. It was previously postulated that myriapods are the most closely related to hexapods (insects). 3. Myriapods are all terrestrial animals and are nearly all herbivores. The exceptions are the predatory centipedes. 4. Myriapods reproduce through sexual reproduction. Sperm is not transferred directly from males to females by copulation. Instead, the male deposits sperm into a packet and leaves it on or near the female.

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5. Millipedes were likely the very first animals to live on land. So far, the first land-dwelling organism to be found in the fossil record is a species of millipede called Pneumodesmus newmani that lived on Earth 428 million years ago. 6. The chelicerate body is organized into two main regions: the cephalothorax and the abdomen.

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1.34 Crustaceans - Advanced

• Understand the different classes that comprise crustaceans. • Learn about the importance of crustaceans in the ecosystem.

What do crabs, lobsters, crayfish, shrimp, krill, and barnacles have in common? These are all crustaceans, a very large crop of arthropods with about 67,000 described species. They are distin- guished, in part, by their biramous (two-parted) limbs.

Crustaceans

There are roughly 40,000 species of crustaceans, and they are the predominant arthropods of the ocean. They include lobsters, shrimp, crabs, and barnacles. There are also some freshwater species, such as crayfish, and even a few terrestrial species like sand fleas. Like myriapods and hexapods, crustaceans have modified feeding appendages called mandibles and maxillae. So what makes a crustacean different from other arthropods? The defining features of crustaceans are two pairs of antennae located in front of the mouth and a unique larval stage that is common to all members of this subphylum. These will be discussed in more detail later in this section.

Structure and Function

Crustaceans have three basic body regions: the head, thorax, and abdomen (although in many species the head and thorax have fused to form a cephalothorax). The head region has two pairs of antennae and three pairs of feeding appendages: a pair of mandibles and two pairs of maxillae. The crustacean head also contains two compound eyes. These eyes may or may not be located at the end of stalk-like projections on the head. The appendages of most crustaceans are branched or biramous, with the exception of the first pair of antennae. The exoskeleton often includes a carapace, or a shield-like plate that covers the dorsal side of the cephalothorax region. This feature is shared with chelicerates. A diagram of the overall body plan of a typical crustacean, highlighting the carapace, is shown in the Figure 1.103.

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FIGURE 1.103 The external anatomy of a crustacean with the carapace highlighted in orange. In this example there are two main body regions, the cephalothorax and the ab- domen. You can see from this di- agram that, unlike chelicerates, crus- taceans have appendages on the seg- ments of their abdomens. You can also see the branching at the end of some of the biramous appendages.

The internal organ systems of crustaceans are much like those of most arthropods. They have an open circulatory system and a centralized nervous system. Coxal glands carry out the process of excretion in crustaceans. Crustaceans respire by diffusion through the surface of their bodies (small species), with gills (most aquatic species), or, rarely, with very primitive lungs. An example of a terrestrial crustacean that respires using lungs is the coconut crab shown in the Figure 1.104. In the next section we will examine different modifications of these basic crustacean features that are found in some of the classes within the crustacean subphylum.

Classification

Most living species of crustaceans fall into one of the following five classes:

• Remipedia. • Cephalocarida. • Branchiopoda. • Maxillopoda (copepods, barnacles). • Malacostraca (amphipods, decapods, krill).

The differences between species of each class are primarily seen in their body forms; for example, the number of segments, the presence or absence of a carapace, and the shape and function of their appendages all vary between the classes. Remipedia, Cephalocarida, and Branchiopoda are all small classes, with roughly 12, 9, and 900 species respectively. The largest classes are Maxillopoda, with over 10,000 species, and Malacostraca, with over 20,000 species. In the next section we will consider some of the details of these two classes.

Maxillopoda and Malacostraca

A typical maxillopod has a head made up of five segments: a thorax consisting of six segments and an abdomen with four segments. There are no appendages on the abdomen. Copepods and barnacles make up the majority of the species in Maxillopoda. Copepods are tiny (most are about the size of a spec of dust) organisms found in all aquatic environments that are distinguished by having a single, simple eye. Copepods are abundant in number and are enormously important members of the food chain. This is due to the fact that they consume large amounts of organisms that occupy the base of the aquatic food chain, such as phytoplankton (tiny, drifting plants), and in turn

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FIGURE 1.104 A coconut crab is a terrestrial member of the class Crustacea that respires using primitive lungs.

are a major source of food for organisms further up in the food chain. Copepods include filter feeders that consume other plankton, predators that feed on small prey such as insect larvae, and many symbiotic and parasitic species. The word copepod means “paddle foot,” and the appendages of copepods are modified for efficient swimming. The Figure 1.105 shows a copepod. Barnacles have a very different body plan and life style from most arthropods. On the surface, they look much more like mollusks than arthropods, as shown in the Figure 1.106. However, they go through a specific larval stage during their development that is a defining characteristic of crustaceans. You will learn more about this stage in the next section. Barnacles attach themselves to a solid substrate and live a sessile existence. Unlike most arthropods, they are usually hermaphrodites, meaning that an individual can generate both sperm and eggs. Malacostraca includes the species that most people think of when they hear the word crustacean. They include decapods (having 10 feet) such as lobsters, crabs and shrimp, amphipods, isopods, and krill. Species within these different orders vary primarily on the number and structure of their appendages. Isopods are the most diverse group, and they include a number of terrestrial species. Most Malacostraca have three body regions: a head of five segments fused to a thorax of eight segments and an abdomen of six segments. The overall body plan of a Malacostraca is

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FIGURE 1.105 A species of copepod within the class Maxillopoda of the subphylum Crustacea. Notice that there are no appendages on the abdomen. These tiny animals play a major role near the base of the aquatic food chain.

FIGURE 1.106 A mass of barnacles surrounding a group of mussels. The barnacles are lightly colored, and the mussels are dark. Notice that the barnacles look like they might be more closely related to the mussels (mol- lusks) in this photograph than to spiders (arthropods), but the opposite is true.

shown in the Figure below.

Development and Ecology

Crustaceans reproduce sexually, and, in most species, male and females are separate sexes. Unlike the myriapods, development in crustaceans is considered indirect. That means that there are intermediate juvenile stages called larval stages that are very different in form from the adult. One of the unifying features of all crustaceans is that

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FIGURE 1.107 The external anatomy of a typical mem- ber of the crustacean class Malacos- traca. Pereiopods function as walking legs, while pleopods are swimming legs. The uropods act as tail fins along with the telson. The rostrum is a ledge region at the front of the carapace that extends over the head to protect it. Notice the stalked eye tucked in under the rostrum. Another interesting feature that can be seen in this diagram is that the first antennae are biramous. This is distinct from most crustaceans, in which the first antennae are uniramous.

they all develop through a larval stage called nauplius (plural, nauplii) that is unique to this group. A nauplius larva is shown in the Figure 1.108. The defining feature of nauplii is that they use appendages attached to the head as swimming appendages. These appendages change considerably as they develop into the antennae of the adult crustacean. This is characteristic of indirect development; the transition from larvae to adults usually involves a fairly drastic change in form called metamorphosis. You will see in the Insect concepts that indirect development is also common for the insects of Hexapoda although they do not go through a nauplius larval stage.

FIGURE 1.108 A nauplius larva. This larval stage is unique to the subphylum Crustacea.

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Most crustaceans are free-living, but there are some sessile (unable to move from place to place) species such as the barnacles mentioned earlier. Crustaceans obtain food by predation, filter feeding, scavenging, or through parasitism. Members of the class Malacostraca are a major source of food for humans and are also an important part of the economy. Many crustaceans, such as copepods and krill, are extremely important components of the marine food chain. Krill, like the one shown in the Figure 1.109, are found in all oceans of the world where they constitute a large portion of the overall biomass. One particular species, the Antarctic Krill (Euphausia superba), compose a biomass of over 500 million tons, roughly twice that of humans. Like copepods, they are considered key species near the bottom of the food chain because they feed on phytoplankton and other tiny, drifting aquatic organisms. Other organisms higher up in the food chain, such as whales, seals, penguins, and squid, rely on krill as a major food source and indirectly obtain the nutrients originally generated by phytoplankton through eating krill.

FIGURE 1.109 A species of krill. A common theme in studying arthropods is their massive numbers –either number of species or number of individuals within a species. This holds true for krill as well. If all the individuals of all the krill species were gathered together, their mass would be well over twice the mass of all of the humans on Earth. It is sometimes difficult to fathom the vastness of the ocean and the importance of the many tiny species that inhabit the seas.

Vocabulary

• biramous: An appendage that divides to form two branches, found mainly in arthropods.

• carapace: A hard, bony or chitinous outer covering such as the fused dorsal plates of a turtle or the portion of the exoskeleton covering the head and thorax of a crustacean.

• compound eyes: An eye consisting of an array of numerous small photoreceptor units that each form a portion of an image, as found in insects and crustaceans.

• mandible: A jaw-like structure that is used to grasp, bite, and even chew food.

• maxillae: One of the paired appendages immediately behind the mandibles of arthropods that are used for manipulating food in the mouth and swallowing.

• nauplius: The first larval stage of many crustaceans, which has an unsegmented body and a single eye.

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Summary

• There are roughly 40,000 species of crustaceans, and they are the predominant arthropods of the ocean. • Crustaceans have three basic body regions: the head, thorax, and abdomen (although in many species the head and thorax have fused to form a cephalothorax). • The differences between species of each class are primarily seen in their body forms; for example, the number of segments, the presence or absence of a carapace, and the shape and function of their appendages all vary between the classes. • Malacostraca includes the species that most people think of when they hear the word crustacean. They include decapods (having 10 feet) such as lobsters, crabs and shrimp, amphipods, isopods, and krill. • One of the unifying features of all crustaceans is that they all develop through a larval stage called nauplius (plural, nauplii) that is unique to this group. • The defining feature of nauplii is that they use appendages attached to the head as swimming appendages. These appendages change considerably as they develop into the antennae of the adult crustacean.

Practice

Use this resource to answer the questions that follow.

• What is a Crustacean? at http://eol.org/info/444 .

1. What characteristics of crustaceans differentiate them from other arthropods? 2. Where can crustaceans be found? 3. What distinguishes krill from decapods? 4. How have ostracods contributed to the study of climate change?

Practice Answers

1. Crustaceans have several characteristics that distinguish them from other arthropods. Crustaceans usually have biramous appendages, a nauplius larval stage, a naupliar eye that differs from compound eyes, a labrum structure anterior to the mouth, a five-segmented head, and a naupliar arthrite that is lost when crustaceans mature. 2. Crustaceans are primarily found in marine habitats, although there are freshwater species as well as terrestrial species such as the pillbug. 3. Krill resemble decapods, but their appendages are different, they lack maxillipeds, and their gills are outside the carapace. 4. Ostracods have calcified carapaces that are easily fossilized. Paleontologists study these fossils to understand our planet’s ecological past and how it changed through time.

Review

1. What are the three major body regions of a crustacean? 2. Which two classes make up the majority of crustaceans? 3. What class do copepods belong to, and what is their importance to the ecosystem? 4. What is an unifying characteristic of all crustaceans? 5. Discuss the importance of krill.

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Review Answers

1. The three major body regions are the head, thorax, and abdomen (although some crustaceans have their head and thorax fused into a cephalothorax). 2. The largest classes of crustaceans are Maxillopoda, with over 10,000 species, and Malacostraca, with over 20,000 species. 3. Copepods are members of the class Maxillopoda. They are very important to the ecosystem because they consume large amounts of organisms that occupy the base of the aquatic food chain, such as phytoplankton (tiny, drifting plants), and in turn are a major source of food for organisms further up in the food chain. 4. All crustaceans go through a larval stage known as a nauplius. The defining feature of nauplii is that they use appendages attached to the head as swimming appendages. These appendages change considerably as they develop into the antennae of the adult crustacean. 5. Organisms higher up in the food chain, such as whales, seals, penguins, and squid, rely on krill as a major food source and indirectly obtain the nutrients originally generated by phytoplankton through eating krill.

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1.35 Arachnids - Advanced

• Differentiate between different arachnids, and focus on the largest group: spiders. • Discuss the impact arachnids have on humans.

Is this really a spider? Yes, it is. The Mediterranean Jumping Spider has four bright eyes and distinct coloration differences between the sexes. This would be a male. When you have attracted their attention, they follow you with their big eyes. Their life in the warm sun has produced a variety and coloration not matched by any other spider.

Arachnids

Arachnids include species, such as spiders and scorpions, that instill fear and sometimes horror in many people. This is partly due to their portrayal as human predators in movies and books. In real life, spiders are hardly a threat to humans, and it is actually another type of arachnid that poses a greater threat: ticks. Ticks are parasites that can act

184 www.ck12.org Chapter 1. Invertebrates - Advanced as vectors to carry and transmit diseases between their prey. The consequences of this process on human health will be discussed in a section below. There are over 70,000 described species of arachnids, and more than 40,000 of them are spiders. The rest include scorpions, ticks, and mites. Examples of each of these groups are shown in the Figure 1.110. Almost all arachnids are terrestrial, carnivorous animals.

FIGURE 1.110 Examples of the different arachnid groups. (a) A spider, (b) a scorpion, (c) a mite, and (d) a tick.

Structure and Function

Spiders are usually thought of as 8-legged organisms. It is true that they have four pairs of appendages modified to function as walking legs, but in all they have six pairs of appendages. The other two pairs include the chelicerae and the pedipalps that were discussed earlier in the Arthropods: Classification (Advanced) concept. Like all chelicerates, arachnids do not have appendages modified for biting and chewing such as the mandible and the maxillae. When attacking prey, they use their chelicerae to pierce and pulverize the organism. Many spider chelicerae have venom glands that allow them to poison prey during the attack. In species that do not have chelicerae strong enough to masticate their prey, they usually secrete digestive enzymes to carry out external digestion. Once the prey has been sufficiently liquefied, the spider can then feed by sucking the liquid into its mouth. As mentioned above, pedipalps may be modified for use in feeding, locomotion, or defense depending on the species. In many spider species, the pedipalps of the males are modified at the ends to function in transferring sperm to the females. Another feature of the pedipalps of many species is the presence of sensory organs. These are important

185 1.35. Arachnids - Advanced www.ck12.org adaptations because, unlike other arthropods, arachnids (and other chelicerates) generally do not have antennae. Scorpions, mites, and ticks may have varying numbers of appendages. The body plan of most arachnids is made up of a cephalothorax, which forms a base for all of the appendages, and an abdomen. Ticks and mites differ because the cephalothorax and abdomen are not distinct regions, and they form one main body region. A distinct feature of spiders is the presence of glands called spinnerets located on the underside of the rear end of the abdomen. Spinnerets are used to secrete silk, which can be used for a number of purposes. These include spinning webs to trap prey, forming cocoons for protection, and forming a dragline to hang from when descending from a high object. In the Figure 1.111 you can see silk coming from the spinnerets of a spider in the process of spinning a web.

FIGURE 1.111 A spider in the process of spinning its web. Notice the spinnerets secreting strands of silk at the very end of this spider’s abdominal region.

Spider webs come in a variety of shapes and sizes depending on the species, including funnels, flat horizontal sheets, and tangled mazes. The spider webs that most people are familiar with, the orb webs like the one shown in the Figure 1.112, are just one of these many variations. Most of the internal organ systems of arachnids are similar to those of most other arthropods, including an open circulatory system, a centralized nervous system, and a complete digestive tract. However, arachnids are unique among arthropods in using book lungs for respiration. Book lungs are made up of stacked folds of tissue with air pockets in between the folds. They are usually found in spiders and scorpions. Mites and ticks respire either through a tracheal system (branched tubes that bring in air and distribute it throughout the organism) or by diffusion through their skin. The excretory systems in arachnid species consist of Malphigian tubules, coxal glands, or both in some species. The type and number of eyes vary greatly between arachnid groups. Spiders, for example, generally have eight eyes.

Arachnids and Human Health

Although there are some species of spiders that can cause harm to humans with a poisonous sting, and there are several human deaths each year attributed to one species of scorpion, the threat posed by spiders and scorpions is really quite small. Overall, ticks have a much greater impact on human health, primarily as disease-carrying organisms. Ticks can deliver a number of human pathogens in a single bite. These are some of the diseases transmitted through tick bites:

• Lyme disease.

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FIGURE 1.112 A familiar orb web spun by an orb weaver spider species. This web design is just one of several designs used by different species of spider.

• Human granulocytic and monocytic ehrlichiosis. • Babesiosis. • Rocky Mountain spotted fever. • Colorado tick fever. • Tularemia. • Boutonneuse fever. • Tick-borne encephalitis.

Lyme disease, Rocky Mountain spotted fever, boutonneuse fever, human granulocytic and monocytic ehrlichiosis, and tularemia are each caused by different species of bacteria, and the symptoms are often flu-like with a rash or, in the case of tularemia, inflammed lymph nodes. Colorado tick fever and tick-borne encephalitis are caused by viruses, and the symptoms of Colorado tick fever are also flu-like and may include a rash. Tick-borne encephalitis begins with flu-like symptoms that progress to neurological abnormalities such as dizziness and paralysis. Babesiosis is caused by a protozoan parasite and manifests as a fever and anemia. These diseases can be very serious if left untreated, however, most of them are generally highly treatable, particularly if caught early. It is important to remember, however, that tick bites are fairly common, and most bites do not lead to these infections. What should you do if you find a tick on your body? Use forceps to grasp the tick as close to the skin as possible. Try not to squeeze the tick so that fluids are not forced into your skin from the tick’s body. Pull it out slowly but firmly. Disinfect the bite area with alcohol. If possible, you should wear gloves for this procedure. You should then call a doctor to find out the risk of infection from ticks in your area and to find out if antibiotics should be used. The Figure 1.113 shows a rash that commonly develops following a bite from a tick carrying the bacteria that cause Lyme disease.

Vocabulary

• book lungs: Respiratory organs found in some arachnids, such as scorpions and spiders, that consist of several parallel, membranous folds arranged like the pages in a book.

• chelicerae: Either of the first pair of fang-like appendages near the mouth of an arachnid, such as a spider, that is often modified for grasping and piercing.

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FIGURE 1.113 The characteristic bulls-eye rash that of- ten occurs following a bite with a tick car- rying the Lyme disease-causing bacteria, Borrelia burgdorferi. The rash represents a skin infection caused by the bacteria, and it can appear anywhere from 1 day to 1 month following a bite from a tick carrier. Diseases such as Lyme disease that are transmitted by arachnids are a much greater threat to human health than the arachnid bites themselves.

• Malphigian tubules: Excretory organs in insects and many other arthropods; they are narrow tubules opening into the anterior part of the hindgut.

• mandible: A jaw-like structure that is used to grasp, bite, and even chew food.

• maxillae: One of the paired appendages immediately behind the mandibles of arthropods, which are used for manipulating food in the mouth and swallowing.

• pedipalps: A pair of appendages near the mouth of a spider or other arachnid that is modified for various reproductive, predatory, locomotory, or sensory functions.

• spinnerets: Any of the various tubular structures from which spiders and certain insect larvae, such as silkworms, secrete the silk threads from which they form webs or cocoons.

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Summary

• There are over 70,000 described species of arachnids, and more than 40,000 of them are spiders. The rest include scorpions, ticks, and mites. • Almost all arachnids are terrestrial, carnivorous animals. • Arachnids are unique among arthropods in using book lungs for respiration. • Although there are some species of spiders that can cause harm to humans with a poisonous sting, and there are several human deaths each year attributed to one species of scorpion, the threat posed by spiders and scorpions is really quite small. Overall, ticks have a much greater impact on human health, primarily as disease-carrying organisms.

Practice

Use this resource to answer the questions that follow.

• Spiders: Prey Capture and Feeding at http://www.australianmuseum.net.au/Prey-capture-and-feeding .

1. How do knockdown webs help spider capture prey? 2. Describe how orb webs have adapted to the defenses of prey such as moths. 3. How has the Phrynarachne decipiens species adapted unique characteristics that allow it to hunt down prey without the use of a web? 4. How does spider venom immobilize prey?

Practice Answers

1. Knockdown webs have a maze of web lines above a silk sheet. When prey hits the web lines, they become disorientated and fall down into the silk sheet, where they are captured and then eaten. 2. Moths are covered in hairs that are easily shed, allowing them to escape some webs. Spiders have adapted by modifying their orb webs into long ladder-like structures that capture moths even after the moths have shed all their hair. 3. Spiders of the species Phrynarachne decipiens actually look and smell like bird dung. This attracts insects, which the spider ambushes. 4. Spider venom affects the nervous systems of arthropod prey and interferes with nerve-muscle impulse trans- mission.

Review

1. How many appendages do spiders have? 2. What mouth parts do arachnids lack? 3. How do some spiders deal with prey that they cannot physically tear apart using their chelicerae? 4. What are some common functions that the pedipalps serve in spiders? 5. What are some diseases that ticks are likely to transmit to humans?

Review Answers

1. Spiders have four pairs of appendages modified to function as walking legs, but they have six pairs of appendages in total. The other two pairs include the chelicerae and the pedipalps. 2. Like all chelicerates, arachnids do not have appendages modified for biting and chewing such as mandibles and maxillae. When attacking prey, they use their chelicerae to pierce and pulverize the organism.

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3. In species that do not have chelicerae strong enough to masticate their prey, they usually secrete digestive enzymes to carry out external digestion. 4. In many spider species, the pedipalps of the males are modified at the ends to function in transferring sperm to the females. Another feature of the pedipalps in many species is the presence of sensory organs, since arachnids do not have antennae. 5. Ticks are responsible for diseases such as Lyme disease, human granulocytic and monocytic ehrlichiosis, babesiosis, Rocky Mountain spotted fever, Colorado tick fever, tularemia, boutonneuse fever, and tick-borne encephalitis.

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1.36 Insects - Advanced

• Describe the characteristics common to all insects. • Understand how insects are classified into different groups.

What dominates life on Earth? Well, by numbers, it’s not humans. This may look like a scary creature from your worst nightmare, but it wouldn’t hurt a fly. In fact, it is a fly! This picture shows the charming portrait of a horsefly, up close and personal. Those big, striped, colorful orbs are its eyes. Did you ever look through a kaleidoscope? If so, then you have an idea of what the world looks like to a horsefly. What other organs do insects, like this horsefly, have? Besides sensing their environment, what other functions do their organs serve?

Characteristics and Classification of Insects

It is humbling to realize that it is insects, not humans, that dominate life on this planet. More than half of all known living organisms are insects. There are over one million described species, and it is estimated that there may be over ten million species of insects that have yet to be identified or described. There are many thousands of species of the common insects that are most familiar to us. For example, there are roughly 20,000 grasshopper species, 170,000 butterfly and moth species, 120,000 species of flies, 360,000 beetle species, and 110,000 species of bees, wasps, and ants. Insects make up a class within the subphylum Hexapoda, and they include most hexapod species. They range in size from less than a millimeter to about 55 centimeters (approximately the length of your arm). An example of a large insect, the walking stick, is shown in the Figure 1.114. Although insects can be found in most habitats on Earth,

191 1.36. Insects - Advanced www.ck12.org the majority of species are either terrestrial or aerial. The study of insects is called entomology. Like all arthropods, insects have segmented bodies, jointed appendages, and an exoskeleton made mostly of chitin. As members of the subphylum Hexapoda, insects have six legs. They also have additional appendages that are modified for other functions, and we will look at these in more detail in the Insects: Structure and Function (Advanced) concept.

FIGURE 1.114 Walking stick insects are among the longest insects on Earth.

Body Plan

The body of an insect is divided into three regions: the head, the thorax, and the abdomen. The Figure 1.115 shows the external anatomy of a typical insect. The head houses three sets of appendages modified for feeding (mouthparts), one set of sensory antennae, a pair of compound eyes, and several simple eyes called ocelli. The six legs are attached to the thorax, and the abdomen contains most of the internal organ systems. If there are wings present, they are also attached to the thorax. The abdomen is made up of eleven segments, and these may be reduced or fused in some species. See also http://www.earthlife.net/insects/anat-abdomen.html .

Classification

Like the phylum Arthropoda in general, the classification of insects is in a state of flux. Classification of insects is also particularly complex because of the vast number of species. The Figure 1.116 shows the subdivisions of the

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FIGURE 1.115 The body plan of a typical insect, showing the division of the body into three major regions: head, thorax, and abdomen. Ap- pendages modified as legs all protrude from the thorax as do the wings. The head includes mouthparts and sensory organs.

class Insecta. You will notice that there are more subdivisions than simply class and order, as is the case with the subphyla in Arthropoda. These extra layers of phylogenetic classification are due to the large diversity of species within this class.

FIGURE 1.116 The classification of insects. Common members of each of these groups are in parenthesis: Apterygota (bristletails), Paleoptera (dragonflies, mayflies, and damselflies), Exopterygota (grasshop- pers, termites, cockroaches, and lice), Endopterygota (ants, bees, flies, beetles, fleas, butterflies, and moths).

Currently, there are two major subclasses within the insect class: Apterygota and Pterygota. Pterygota includes winged insects and insects that have lost their wings during the course of evolution but arose from a winged ancestor (described as secondarily wingless). Almost all insect species are found in the subclass Pterygota. Apterygota is a small subclass that includes species, such as bristletails, that undergo very little change during development aside from a size increase and sexual maturation. The subclass Pterygota is further subdivided into two infraclasses: Palaeoptera and Neoptera. This division is based on the complex folding pattern of the wings of insects in the Neoptera infraclass. The wings of insects in the Palaeoptera infraclass do not fold back onto the abdomen. These include mayflies, dragonflies, and damselflies. Insects within the infraclass Neoptera are further subdivided into two superorders: Exopterygota and Endopterygota. The difference between these two groups is related to how they change during development. Species within

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Exopterygota undergo moderate changes during their development from young, immature organisms to adults, whereas Endopterygota species exhibit rather drastic changes during development. This process of developmental transformation, called metamorphosis, is a prominent feature in the insect class, and you will learn more about the details of insect metamorphosis in a later concept. Exopterygota members include grasshoppers, termites, cock- roaches, and lice. Species within the Endopterygota superorder include ants, bees, beetles, fleas, flies, butterflies, and moths. Members of the subphylum Chelicerata, such as spiders and ticks, are often confused with non-flying insects. However, as you learned in the last lesson, chelicerates have four pairs of appendages that are modified to function as legs, while insects have only three pairs. Also, most insects have three distinct body regions, while chelicerates generally have only two.

Vocabulary

• antennae: A pair of long, thin sensory appendages on the heads of insects, crustaceans, some annelids, and some other arthropods.

• compound eyes: An eye consisting of an array of numerous small photoreceptor units that each form a portion of an image, as found in insects and crustaceans.

• entomology: The branch of zoology concerned with the study of insects.

• metamorphosis: The process of transformation from an immature form to an adult form in distinct stages.

• ocelli: Simple light sensing tissues.

Summary

• More than half of all known living organisms are insects. • As members of the subphylum Hexapoda, insects have six legs. • The body of an insect is divided into three regions: the head, the thorax, and the abdomen. • Like the phylum Arthropoda in general, the classification of insects is in a state of flux. • Currently, there are two major subclasses within the insect class: Apterygota and Pterygota. • The subclass Pterygota is further subdivided into two infraclasses: Palaeoptera and Neoptera. • Insects within the infraclass Neoptera are further subdivided into two superorders: Exopterygota and En- dopterygota.

Practice

Use this resource to answer the questions that follow.

• Insect at http://www.britannica.com/EBchecked/topic/289001/insect .

1. How have insects, specifically the fruit fly, contributed to biology? 2. Why is it so difficult for scientists to estimate the population of insects in large areas? 3. What kind of habitats do insects inhabit? 4. What important roles do insects play in nature?

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Practice Answers

1. The fruit fly has contributed greatly to our understanding of genetics. Investigations into population biology, hormonal action, organ function, and other processes have also been carried out on insects. 2. The number of insects becomes hard to calculate beyond a few square miles because they are so numerous. Their small size, high rates of reproduction, and wide variety of food all contribute to their large populations. 3. Insects are found in almost all habitats, ranging from deserts to jungles, glacial fields, cold mountain streams, and hot springs. They are mostly found in freshwater or terrestrial environments. Some have even adapted to living in petroleum. 4. Insects aid in the decomposition of organic material into soil. Many plants also rely on insects for pollination.

Review

1. Where do appendages that function as legs attach to the body of an insect? How many legs do insects have? 2. How many sections make up the typical insect body? How does this differ from arachnids? 3. Which group of insects is not included in the large subclass Pterygota? 4. Which infraclass are dragonflies classified under? How do they differ from the other infraclass? 5. Which superorder contains insects that undergo metamorphosis?

Review Answers

1. As members of the subphylum Hexapoda, insects have six legs. The six legs are attached to the thorax, and the abdomen contains most of the internal organ systems. If there are wings present, they are also attached to the thorax. 2. The body of an inset is divided into the head, thorax, and abdomen. Arachnids, on the other hand, often have a cephalothorax and an abdomen. 3. Apterygota is a small subclass that includes species, such as bristletails, that undergo very little change during development aside from a size increase and sexual maturation. The Apterygota descended from an ancestor that did not have wings, whereas the Pterygota all descended from ancestors that had wings. 4. Dragonflies fall under the Palaeoptera infraclass, which have wings that do not fold back onto the abdomen. The Neoptera infraclass has species that exhibit a complex wing folding pattern. 5. Species within Exopterygota undergo moderate changes during their development from young, immature organisms to adults, whereas Endopterygota species exhibit rather drastic changes during development (meta- morphosis).

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1.37 Insect Structure and Function - Advanced

• Describe the structures common to all insects. • Understand how different insects have evolved different mouthparts.

Is the structure-function relationship obvious? Yes, especially in this close-up of a grasshopper. The structure of this insect is apparent: there are distinct head, thorax, and abdomen regions. The enlarged hind femora, with powerful muscles, stand out. Grasshoppers and other jumping insects share these enlarged muscles.

Structure and Function in Insects

The basic structures of insects are similar to those of all arthropods. They have a complete digestive tract, an open circulatory system, and a central nervous system. There are some features that they share only with terrestrial arthropods such as a tracheal respiratory system. There are still other features that are truly unique to insects. The most obvious of these are the wings. Insects are the only invertebrates that are capable of flight. In this section we will examine the details of insect structure and function, including a look at the insect wing and how it has contributed to the success of this organism.

Feeding, Digestion, and Excretion

There is a variety of types of feeding among insects. There are carnivores, herbivores, scavengers, and parasites. One parasitic insect that you are probably very familiar with is the mosquito. Mosquitoes feed on mammalian blood. The different insect diets require different types of feeding appendages, or mouthparts. The mouthparts may be modified to obtain nutrients in several different ways:

• Chewing (grasshoppers and beetles). • Siphoning (butterflies and moths). • Piercing and sucking (mosquitoes).

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• Sponging (flies).

Insects have three sets of mouthparts that include one pair of mandibles and two pairs of maxillae. You may recall from a previous lesson that mandibles are generally used for biting and chewing food, while maxillae are used for manipulating and swallowing food. Many adaptations of these three pairs of mouthparts are found in the insect class to accommodate the different ways of obtaining nutrients listed above. For example, in chewing insects, such as dragonflies and beetles, the mandibles are often quite large and protrude out from the head. In carnivorous chewing insects, the mandibles may be modified to function as weapons, while herbivorous chewing insects generally have flattened mandibles suitable for grinding plant tissue. An example of a species of beetle with mandibles modified as hunting weapons is shown in the Figure 1.117.

FIGURE 1.117 A predatory beetle with mandibles modified for defense and attack. In insects such as this one, the mandibles are strictly used as weapons and no longer function in feeding.

FIGURE 1.118 Mouthpart Specialization in Insects. The mouthparts of insects are adapted for different food sources. How do you think the different mouthparts evolved?

In contrast, siphoning insects, such as moths or butterflies, do not have much use for a chewing appendage, and the mandible is often reduced or even lost in these species. Instead, they have a long tubular proboscis that extends out from the mouth, as shown in the Figure 1.118 and the Figure 1.119. This allows the insect to suck nectar from flowers. Piercing and sucking insects, such as mosquitoes, also have a modified mouthpart called a stylet that allows them to both pierce the skin of their prey and then suck food from the organism. Two other insect mouthparts are called the labium and labrum. They essentially form the base (or floor) and the top (or roof) of the mouth respectively. In sponging insects, such as flies, digestive enzymes are secreted onto the solid food source to allow it become somewhat of a liquid. The insect then uses a modified labium to “sponge up” and deliver the food to the esophagus. Sponging insects often have reduced mandibles. The relative positions of the insect mouthparts are shown in the Figure 1.120. Insects have a complete digestive tract that is divided into three main regions that follow the pharynx and esophagus: a foregut, midgut, and hindgut. The forgut is basically a crop-like region that functions to grind and pulverize

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FIGURE 1.119 A siphoning insect with a long coiled pro- boscis used for feeding. Insects that feed by piercing and sucking nutrients from their prey have a modified proboscis with a sharp end called a stylet used for pene- trating the skin of prey.

FIGURE 1.120 The mouthparts of a grasshopper. The top diagram represents the head of a typical grasshopper. Each mouthpart is shown separately below with labels as follows: lr = labrum, md = mandibles, mx = maxillae, lb = labium, hp = hypopharynx (modified tongue-like region of the labium).

ingested food particles. The midgut is where most of the food is digested with enzymes and nutrients are absorbed. Most insects use Malphigian tubules as excretory organs to regulate salt balance in their bodies. As you learned in the Arthropods: Structure and Function concept, Malphigian tubules do not exit the body directly but instead empty into the hindgut of the digestive tract. As a result, the hindgut of insects also carries out some excretory functions. In the hindgut, water and salts may be absorbed from both the digested material and the urine intake from the Malphigian tubules in order to regulate osmotic balance in the organism and concentrate waste material for excretion. Osmotic balance is the control of salt concentrations in tissues and fluids of the body, and it is critical that these levels are kept within a certain range in order for cells to function properly. The Figure 1.121 shows the internal anatomy of a typical insect. The digestive system is shown in green, and the Malphigian tubules are shown in yellow.

Respiration and Circulation

The circulatory system of insects serves a number of functions, including the transport of nutrients, salts, waste, and hormones throughout the body. It also acts like a very primitive immune system to generate wound-healing clots

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FIGURE 1.121 The internal anatomy of insects. The main body regions are labeled: A - Head, B - Thorax, C - Abdomen. Various organs and body parts are labeled as follows: 1. antenna, 2. ocelli (lower), 3. ocelli (upper), 4. compound eye, 5. brain (cerebral ganglia), 6. prothorax, 7. dor- sal artery, 8. tracheal tubes (trunk with spiracle), 9. mesothorax, 10. metathorax, 11. first wing, 12. second wing, 13. mid-gut (stomach), 14. heart, 15. ovary, 16. hind-gut (intestine, rectum anus), 17. anus, 18. vagina, 19. nerve cord, 20. Malpighian tubes, 21-26 segments of the leg, 27. fore-gut (crop, gizzard), 28. thoracic ganglion, 29. coxa (base of the legs), 30. salivary gland, 31. subesophageal ganglion, 32. mouthparts. Notice that most of these systems are similar to those of a typical arthropod, shown in lesson one. Major differences include Malphigian tubules, trachea, and wings.

and to attack cells of infectious organisms that have entered the body such as bacteria. Like other arthropods, insects have an open circulatory system in which the blood (called hemolymph) spends most of its time in body cavities called hemocoels and only a small amount of time flowing through blood vessels. The Figure 1.121 shows the insect circulatory system in red. One interesting feature of the insect circulatory system is that it does not contain oxygen-transporting molecules. Unlike most invertebrates (and vertebrates) that use respiratory organs to absorb oxygen from their environments and deliver it to the circulatory system for dispersal throughout the body, insects (and some other arthropods) do not transport oxygen via the circulatory system. Instead, insects have a highly efficient respiratory system that is composed of trachea. The tracheal system is composed of tubules that take in air from the environment through spiracles and carry it throughout the organism to bring oxygen directly to various tissues. Spiracles are small openings on the outside of the body. The tracheal system is labeled with the number “8” in the Figure 1.121. The Figure below shows the spiracles of a caterpillar.

Nervous System

Insects, like other arthropods that you have learned about in this chapter, have a central nervous system with a brain located in the head region and a pair of ventral nerve cords that run along the bottom interior of the body. Along the nerve cord there are pairs of ganglia located in each segment that function to innervate organs and tissues within that segment. The sensory capabilities of insects include touch, sound, sight, and smell (called chemoreception). A few insects, such as bees and butterflies, actually have color vision. Most of these capabilities stem from sensory organs found in

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FIGURE 1.122 The spiracles of a caterpillar. Spiracles are small openings on the outside of an insect’s body that take air into the tracheal respiratory system.

the head region. You have already learned that arthropods may have simple eyes and compound eyes for sensing light and forming images as well as bristles on the surface of their bodies for sensing touch. The simple eyes, called ocelli, and compound eyes of an insect are shown above in the Figure 1.121. You have also learned that most arthropods, with the exception of chelicerates, have antennae for sensory perception. What exactly do they sense? The answer is that the antennae can sense a number of different environmental stimuli. They often have chemoreceptors that are capable of detecting, or “smelling,” certain molecules present in the air around them. Some of these receptors can also detect, or “taste,” the molecules of liquid or solid substances. The Figure 1.123 shows the head region of various insects, highlighting the variations in insect antennae.

FIGURE 1.123 The antennae of various insects. Note the differences in length and shape between each pair of antennae.

In addition, some antennae have organs called Johnston’s organs that detect the position of the antennae and, in some cases, can detect sound vibrations. However, insects that can detect sound, such as grasshoppers, moths, and butterflies, generally use tympanal organs located on various parts of the body, depending on the species. The tympanal organs are made up of an eardrum-like membrane. The ability to sense many aspects of their environment is a fundamental component of the ability of some insects to have fairly complex responses to or interactions with their environments. These will be discussed in the Insects: Behavior (Advanced) concept.

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Vocabulary

• chemoreceptor: A sensory receptor that responds to chemical stimuli.

• hemocoel: A blood-filled body cavity that is distinct from the fluid-filled coelom.

• hemolymph: The circulatory fluid of certain invertebrates, which is analogous to blood in arthropods.

• Johnston’s organ: A sensory organ in the second antennal segment of insects that responds to movements of the antennal flagellum and serves as a flight-speed indicator; it may also detect sound.

• labium: A fused mouthpart that forms the floor of the mouth of an insect.

• labrum: A structure corresponding to a lip; it is the upper border of the mouthparts of a crustacean or insect.

• Malphigian tubules: Excretory organs in insects and many other arthropods; they are narrow tubules opening into the anterior part of the hindgut.

• mandible: A jaw-like structure that is used to grasp, bite, and even chew food.

• maxillae: One of the paired appendages immediately behind the mandibles of arthropods, which are used for manipulating food in the mouth and swallowing.

• proboscis: An elongated sucking mouthpart that is typically tubular and flexible.

• spiracle: Any of several tracheal openings in the exoskeleton of an insect or a spider; it is also an opening behind each eye that is used to take in water and pump it over the gills in cartilaginous fish.

• stylet: A small, stiff, needlelike organ or appendage used to both pierce the skin of prey and suck food from the prey.

• tracheal system: An respiratory system consisting of trachea, fine chitinous tubes in the body of an insect, that convey air directly to the tissues.

• tympanal organs: A hearing organ in insects consisting of a membrane (tympanum) stretched across a frame backed by an air sac and associated sensory neurons.

Summary

• Insects are the only invertebrates that are capable of flight. • Insects have three sets of mouthparts that include one pair of mandibles and two pairs of maxillae. • Depending on how an insect feeds, the mouthparts become adapted to specific functions. • Insects have a complete digestive tract that is divided into three main regions that follow the pharynx and esophagus: a foregut, midgut, and hindgut. • Insects, like other arthropods, have a central nervous system with a brain located in the head region and a pair of ventral nerve cords that run along the bottom interior of the body. • Most arthropods, with the exception of chelicerates, have antennae for sensory perception.

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Practice

Use this resource to answer the questions that follow.

• Insect Structure and Function at http://www.biology-resources.com/insect-structure.html .

1. How many wings do insects typically have? How have the wings changed over time? 2. Do insects ever stop molting, or going through ecdysis? 3. How does air move through the tracheal system of insects? 4. How do insects detect sound? 5. Describe the structure of a compound eye.

Practice Answers

1. Insects usually have two pairs of wings. Some insects lose one or both pairs of wings through evolution. Others, such as the grasshopper, have one pair of wings that form a hard outer covering over the second pair. 2. Insects molt only in the larval and pupal forms and not in adults. They therefore do not grow as adults. 3. For small, less active insects, air moves through the tracheal system by simple diffusion. For more active insects, muscles contract in the abdomen, compressing the tracheae and expelling air. The positive muscular action is associated with expiration in insects, whereas mammals use positive muscular action for inhalation. 4. Some of the bristles on the cuticle of insects might be able to sense low-frequency vibrations. Other insects have more sensitive tympanal organs on the thorax or abdomen that are sensitive to sounds of high frequency. 5. Inside a compound eye, there are 2,000 to 10,000 ommatidia, which consist of a lens system that concentrates light on to a transparent rod known as the rhabdom. Light on the rhabdom stimulates retinal cells that sends impulses to the brain.

Review

1. Which mouthparts are present in almost all insects? 2. Describe how insects have adapted their mouthparts for different functions. 3. What is the function of the forgut, midgut, and hindgut? 4. How does the respiratory system of insects differ from most animals? 5. What kind of sensory input do antennae provide for insects?

Review Answers

1. Insects have three sets of mouthparts that include one pair of mandibles and two pairs of maxillae. Insects have developed different uses for these mouthparts depending on how they feed. 2. In chewing insects, the mandibles are often quite large and protrude out from the head. In carnivorous chewing insects, the mandibles may be modified to function as weapons, while herbivorous chewing insects generally have flattened mandibles suitable for grinding plant tissue. Siphoning insects do not have much use for a chewing appendage, and the mandible is often reduced or even lost in these species. Instead, they have a long tubular proboscis that extends out from the mouth. Piercing and sucking insects also have a modified mouthpart called a stylet that allows them to both pierce the skin of their prey and then suck food from the organism. In sponging insects, such as flies, digestive enzymes are secreted onto the solid food source to allow it become somewhat of a liquid. The insect then uses a modified labium to “sponge up” and deliver the food to the esophagus. 3. The forgut is basically a crop-like region that functions to grind and pulverize ingested food particles. The midgut is where most of the food is digested with enzymes and nutrients are absorbed. In the hindgut, water

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and salts may be absorbed from both the digested material and the urine intake from the Malphigian tubules in order to regulate osmotic balance in the organism and concentrate waste material for excretion. 4. The insect circulatory system does not contain oxygen-transporting molecules. Oxygen is delivered purely by diffusion through the tracheal system. 5. The antennae can sense a number of different environmental stimuli. They often have chemoreceptors that are capable of detecting, or “smelling,” certain molecules present in the air around them. Some of these receptors can also detect, or “taste,” the molecules of liquid or solid substances.

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1.38 Insect Flight - Advanced

• Understand how flight contributed to the success of insects. • Differentiate between two types of insect flight.

What makes insects so successful? One characteristic that helps is the ability to fly, as shown with this ladybug. This ability helps tremendously. Why do you think flight has made these invertebrates so successful?

A Flight Plan for Success

Insects owe much of their success to the evolution of flight. They are the only invertebrates that can fly, and the fossil record indicates that they had been enjoying the benefits of flight for over 100 million years before reptiles joined them in their aerial habitats. What are the benefits of flight? It is a guaranteed means of escape from predators that are restricted to land and water. The great speeds and vast distances traveled by insects have also provided for efficient dispersal. Flying insects can easily seek new habitats and additional food sources when the need arises. And flight aids in the search for a mate. What is the overall structure of an insect wing? How does the organism control the wings to enable flight? These questions will be answered in the remainder of this section. Insects generally have two pairs of wings, and they are actually part of the exoskeleton. They are formed by out- folds of the exoskeleton on the thorax during development. Although wings primarily function to enable flight, some wings are structured to serve other functions as well:

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FIGURE 1.124 Form and Function in Insect Wings. Bee- tles, butterflies, and katydids all have two pairs of wings that they use for flight. However, their wings are very different be- cause they have other functions as well.

• Protective body covering (beetles). • Sound production (cicadas). • Heat retention. • Visual communication (butterflies). • Orientation.

The structure of the wings varies from the thin, paper-like flaps of butterflies to the tough, armor-like coverings of beetles. Examples of different wing types are shown in the Figure 1.125.

FIGURE 1.125 Insect wings can have very different struc- tures that allow them to be used for pur- poses in addition to flight. (a) The tough wings of beetles serve as a protective covering when the animal is not flying. (b) The papery and often colorful wings of a butterfly also function as visual cues in communication.

The circulatory, respiratory, and nervous systems all extend into the wings. Components of these systems, such as blood vessels, trachea, and nerves, make up the veins that can be seen branching throughout the wings of species with thin wing tissue. The cells of the wing are separated by and named after the veins. This is shown in the Figure below. There are two different mechanisms of insect flight: indirect flight and direct flight. These two types of flight differ in how the insect’s muscles cause the wings to flap. For direct flight, the muscles insert into the hinged base of the wing and cause the wings to flap by initiating movements in the base. This is considered an earlier version of insect flight that likely evolved before indirect flight. For insects with an indirect flight mechanism there are no muscles attached directly to the wings. For indirect flight, the muscles do not attach to the wings. Instead, they attach to the walls of the thorax, as shown in the Figure below. Muscle contractions distort the shape of the thorax, and this in turn causes the wings to move. Recall that the wings are extensions of the thoracic exoskeleton. A link to an animated version of indirect flight that shows how the distortion of the thorax causes the wings to move can be found at http://www.cronodon.com/BioT ech/insect_locomotion_2.html . Some insects also use their abdomen as a sort of rudder while they fly. For more information on how insects control themselves in the air, visit Roboticists discover the secret of insect flight, and it’s

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FIGURE 1.126 A diagram of the different cells that make up an insect wing. The labels correspond to the following cell names: C = costa, Sc = subcosta, R = radius, M = media, Cu = cubitus, and A = anal. These cells are named according to a system of insect wing nomenclature called the Comstock- Needham system. They are based on the names of the veins that separate the cells from each other. In this diagram, the veins are shown as black lines. The subscript numbers indicate cells produced by differ- ent branches of the same vein.

not wings at http://io9.com/roboticists-discover-the-secret-of-insect-flight-and-i-476242076 .

FIGURE 1.127 A diagram showing the organization of muscle attachments for indirect flight. This figure shows a cross-section of an insect that moves by indirect flight. The labels are as follows: a = wings, b = joints, c = dorsoventral muscles, and d = longitudinal muscles. The combined ac- tions of the dorsoventral and longitudinal muscles deform the shape of the thorax. This shape distortion causes to wings to move indirectly, meaning that the muscles have no direct attachments to the wings.

Insects that use indirect flight generally have more fine control over their movements. They are often able to hover and fly backwards, something not usually accomplished by direct flight insects. The Figure 1.128 shows examples of an insect flying by a direct mechanism and an insect flying by an indirect mechanism.

Vocabulary

• direct flight: Flight achieved by muscles inserted into the hinged base of the wing, which cause the wings to flap by initiating movements in the base (associated with certain insects).

• indirect flight: Flight achieved by muscles attached to the thorax, which distort the shape of the thorax and cause the wings to move (associated with certain insects).

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FIGURE 1.128 A dragonfly flying by a direct mechanism (a) and a bee flying by an indirect mecha- nism. One difference between these two forms of flight is that in the direct mech- anism the two sets of wings usually func- tion independently of each other, whereas insects that carry out indirect flight often have fused both sets of wings so that they function together.

Summary

• Insects are the only invertebrates that can fly, and the fossil record indicates that they had been enjoying the benefits of flight for over 100 million years before reptiles joined them in their aerial habitats. • Insects generally have two pairs of wings, and they are actually part of the exoskeleton. • Benefits of flight include evading predators, dispersal, and finding a mate. • Other functions of wings include protection, sound production, heat retention, visual communication, and orientation. • There are two different mechanisms of insect flight: indirect flight and direct flight.

Practice

Use this resource to answer the questions that follow.

• How Insects Fly at http://insects.about.com/od/insects101/qt/How-Insects-Fly.htm .

1. What technology allowed scientists to study insect flight in more detail? 2. Describe how the flight muscles in an insect coordinate to achieve direct flight. 3. Why does indirect flight conserve energy for insects?

Practice Answers

1. Due to the small size of insects and their fast wing beats, scientists have had trouble observing their flight. The invention of high-speed film allowed scientists to watch insect flight at super slow speeds and understand the mechanism. 2. An insect that flies using direct flight has two sets of flight muscles. One set attaches inside the base of the wing, while the other attaches slightly outside the wing base. The two sets of muscles work by alternating contractions to move the wings up and down. 3. Indirect flight allows insects to conserve energy because the thorax has a certain amount of elasticity that returns it to its natural shape. Muscle contractions are needed to distort the thorax, but muscles can relax as the thorax springs back into place.

Review

1. Which invertebrates evolved the ability to fly? Did this occur before or after vertebrates evolved the ability to fly?

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2. List some benefits of flight. 3. What makes up the wings of insects? 4. What are some other functions of insect wings excluding flight? 5. Explain the difference between direct and indirect flight. Which one is superior, and which one evolved first?

Review Answers

1. Insects are the only invertebrates that can fly, and the fossil record indicates that they had been enjoying the benefits of flight for over 100 million years before vertebrates joined them in their aerial habitats. 2. Flight allows insects to escape predators that are restricted to land or water. It also allows them to spread quickly to different areas and find mates in a more efficient manner. 3. Insect wings are formed by out-folds of the exoskeleton on the thorax during development. 4. Other functions of wings include protection (beetles), sound production (cicadas), heat retention, visual communication (butterflies), and orientation. 5. Insects first evolved direct flight, where muscles are directly attached to the wings in order to generate movement. Insects that fly using indirect flight have muscles that distort the shape of their thorax, causing their wings to move in the process. Insects that use indirect flight generally have more fine control of their movements. They are often able to hover and fly backwards, something not usually accomplished by direct flight insects.

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1.39 Insect Reproduction and Development - Advanced

• Learn about the sexual and asexual forms of reproduction present in insects. • Understand the different types of metamorphoses and how insects develop.

Where do butterflies come from? Most everyone knows they are not born as butterflies but as caterpillars. After mating, the black swallowtail butterfly lays small, yellow eggs. The caterpillars grow to about two inches and are green and black banded with yellow spots around every second black band. They have short, black spikes around some of the black bands, although these disappear as the larva nears pupation and undergoes a complete metamorphosis.

Reproduction and Development in Insects

Nearly all insects reproduce by sexual reproduction. This involves the formation and fusion of gametes: sperm from the testes and eggs from the ovaries. There are some species of insects that can also reproduce asexually. They do this by a process called parthenogenesis. During parthenogenesis, a female’s egg can produce a new organism without being fertilized by sperm. This usually requires that the females can produce eggs that have two copies of each chromosome (diploid) instead of the one copy that an egg normally contains (haploid). Parthenogenesis generates an exact clone of the female. There are several other forms of parthenogenesis found in some insect species. One of them is extremely interesting because it allows the female to choose whether she will produce male or female offspring. In these species, including some ants, bees, and wasps, females are diploid and are the result of sexual reproduction (fusion of egg and sperm), whereas males are haploid and are the result of parthenogenesis. The female produces haploid eggs, and she can choose whether or not to fertilize them with sperm obtained from a male and stored in her body. If she fertilizes the eggs by allowing them to fuse with sperm, they will become diploid and produce female progeny. If she chooses not to fertilize the eggs, they will still develop, but they will produce haploid male progeny. Unlike aquatic organisms that can release their sperm into the open ocean where they will swim to the eggs in the female’s body, most insects have to either deliver sperm directly or indirectly into the female’s body for internal

209 1.39. Insect Reproduction and Development - Advanced www.ck12.org fertilization. There are some species that package their sperm into sealed pouches that they leave on the ground for females to find. The females then use the packaged sperm to fertilize their eggs. This is less efficient than directly delivering sperm into the female’s body by copulation, and it runs the risk of the pouch not being found.

FIGURE 1.129 Insect Life Cycle. This diagram repre- sents the life cycle of a mosquito. Most insects have a similar life cycle.

Metamorphosis

Many of the invertebrates that we have examined so far go through one or more larval stages during their development from an embryo to an adult. These larval stages are often very different in form from the adult animal. This is called indirect development, and it differs from direct development in which the young organisms hatch resembling smaller versions of the adult. The process of developing from a larva to the recognizable adult form is called metamorphosis. Most insects go through some form of metamorphosis. In some insect species, the transition to adulthood involves relatively minor changes, but, in the majority of species, it is a drastic and profound transformation. Insects that do not undergo a metamorphic change during their development are called ametabolous insects. These include members of the Apterygota subclass and Paleoptera infraclass. The only change that these species undergo is an increase in size and the maturing of sexual organs. Among insects that experience metamorphosis, there are two types:

• Hemimetaboly (superorder Exopterygota). • Holometaboly (superorder Endoterygota).

Hemimetabolous insects, such as cockroaches, grasshoppers, and dragonflies, go through gradual changes as they slowly develop from juvenile forms, called nymphs, to the adult form. These changes often include the budding and growth of their wings. The overall forms of the nymphs and the adults are not drastically different from each other, and most of the body parts are the same between the two. The stages of development of a hemimetabolous insect, the grasshopper, are shown in the Figure 1.130.

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FIGURE 1.130 A grasshopper undergoing hemimetabolous development. Notice that the overall features do not drastically change during the grasshopper’s development from a nymph to an adult, but wings begin to emerge and develop during this process, and the grasshopper increases in size.

Holometabolous insects are another story altogether. Think of the difference between a maggot and a fly. The larval stages are distinctly different from the adult stage. Insect larvae are focused on two main activities: eating and growing. At the end of the larval stage, they still do not have any obvious adult features. They then enter a transition stage called a pupa (plural pupae). Pupae are immobile, and sometimes they are encased in a cocoon made of silk or a hard shell. During the pupal stage, the tissues and appendages of the larval stage are broken down and reorganized into new adult tissues, organs, and limbs. The Figure 1.131 shows an insect at various times during the pupal stage. The mature adults emerge from the pupal stage able to move and reproduce, looking nothing like the preceding larval stage. As with molting, this process is controlled by hormones that trigger the drastic internal changes needed to undergo metamorphosis.

FIGURE 1.131 A honeybee shown in different phases of the pupal stage. Notice the progressive formation of the adult limbs and body regions as the pupa develops.

How is this complex process of metamorphosis advantageous to the animals? One advantage is that it allows them to divide certain functions between different stages of the life cycle. In some species, the extensive feeding and

211 1.39. Insect Reproduction and Development - Advanced www.ck12.org energy storage of the larval stage is adequate to last the lifetime of the insect. The main benefit of this process is that these stages in the life cycle can adapt independently of each other. Evolutionary changes can occur that modify the larval stage, to improve feeding and nutrient uptake, for example, without affecting the adult stage and vice versa. Not all aspects of the organism’s form and function are affected by changes that are compartmentalized in this way. Let’s take a specific example. The mouthparts of a larva can be modified to adapt to a different type of food source that has become more available. This change will not affect the mouthparts or head appendages of the adult form, allowing them to adapt independently for other functions such as sensory perception. Despite the type (or absence) of metamorphosis during development, all insects periodically shed their exoskeleton through the process of molting that you learned about in previous lessons of this chapter. Molting is a type of behavior that is carried out by insects. This might be considered a fairly simple behavior, particularly if you compare it to building a pyramid or competing in a chess tournament. However, even insects are capable of some fairly complex behaviors or interactions with their environments. These will be the topics of the next concept.

Vocabulary

• hemimetabolous: An insect that undergoes a metamorphosis that lacks a pupal stage.

• holometabolous: An insect that undergoes complete metamorphosis, in which it passes through four separate stages of growth: embryo, larva, pupa, and imago.

• metamorphosis: The process of transformation from an immature form to an adult form in distinct stages.

• parthenogenesis: A form of asexual reproduction where growth and development of embryos occur without fertilization.

• pupa (plural, pupae): A life cycle stage of many insects; it occurs between the larval and adult stages. During this time the insect is immobile, may be encased within a cocoon, and changes into the adult form.

Summary

• Nearly all insects reproduce by sexual reproduction. • In some insect species, the female can choose to produce male or female offspring. • Insects that do not undergo a metamorphic change during their development are called ametabolous insects. • Among insects that experience metamorphosis, there are two types: hemimetaboly and holometaboly. • Insect larvae are focused on two main activities: eating and growing. • During the pupal stage the tissues and appendages of the larval stage are broken down and reorganized into new adult tissues, organs, and limbs.

Practice

Use this resource to answer the questions that follow.

• How did Insect Metamorphosis Evolve? at http://www.scientificamerican.com/article.cfm?id=insect-meta morphosis-evolution .

1. How many species of animals go through metamorphosis? 2. What cells form the adult structures of insects during metamorphosis? 3. When did insects that go through metamorphosis first appear in the fossil record? 4. What is one theory on how metamorphosis evolved? 5. What are the benefits of complete metamorphosis?

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Practice Answers

1. Scientists estimate that 45-65% of all species are metamorphosing insects. 2. Clusters of cells called imaginal discs first form when an insect embryo develops in its egg. These cells remain dormant until the pupal stage when they dissolve larval cells that fuel the imaginal discs to form adult structures. 3. Early fossils show that insects developed like modern ametabolous and hemimetabolous species. Insects that go through complete metamorphosis first appear in fossils dating back 280 million years ago. 4. A modern theory suggests that the larval stage is a pro-nymphal stage that developed the ability to feed on its own. The pupal stage of metamorphosis probably developed later, as a condensed nymphal stage before the insects developed into adults. 5. The benefits of metamorphosis are mainly based on the fact that adult and larval stages occupy different ecological niches. More larvae and adults can coexist because they are not competing for the same resources.

Review

1. What does parthenogenesis allow insects to do? 2. One type of parthenogenesis found in insects involves the female choosing the gender of their offspring. What kind of mechanism does the female use to control the gender of her offspring? 3. Some insects simply deposit pouches of sperm for females to find. Why is this form of fertilization less efficient? 4. Which insects undergo metamorphosis? Which do not? 5. Describe the differences between hemimetabolous insects and holometabolous insects.

Review Answers

1. Insects that can reproduce through parthenogenesis do not require sperm in order to fertilize eggs. The female is able to produce eggs with a diploid number of chromosomes that are exact clones of herself. 2. Some insects go through a type of parthenogenesis where the female produces a female if she fertilizes her egg. If her egg is not fertilized, the egg becomes a male. Males, therefore, only have a haploid number of chromosomes, whereas females have a diploid number. 3. This type of fertilization is less efficient than directly delivering sperm into the female’s body by copulation because it relies on the female finding the pouch. 4. The superorder Exopterygota and superorder Endoterygota go through some form of metamorphosis. Insects that do not undergo a metamorphic change during their development are called ametabolous insects. These include members of the Apterygota subclass and the Paleoptera infraclass. 5. Hemimetabolous insects, such as cockroaches, grasshoppers, and dragonflies, go through gradual changes as they slowly develop from juvenile forms, called nymphs, to the adult form. These changes often include the budding and growth of their wings. Holometabolous insects have larval stages that are distinctly different from the adult stage.

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1.40 Insect Behavior - Advanced

• Discuss some innate and learned behaviors that certain insects exhibit. • Understand how insects communicate in a social group.

Teamwork? Yes, many insects, especially bees and ants, work as a team for all to survive. Each individual has a specific job and relies on other individuals for survival. Teamwork is a type of behavior.

Insect Behavior

What exactly is behavior? It is defined as the way that organisms respond to their environment and to internal signals. In addition to many basic behaviors that are shared by other invertebrates, such as mating, there are some insect species that are capable of more advanced forms of interaction with each other and with their environments. There are two types of behavior that can be observed in organisms: innate behavior and learned behavior. Innate behavior is genetically encoded. Flight and mating habits are considered innate behaviors. You have probably seen a clear example of innate insect behavior called the dorsal light reaction. Flying insects will sense the direction of light coming from the sun and fly in a way that keeps the sun overhead, or on their dorsal side. This is a means for the insect to maintain a flight plan that is parallel to the ground. You may have witnessed that this innate behavior is not so helpful when a moth encounters an artificial light source and flies in continuous circles around it to keep the light on its dorsal surface. At times, the moth is not able to fly away from that light source and, in essence, becomes trapped. Another insect behavior that you have probably experienced firsthand is sound production. Some insects produce sounds in order to communicate. An example is the chirping of crickets that often lulls us to sleep on warm summer nights. Learned behaviors are those that are not encoded genetically and are not present in the organism at birth. They are obtained through life experiences, and they can change or improve over time. Learned behaviors require acute sensing of environmental signals and a fairly complicated network of nerve cell connections for transmitting those signals in order to process the information and modify or initiate a behavior. Insects are capable of this level of behavior. In order to forage for food and return to the same food source repeatedly, they use a type of learning called

214 www.ck12.org Chapter 1. Invertebrates - Advanced associative learning. Associative learning is when separate ideas or environmental stimuli are connected to each other. For example, the location of a food source can be associated with a series of visual cues seen on the way to the source. In this way, the organism learns that if it follows a path that includes all of those visual cues, it will again find the food source. There are examples of this type of learning that can be tested experimentally. For example, honeybees can be taught to obtain their food from a particular source based on color cues, even when the location of that source changes. They can learn that their food (sugar water) is located on a yellow dish next to a blue dish containing only water. If the dish positions are switched, the bees remember which color has the food, and they seek that dish. There are several groups of insects that have evolved complex social networks that involve elaborate patterns of communication between individuals within the community. These species are called social insects, and we will examine their behavior in the next section.

Social Insects and Communication

Insects and other organisms that live together in well-organized and tightly integrated colonies are called eusocial animals. Eusocial insects include species of ants, termites, bees, and wasps. Some colonies can include millions of individual animals. These are two of the major features of eusocial insects:

• Division of reproductive labor. • Cooperative care of the young members of the colony.

Social ants are a good example of the division of reproductive labor. The individual ants within a colony divide up into three major groups:

• Fertile females (queens). • Infertile (sterile) females (workers). • Fertile males (drones).

Fertile males and queens carry out the reproductive activities of the colony, while workers focus on obtaining food as well as building and maintaining the nest or hive. In some social insect colonies there are other specialized individuals; nurses, for example, feed and care for young larvae. Often times, some individuals in a colony will form a defensive army with the main purpose of defending the nest. Communication between members of a colony can take several different forms. Ants generally communicate using pheromones. Pheromones are hormones that are released by one individual to be sensed and responded to by another individual. You may have noticed how a group of ants making their way to a crumb on the ground will generally travel lined up one in front of the other. This is because they are sensing and following a trail of pheromones that was laid down by an earlier ant that discovered the crumb. On its way back to the nest, this little pioneer (called a forager) left a trail of pheromone for other colony members to follow. Honeybees have evolved a fascinating form of communication using body movement. These insects perform an elaborate dance called the “waggle dance” to tell other colony members where to find a source of food. The angle of the dance indicates the particular direction of the food source relative to the sun, and the length of the dance correlates with how far away the food is, as shown in the Figure 1.133. This is considered a form of abstract symbol communication, meaning that they use a behavior to represent information (in this case a location) about something in the environment. For more about this unique behavior, see The Waggle Dance video, found at http://video.pbs.org/video/230084618 3/ (3:25).

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FIGURE 1.132 A Termite Nest. This cathedral-like struc- ture is the nest of a huge colony of ter- mites in Australia. In fact, it is the world’s largest known termite nest. It towers 7.5 meters (25 feet) above the ground and houses millions of termites.

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In addition to the ability to learn, some species of social insects have also been shown to teach behaviors to other individuals. An example of this is seen in one ant species where the forager ant (the one who ventures out to find food) will actually take the time to lead a nest-mate to a new food source, in a way teaching the nest-mate how to forage. Altruism is another feature of many social insects. Altruism is the act of self-sacrifice for the benefit of others. A worker bee, for example, forfeits her own potential for reproduction in order to obtain food and provide shelter for the benefit of the queen bee. This allows the queen bee to focus on the reproduction of her genetic material at the cost of the worker bee reproducing her own genetic material. This complex phenomenon is a favorite topic of study among philosophers and sociologists, and there are a number of different theories on the evolutionary advantage of altruism that you can read about in the Animal Behavior: Evolution (Advanced) concept. What do you think might be the evolutionary driving force behind altruism?

KQED: Ants: The Invisible Majority

Most of us think ants are just pests. But not Brian Fisher. Known as “The Ant Guy,” he’s on a mission to show the world just how important and amazing these little creatures are. In the process, he hopes to catalog all of the world’s 30,000 ant species before they become casualties of habitat loss. See http://www.kqed.org/quest/television/ants-th e-invisible-majority2 (10:36) for more information.

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FIGURE 1.133 Bee communication. Some bees can tell each other the location of a food source by performing a “waggle dance.” The dance is outlined in this diagram. The angle (alpha) of the “waggle,” represented by the wavy section of the bee’s path, from the sun indicates the direction of the food source. The length of the “waggle dance” indicates how far away the food source is.

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KQED: Ladybugs: A Population of Millions

Ladybugs, also known as ladybird beetles, have a life cycle of four to six weeks. In one year, as many as six generations of ladybird beetles may hatch. In the spring, each adult female lays up to 300 eggs in small clusters on plants where aphids are present. After a week the wingless larvae hatch. Both the ladybird beetle larvae and adults are active predators, eating only aphids, scales, mites, and other plant-eating insects. The ladybugs live on the vegetation where their prey is found, which includes roses, oleander, milkweed, and broccoli. Adult ladybugs don’t taste very good. A bird careless enough to try to eat one will not swallow it. By late May to early June, when the larvae have depleted their food supply, the adults migrate to the mountains. There, they eat mainly pollen. The ladybugs gain fat from eating the pollen, and this tides them over during their nine-month hibernation. Thousands of adults hibernate overwinter in tight clusters, called aggregates, under fallen leaves and ground litter near streams. In the clear, warmer days of early spring, the ladybugs break up the aggregates and begin several days of mating. Learn about ladybugs at http://www.kqed.org/quest/television/ladybug-pajama- party (3:08).

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Vocabulary

• altruism: Doing something for others that puts oneself at risk.

• associative learning: A type of learning in which two different ideas or stimuli are connected to each other.

• eusocial: Describes animals that have an extreme form of social living in which individuals become special- ized for specific tasks.

• innate behavior: Behavior that occurs naturally in all members of a species; behavior triggered by a particular stimulus.

• learned behavior: Behavior that occurs as a result of experience.

• pheromones: One of many chemicals, secreted by special glands, that trigger responses in other organisms.

Summary

• There are two types of behavior that can be observed in organisms: innate and learned.

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• Insects are capable of learned behavior. • Two of the major features of eusocial insects are division of reproductive labor and cooperative care of the young members of the colony. • Communication between members of a colony can take several different forms such as "dances" or pheromones. • In addition to the ability to learn, some species of social insects have also been shown to teach each other various behaviors.

Practice

Use this resource to answer the questions that follow.

• Ants Use ’Math’ to Find Fastest Route, Study Shows at http://www.huffingtonpost.com/2013/04/18/ant s-math-fastest-route-fermats-theorem_n_3108273.html .

1. What did scientists realize the ants were doing in this experiment? 2. How do ants eventually hone in on the most efficient path? 3. Why are ants less efficient at finding the optimal path across short distances?

Practice Answers

1. Scientists realized that ants eventually optimize the time it takes to get to their food source rather than the actual distance it takes. Ants will go in a roundabout way if the terrain is easier for them to transverse. 2. Ants communicate using pheromones. Although the initial paths may be random, eventually the chemical trails converge on a single optimal path. 3. Scientists believe that ants cannot navigate short distances as effectively because there is more pheromone in each area.

Review

1. In every day life, we often see insects circling a light source at night. What is this behavior called, and why has it evolved in insects? 2. Bees, ants, and termites are all considered eusocial animals. What characteristics distinguish eusocial insects? 3. How do species such as ants and bees communicate with each other within their complex social structure?

Review Answers

1. This behavior is known as the dorsal light reaction. Flying insects will sense the direction of light coming from the sun and fly in a way that keeps the sun overhead, or on their dorsal side. This ensures that insects fly parallel to the ground. 2. Two of the major features of eusocial insects are division of reproductive labor and cooperative care of the young members of the colony. 3. Ants usually communicate with each other using pheromones. The foragers, for example, lead others to a food source by leaving a trail of pheromone for other colony members to follow. Honeybees perform an elaborate dance called the “waggle dance” to tell other colony members where to find a source of food. The angle of the dance indicates the particular direction of the food source relative to the sun and the length of the dance correlates to how far away the food is.

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1.41 Humans and Insects - Advanced

• Understand the relationships between humans and a variety of insects.

What snack do you have after school? Maybe not these silkworms, but, to many people, these skewered silkworms are considered a delicacy. The con- sumption of insects feeds millions or billions of people. Popular bugs to eat include beetles, butterflies and moths, bees and wasps, ants, grasshoppers, crickets and locusts, and stinkbugs.

Humans and Insects

Most humans interact with insects on a daily basis. In addition to encountering insects outside, many insects live in your homes with you. Most human-insect interactions are benign, that is, no one gets hurt. Some are really just a nuisance; being bit by a mosquito or seeing a spider crawling on your bed can be quite irritating. However, there are other interactions between humans and insects that are enormously devastating, and there are also some that are extremely beneficial. We will consider the relationships of humans and insects in this section.

Insects and Crops

Other than the ants that may try to invade your picnic lunch, you have probably never considered insects to be major competitors for your food sources. In fact, insects are considerable competitors for human food sources, particularly crop plants. It takes a lot of plant material to feed the herbivorous insects that make up roughly half of all insect species. The fight to protect agricultural crops from destruction by insects is an ongoing battle for humans. Sometimes insects can impart sudden and devastating damage to plant resources. An example of this occurred as recently as 2004 when swarms of desert locusts caused over $2.5 billion worth of crop damage to over 20 countries in West and North Africa. The threat of these swarms continues today in many regions of Africa and Asia. The desert locust is shown in the Figure 1.134. Insecticides are one possible solution to the problem of some crop-damaging insects, but there are a number of serious problems with this approach. One is the potential toxicity of these compounds, which harms both the farm

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FIGURE 1.134 A desert locust. The desert locust actually has two different adult life forms. The solitary life form is not harmful to human crops, but the gregarious form congre- gates in extremely large swarms as a result of changes in the hormonal signals that they emit. These swarms fly over long distances and feed voraciously on both crop and non-crop plants.

FIGURE 1.135 A Locust Swarm. A swarm of locusts in the African country of Mauritania darkens the mid-day sky. The hungry insects will eat virtually all the plants in their path.

workers who are exposed to large quantities and the consumers who eat plant material that may have traces of insecticide on them. Another problem with insecticides is that insects often reproduce rapidly enough to evolve pesticide resistance fairly quickly. Instead of this method, many other methods of insect pest control, which are more environmentally sound, can be used. These include crop rotation, the introduction of natural predators of the offending insects, and breeding to select for plants that are resistant to certain insect infestations. The other side of this issue is that most flowering plants, including many that are components of agricultural crops, are critically dependent on insects to mediate their reproduction through pollination. Therefore, we are critically dependent on insects for our survival. Flowering plants make up a large fraction of the primary producers on Earth. Primary producers harvest the energy of the sun and turn it into an energy source that the rest of the organisms on the planet, including ourselves, need in order to survive. Even if you obtain a lot of your nutrition from meat sources, those animals (cows, pigs, and chicken) gained all of their nutrition from the plants they ate.

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Insects and Human Disease

Although the bites of insects, including poisonous species, are almost never fatal in and of themselves, insects are the cause of an extremely large number of human deaths - over one million each year. These fatal consequences are largely the result of bites from hematophagous (blood eating), parasitic species that feed on human blood and act as vectors for disease transmission. Examples include the spread of the bacteria that cause bubonic plague by fleas during a number of pandemics throughout history and the continued spread of malaria by mosquitoes. Another example is the spread of the protozoa that cause sleeping sickness by tsetse flies in Africa. It is estimated that up to four million people die each year as a result of these insect-borne diseases spread by the organisms shown in the Figure 1.136.

FIGURE 1.136 Indirectly harmful insects. The three in- sects shown here, (a) a flea (microscopic image), (b) a mosquito, and (c) a tsetse fly, are not harmful in and of themselves, but by feeding on human blood they can transfer dangerous disease-causing or- ganisms and lead to millions of infections each year, many of which are deadly.

Stings from stinging insects, such as bees, occasionally result in human deaths (about 20 per year in the U.S.), which are not caused by the sting itself but by a severe allergic reaction to components of the insect’s venom. This reaction is called anaphylactic shock, and it requires immediate medical attention.

An Early Model Organism

In the Roundworms: Classification (Advanced) concept you learned about a particular roundworm species called C. elegans that has served as a model organism in biological research. A model organism is a species that is selected by the scientific community for extensive research study. These organisms are usually selected based on their ease of use in the laboratory (inexpensive to maintain, short life cycle so many generations can be studied, etc.) and the similarity of their systems to other organisms of biological significance such as humans. The insect class includes a model organism that has made an even more significant contribution to our understanding of animal biology than C. elegans: the fruit fly Drosophila melanogaster (commonly referred to simply as drosophila), shown in the Figure 1.137. Studies first began on drosophila genetics in the early 1900s, led by a scientist named Thomas Hunt Morgan in what became known as the fly room at Columbia University. Drosophila is one of the most studied organisms in biological research, and these studies have made particularly important and extensive contributions to our understanding of the molecular genetics and development of animals. Much of this information has been readily applicable to understanding human biology. For instance, approximately 75% of known human disease genes have a related gene in the drosophila genome. Studying these genes in an organism where mutations can be generated and their effects analyzed in a lab is an efficient way to gain an understanding of how these genes might work in humans. Just to give you some examples of the many human conditions that can be modeled in an organism as seemingly simple as a fly, drosophila is used as a model organism for:

• Parkinson’s Disease. • Huntington’s Disease.

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FIGURE 1.137 An extremely beneficial insect: Drosophila melanogaster. As a model organism for research in the fields of genetics, developmental biology, and neurobiology, this little fly has made enormous contributions to our knowledge of the biology of animals.

• Alzheimer’s Disease. • Diabetes. • Cancer. • Drug abuse (including cocaine and alcohol).

One major discovery that was first found in Drosophila was a family of genes called homeobox genes. The proteins encoded by this gene family all have a specific region called the homeodomain that can bind to DNA and cause other genes on the chromosome to become activated. These genes have since been found in almost all animal phyla, and they are now known to be incredibly important for generating the different regions of an animal’s body such as the thorax and the abdomen (this is called patterning the body). The identity of each of these regions is important for the proper placement of limbs and other body parts. For example, a mutation in just one of the Drosophila homeobox genes can cause a leg to form on the head region in place of an antenna. This type of anomaly is rarely seen in humans and other vertebrates because mutations in homeobox genes are so detrimental that they usually lead to a miscarriage. The study of model organisms is a powerful field of biological research, and, through the contributions of Drosophila melanogaster, insects have been a major force in this work.

Forensic Entomology

An interesting use of insects by humans that you may never have thought about before is in forensic entomology. Forensic entomologists use their expertise on the life cycles and habitats of insects to help solve crimes, particularly homicides. How do insects provide information about a crime scene? The feature of insect species that is most useful to forensic entomologists is that they feed on dead vertebrates (like humans) or on other organisms that feed on dead vertebrates. By examining the life stage of insects found on a murdered body (stage of embryogenesis or larval stage) the time of death can often be determined. A difference between the species found on the body and those inhabiting the area in which the body was found can also sometimes indicate that the body was moved after death. Surprisingly, in some instances, studying the insects found on the body may also lead to the cause of death. This may be the case when the person was killed using poisons that are no longer detectable in their system. Insect larva found on the body may still contain traces of these substances. This may sound like some gruesome stuff, but for the families of victims and potential future victims of a predator, any information that can help the investigation of the crime is vital. The death may also have been accidental or caused by suicide. Either way, it is important for family and community members to find out what happened. We have both the insects themselves, who are just going about their normal business, and the dedicated scientists who are willing to do this work to thank for these contributions to the justice system.

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KQED: Better Bees: Super Bee and Wild Bee

Honeybees are one of the most well-known insects on the planet. Bees are naturalized on every continent except Antarctica. Honeybees have a highly developed social structure and depend on their community, or colony, for survival. Bee colonies can contain up to 20,000 bees. When bees search plants for nectar, pollen sticks to the fuzzy hairs that cover their hind legs. At the next flower, some of the pollen rubs off and fertilizes that flower. In this way, bees help improve fruit production. Bees pollinate an estimated 130 different varieties of fruits, flowers, nuts, and vegetables in the United States alone. Farmers obviously depend on bees to pollinate crops, such as fruits and nuts, but thousands of bee colonies have disappeared in recent years. This could be a devastating issue for farmers. Can anything be done? Meet two Northern California researchers looking for ways to make sure we always have bees to pollinate crops at http://www.kqed.org/quest/television/better-bees-super-bee-and-wild-bee (11:12).

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Lastly, for insect recipes, see Insects Are Food at http://www.insectsarefood.com/recipes.html .

Vocabulary

• forensic entomology: The application and study of insect and other arthropod biology to criminal matters.

• hematophagous: Pertaining to the feeding on blood by insects or other parasites.

• homeobox: A 180 base pair long, highly conserved segment of DNA; it encodes a 60 amino acid domain within the protein (known as the homeodomain), which can bind DNA.

• model organism: A non-human species that is extensively studied to understand particular biological phe- nomena.

Summary

• Insects are considerable competitors for human food sources, particularly crop plants. • Most flowering plants, including many that are components of agricultural crops, are critically dependent on insects to mediate their reproduction through pollination. • Experiments conducted using the fruit fly drosophila have led to important scientific discoveries in biology. • Hematophagous insects sometimes inadvertently spread diseases that are harmful to humans.

Practice

Use this resource to answer the questions that follow.

• The Impact of Insects at http://www.cals.ncsu.edu/course/ent425/text01/impact1.html .

1. How much of an economic impact do insect pests have? 2. What roles do insects play in the biogeochemical cycle of nutrients? 3. What is honey composed of?

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Practice Answers

1. Economists estimate that insects consume or destroy 10% of gross national product in industrialized nations and up to 25% of gross national product in developing nations. 2. Insects aerate the soil, improve its water retention, redistribute nutrients to the root zones, and speed up decomposition. These all help render the landscape suitable for agriculture and plant life in general. 3. Bees make honey from the nectar that they obtain from flowers. The nectar’s sucrose is broken down into glucose and fructose by enzymes in the bee’s foregut. The evaporation of water leads to its thick consistency.

Review

1. Insects sometimes destroy crop plants. What are some downsides to using pesticides? 2. What are some advantages to doing experiments on drosophila? 3. How has the discovery of homeobox genes in drosophila impacted our understanding of genetics? 4. How have insects proven useful for forensic scientists?

Review Answers

1. Insecticides are one possible solution to the problem of some crop-damaging insects, but there are a number of serious problems with this approach such as the potential toxicity of these compounds and the ability of insects to reproduce rapidly enough to evolve pesticide resistance. Other options include crop rotation, the introduction of natural predators of the offending insects, and breeding to select for plants that are resistant to certain insect infestations. 2. Approximately 75% of known human disease genes have a related gene in the drosophila genome. Experi- ments to understand how genes are expressed in drosophila can lead to an understanding of human diseases. 3. The proteins encoded by homeobox genes all have a specific region called the homeodomain that can bind to DNA and cause other genes on the chromosome to become activated. These genes have since been found in almost all animal phyla, and they are now known to be incredibly important for generating the different regions of an animal’s body such as the thorax and the abdomen (this is called patterning the body). This discovery has led to a better understanding of which genes are responsible for embryonic development. 4. Insects can be used to determine the time of death, whether the body was moved, or whether the victim was poisoned.

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1.42 Echinoderms - Advanced

• Understand the characteristics that distinguish echinoderms from other animals. • Learn about how deuterostomes, such as echinoderms, are different from protostomes.

Believe it or not, this is an animal. See the mouth and arms? It is a sea lily, a crinoid echinoderm. Crinoids are essentially a mouth on the top surface that is surrounded by feeding arms. Although the basic echinoderm pattern of fivefold symmetry can be recognized, most crinoids have many more than five arms. Crinoids usually have a stem used to attach themselves to a surface, but many become free-swimming as adults.

Characteristics of Echinoderms

Echinoderms are marine organisms that make up the phylum Echinodermata. Members of the phylum include sea stars (starfish), sea urchins, sea cucumbers, sand dollars, brittle stars, and feather stars. There are 7000 different living species of echinoderms and 13,000 identified fossil species. Although the number of echinoderm species is very small compared to the number of species within the arthropod phylum, Echinodermata is the largest phylum to lack freshwater and terrestrial species. Another distinction is that echinoderms are among the most distinct organisms within the animal kingdom. You learned in the Evolution concepts that evolution is not a linear series of steps and does not necessarily lead to increasing levels of complexity over time. It is based on the response of populations of species to the adaptive pressures of their environments and favors those that have the greatest reproductive success. The phylum Echinodermata provides an excellent example of the non-linear, bush-like trajectory of evolutionary change. Echinoderms are the most closely related phylum to the phylum Chordata, which includes many complex organisms such as humans. Their shared common ancestor was likely a bilaterally symmetrical organism with a cephalized (centralized in a head region) nervous system. Yet, as you will learn in this lesson, many details of echinoderm structure and function are simpler than those of this likely predecessor. The organisms within this phylum evolved these structures as adaptations to the pressures placed on them by their environments, just as the

226 www.ck12.org Chapter 1. Invertebrates - Advanced various groups within the phylum Chordata did. You will see in this chapter that the results of these two evolutionary paths are very different, despite having a fairly recent common origin. Let’s take a look at some of the distinguishing features of echinoderms, and then we will examine their structures and how they function in more detail. The word Echinodermata means “spiny skin.” If you have ever seen a sea urchin, like the one shown in the Figure 1.138, then this name will certainly seem fitting. Although these spines may look like components of an exoskeleton at first glance, echinoderms do not have an exoskeleton. Instead, the spines are extensions of an internal endoskeleton. The endoskeleton is composed of calcium carbonate plates and spines that are actually covered by a thin layer of epidermis (skin). These plates and spines are held together by mesodermal tissue.

FIGURE 1.138 A spiny sea urchin species. Notice the many long spines that look like exoskele- tal projections. These spines are actually part of the echinoderm’s endoskeleton; a layer of epidermal tissue covers each spine.

As mentioned earlier, echinoderm species, such as those shown in the Figure 1.139, look very different from most other invertebrates. This is partly due to their symmetry and their spiny endoskeleton.

FIGURE 1.139 Various types of echinoderms. (a) Feather star. (b) Sea star (starfish). (c) Brittle star. (d) Sea urchin. (e) Sand dollars. (f) Sea cucumber.

Most of the phyla we have discussed so far (except sponges and cnidarians) exhibit bilateral symmetry, meaning that they can only be divided into two equal halves by a single cut along the middle of the anterior-posterior (front-back) axis. Although echinoderms evolved from a bilateral ancestor, adult echinoderms exhibit five-fold radial symmetry called pentameral radial symmetry, which is depicted in the Figure 1.140. This means that their bodies have five similar regions positioned around a central axis. Unlike the adults, echinoderm larvae, such as the one shown in the Figure 1.141, are always bilaterally symmetrical,

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FIGURE 1.140 A cross section of an apple, illustrat- ing pentameral symmetry. Pentameral symmetry in echinoderms refers to the arrangement of the body parts into five equal sectors around the mouth or central region, just like the seeds are arranged around the core of this apple.

reflecting the fact that radial symmetry evolved secondarily from a bilaterally symmetrical ancestor.

FIGURE 1.141 An echinoderm larva. The larval stages of echinoderms are always bilaterally sym- metrical, like the one shown here, even though adults are radially symmetrical. This reflects the fact that radial symmetry evolved secondarily in echinoderms from a bilaterally symmetrical ancestor.

One highly unique feature of echinoderms is an organized network of canals throughout the body called the water vascular system that functions in locomotion, feeding, excretion, and respiration. The water vascular system uses water pressure generated through muscle contractions to move the organism. We will look at this hydraulic system in more detail in the next section.

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Unlike mollusks, annelids, and arthropods, echinoderms are not protostomes. The species of the phylum Echinoder- mata and the phylum Chordata are all deuterostomes. In the Introduction to Animals and Invertebrates chapter you learned that there are a number of differences between protostomes and deuterostomes, all having to do with their patterns of embryonic development. These are where differences between the two lie:

• Pattern of the first several cell divisions. • When the fate of embryonic cells becomes fixed. • Formation of the coelom. • Formation of the mouth and anus.

In deuterostomes, the first several cell divisions are described as radial cleavage because they take place in parallel to or perpendicular to the axis of the embryo. Another feature of these early cells is that they do not have a set fate. For example, if one cell is removed from the embryo at this stage (8-cell stage), it is still capable of dividing to form all of the cells of an entirely new organism. This contrasts the early cell divisions in protostomes that produce cells with a fixed cell fate. If they are removed from the embryo, they will only be able to divide and differentiate into a few different cell types, so they are not capable of producing a new organism. Another difference between protostomes and deuterostomes is the origin of the mouth. In protostomes, the blastopore becomes the mouth. In deuterostomes, the blastopore becomes the anus, and another opening generated later in development produces the mouth. The blastopore is the first opening of the digestive cavity that is formed during the gastrulation stage of embryonic development, as shown in Figure 1.142.

FIGURE 1.142 Blastopore and coelom formation dur- ing gastrulation. In deuterostomes, the coelom begins to form as mesodermal pouches that fold in from the endoderm, and the blastopore eventually develops into the anus. In protostomes, the blasto- pore develops into the mouth. The deuterostome mouth originates as a sec- ond opening that forms later in develop- ment. Orange = ectoderm, red = endo- derm, and blue = mesoderm.

Finally, protostomes and deuterostomes each have different ways of forming a coelom, or body cavity. In pro- tostomes, the mesoderm (middle tissue layer) splits into two sheets of tissue, and the space between these sheets becomes the fluid-filled body cavity. Deuterostomes form a coelom by invaginating pockets of endodermal (internal tissue layer) tissue from the developing gut cavity formed during gastrulation (see the Figure 1.142). The tissue surrounding the pockets differentiates into mesodermal tissue, and the pockets become the coelom. The echinoderm coelom is well-developed, and it forms two major cavities during development. One cavity becomes the actual coelom that houses internal organs, while the other develops into the water vascular system. In the next section we will take a closer look at the water vascular system, the endoskeleton, and other structural features of echinoderms.

Vocabulary

• blastopore: The opening of an embryo’s central cavity in the early stage of development.

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• coelom: A fluid-filled cavity formed within the mesoderm; it forms between the digestive cavity and the body wall.

• deuterostome: An animal in which the first opening formed during development (the blastopore) becomes the anus.

• endoskeleton: An internal skeleton that provides support and protection.

• gastrulation: The development of different layers of cells in an embryo; in humans, this generally occurs during the second week after fertilization.

• pentameral radial symmetry: Having the five-radial plan of symmetry, in which the body or organ consists of five subequal segments, as seen in the echinoderms, certain corals, and jellyfishes.

• water vascular system: A system of fluid-filled tubes used by echinoderms in locomotion, feeding, and respiration.

Summary

• Members of the phylum Echinodermata include sea stars (starfish), sea urchins, sea cucumbers, sand dollars, brittle stars, and feather stars. • Echinoderms are the most closely related phylum to the phylum Chordata, which includes many complex organisms such as humans. • Echinoderms are deuterostomes that exhibit pentameral radial symmetry. • The water vascular system, used for locomotion, is unique to echinoderms.

Practice

Use this resource to answer the questions that follow.

• Echinodermata at http://tolweb.org/Echinodermata .

1. When did echinoderms first appear in the fossil record? 2. What is the endoskeleton of echinoderms made of? 3. What four features distinguish echinoderms from other animals?

Practice Answers

1. Echinoderms definitely appeared in the fossil record during the mid-Cambrian. An animal named Arkarua could have been one of the first echinoderms, and it appeared in the Vendian period. 2. The endoskeleton of echinoderms is composed of calcium carbonate and several proteins. 3. Echinoderms have a calcitic endoskeleton, a water vascular system, mutable collagenous tissue, and pentara- dial body organization as adults.

Review

1. Although there are not as many echinoderm species as there are arthropods, the echinoderms are still the largest phylum in which environment?

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2. Echinoderms are closely related to the phylum Chordata, which includes humans, even though they look more closely related to cnidarians. What is one clue that support the theory that echinoderms are related to chordates? 3. How does early cell development differ between protostomes and deuterostomes? 4. How does the coelom develop in deuterostomes? 5. Unlike other animals, the coelom of echinoderms forms two cavities. What are these two cavities for?

Review Answers

1. Echinodermata is the largest phylum to lack freshwater and terrestrial species; they live exclusively in marine environments. 2. Echinoderm larvae are always bilaterally symmetrical, suggesting that radial symmetry evolved secondarily from a bilaterally symmetric ancestor. The shared common ancestor of echinoderms and chordates was likely a bilaterally symmetrical organism with a cephalized (centralized in a head region) nervous system. 3. The early cells of deuterstomes do not have a set fate. For example, if one cell is removed from the embryo at the 8-cell stage, it is still capable of dividing to form all of the cells of an entirely new organism. This contrasts the early cell divisions in protostomes that produce cells with a fixed cell fate. If they are removed from the embryo, they will only be able to divide and differentiate into a few different cell types, so they are not capable of producing a new organism. 4. Deuterostomes form a coelom by invaginating pockets of endodermal tissue from the developing gut cavity formed during gastrulation. The tissue surrounding the pockets differentiates into mesodermal tissue, and the pockets become the coelom. 5. One cavity becomes the actual coelom that houses internal organs, while the other develops into the water vascular system that is unique to echinoderms.

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1.43 Echinoderm Structure and Function - Ad- vanced

• Understand the different organ systems present in most echinoderms.

An animal? Really? Yes. This is a feather star, one of 550 species of crinoids. Crinoids are marine animals that make up the class Crinoidea of the echinoderms. Feather stars use their grasping “legs” to perch on sponges, corals (as shown here), or other surfaces and feed on drifting microorganisms, trapping them in their sticky arm grooves.

Structure and Function in Echinoderms

Echinoderms have a distinct endoskeleton, an unusual organ system called the water vascular system, and a number of other less unique features. How is the endoskeleton organized? What are the components of the water vascular system, and how do they function to move the organism? This section will answer these questions and will examine the other organ systems found in echinoderms.

Endoskeleton

In the Echinoderms: Characteristics (Advanced) concept, you read that the echinoderm endoskeleton consists of a meshwork of plates and spines connected by mesodermal tissue. The plates are called ossicles. The organization of the ossicles that compose the endoskeleton of a sea star is shown in the Figure 1.143. The ossicles are made up of microscopic networks of calcium carbonate crystals that form a unique structure referred to as the stereom. The ossicles may be tightly packed together, as they are in sea urchins, or they may be more loosely connected, as they are in sea stars. The same is true for the spines. Sea urchin spines are often quite loosely attached (despite the tight packing of the ossicles), and it is not uncommon to find a sea urchin alive and well but lacking most of its spines. There are several specialized aspects of the echinoderm endoskeleton that vary between species. One example is the presence of pedicellariae on species of sea stars and sea urchins. Pedicellariae, shown in the Figure 1.144, are spines modified as pincer-like structures that can be used to thwart predators. Another example of a modified endoskeletal feature is a feeding organ called Aristotle’s lantern (discussed in the Echinoderms: Classification (Advanced) concept), which are found in sea urchin species.

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FIGURE 1.143 An X-ray of a sea star, showing the en- doskeleton. Notice the many tiny plates (ossicles) that are arranged in a clear pat- tern throughout the body of the organism.

Water Vascular System

The water vascular system of echinoderms is essentially a system of fluid-filled canals that extend along each of the body regions and have many external projections called tube feet. There are several functions of the system, one of which is to use water pressure to mediate movement and assist in feeding. It also carries out respiratory, excretory, and some circulatory functions within the animal. The individual components of the water vascular system are the following:

• Madreporite. • Stone canal. • Circular ring canal. • Radial canals. • Lateral canals. • Tube feet.

What does each of these components do, and how do they contribute to the functioning of the water vascular system? The madreporite is a sieve-like, calcified plate that connects the system to the aquatic environment. Water enters through the madreporite and flows through a tube, called the stone canal, that connects to the circular ring canal surrounding the mouth. From there, water can flow into five radial canals that branch off the circular ring. The radial canals each extend into a different ray, or arm, of the organism along a groove called the ambulacral groove. Numerous lateral canals, leading to rows of tube feet on either side of the ambulacral groove, branch from each side of the radial canals. The organization of the components of the water vascular system is shown in the Figure 1.145. The tube feet extend between the endoskeletal plates to reach outside of the organism. They are basically thin-walled cylinders with muscular bulb-shaped structures called ampullae on the internal end and suckered structures called podia on the external ends. The tubular canals that make up the water vascular system are lined with cilia. Cilia are small hair-like, cellular projections that beat back and forth repeatedly to help maintain water flow through the canals. A diagram showing the water vascular system in the context of an echinoderm body is shown in the Figure 1.146. So how do tube feet work to move the animal? When the muscles of the ampullae contract, water is forced into the suckered podia, which then extend outward. As the podia stretch, they can use their suckers to attach to a location farther away from their previous point of attachment. This results in a slow but powerful form of movement.

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FIGURE 1.144 The pedicellariae found on sea stars and sea urchins. Pedicellariae are modified spines that have a pincer-like structure at one end, as shown in this drawing. They may be used for self-defense against predators. The pedicellariae is shown both closed (left) and open (right).

Extended tube feet can also be used to generate small waves of water current toward the mouth region to assist in food collection. A close-up view of the tube feet of an echinoderm is shown in the Figure 1.147. In addition to the mechanical work it conducts, the water vascular system also provides respiratory and excretory functions. Most echinoderms do not have respiratory or excretory organs, so the thin walls of the tube feet serve this purpose by allowing oxygen to diffuse in and waste to diffuse out. When discussing the bilaterally symmetrical phyla, we have been able to use terms such as anterior, posterior, dorsal, and ventral to refer to the front, back, top, and bottom of the animal. With radially symmetrical animals, such as

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FIGURE 1.145 The components of the water vascular system of echinoderms. Water enters the system through the madreporite, flows through the stone canal, and enters the ring canal. A number of bulbous struc- tures, called polian vesicles, that branch off of the ring canal serve as reservoirs to maintain extra water stores. From the ring canal, water enters the radial canals that extend into each of the arms, or rays. The lateral canals branch from the radial canal and bring the water into the ampullae and podia of the tube feet.

FIGURE 1.146 A diagram of a cross-sectioned sea star arm/ray and central disk.

echinoderms, it is more difficult to specify which region of the animal you are talking about. Generally, the surface of the animal with the mouth opening is called the oral surface, and the one without the mouth opening is called the aboral surface. The madreporite opens into the aboral surface. Because the madreporite is one of the few non-repeating structures of echinoderms and is not directly in the center of the animal, it is used as an orientation point to distinguish between the various radial projections (arms/rays) of echinoderms. For example, echinoderm rays are named A through E in a counter-clockwise direction, and A is defined as the ray opposite the position of the madreporite. This is illustrated in the Figure 1.148.

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FIGURE 1.147 The tube feet of a sea star. If you look closely, you can see the suckered podia at the end of the tube feet protruding from the bottom (or oral) surface of the animal.

FIGURE 1.148 Describing the body parts of an echino- derm. The rays, or units of pentameral symmetry, of an echinoderm are labeled A through E. The ray located opposite the madreporite is called A, and subsequent rays are labeled B, C, D, and E in a counterclockwise direction.

Other Organ Systems

Almost all echinoderms have a simple but complete digestive tract. Although most species lack respiratory and excretory organs, echinoderms do have a circulatory system. They generally have an open circulatory system but lack a distinct heart. In an open circulatory system, circulating blood is not entirely contained within blood vessels. The nervous system is not centralized and usually consists of a network of interconnected nerve cells distributed throughout the body. There is a nerve ring surrounding the mouth in the central region and nerves that extend from the ring into each arm. Echinoderms are able to sense chemical and physical stimulation using sensory cells located on the surface of their bodies. Some of these cells have chemoreceptors that bind to molecules called pheromones. Individuals within a species secrete pheromones that bind to the receptors found on other individuals as a means of communication. One example of this is the signaling of metamorphosis. Pheromones secreted by a cluster of adult echinoderms may indicate to larvae swimming nearby that they have found a good place to settle. The larvae sense the pheromone and respond by settling and initiating the process of metamorphosis (changing from a free-swimming

236 www.ck12.org Chapter 1. Invertebrates - Advanced larval form to a more sedentary adult form). In the next section we will examine the development of echinoderms and the nature of their metamorphic changes.

Reproduction and Development

Most echinoderms reproduce by sexual reproduction through the fusion of sperm and eggs. They generally have separate sexes, and fertilization is usually external. Sea cucumbers, for example, release sperm and eggs into the open ocean where they may come in contact and undergo fertilization. The majority of echinoderm species undergo indirect development with a free-swimming bilaterally symmetrical larval stage. The bilateral symmetry of the larvae is strikingly different from the radial symmetry of the adult stage. Larvae undergo a process called metamorphosis, in which the organization of the body shifts from bilateral to radial symmetry. Some species, however, undergo direct development, where the young are simply smaller, sexually immature versions of their parents. There are also a few species, particularly among those living in extreme environments such as polar regions, that exhibit parental care behavior. They nurture the young until they are old enough to fend for themselves. Some species of sea stars and brittle stars are also capable of asexual reproduction by fission. In this process, the animal splits itself into two parts and each part regenerates the missing regions to produce a complete individual. Echinoderms have the ability to regenerate a missing arm, or ray. The limb may be lost as the result of a bite from a predator, or in some cases it is purposely released by the echinoderm in a process called self-amputation, or autotomy. This is usually done to escape a predator that has a hold of that particular limb. Most echinoderms can only undergo regeneration if a certain portion of the animal remains intact, usually including the central disk region. However, sea stars have quite extensive powers of regeneration, and they can often produce an entire organism from a small segment such as an arm. A sea star in the process of regenerating a missing limb is shown in the Figure 1.149.

FIGURE 1.149 A sea star in the process of regenerating a missing limb. Echinoderms, such as this sea star, have the amazing ability to regenerate lost limbs, and, in some cases, almost the entire body can be regenerated from one limb.

Vocabulary

• aboral surface: The front side of the starfish on the opposite side of the mouth.

• ambulacral groove: One of the five grooves arranged as a star, which are found in starfish, echinoids, crinoids, and related animals; they lead to the mouth, serving as a passage down for food.

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• ampullae: Thin-walled cylinders with muscular bulb-shaped structures that make up the tube feet of echino- derms.

• Aristotle’s lantern: The five united jaws and accessory ossicles of certain sea urchins.

• autotomy: The spontaneous casting off of a limb or other body part, especially when the organism is injured or under attack.

• cilia (sing., cilium): Short hairlike projection, similar to flagella, that allows some cells to move.

• oral surface: The side of the starfish with the mouth.

• ossicles: Small pieces of calcified material that form part of the skeleton of an invertebrate animal such as an echinoderm.

• pedicellariae: A defensive organ like a minute pincer present in large numbers on an echinoderm.

• pheromones: One of many chemicals, secreted by special glands, that trigger responses in other organisms.

• podia: Suckered structures at the end of tube feet.

• regenerate: To replace (a lost or damaged organ or part) by formation of new tissue.

• stereom: A microscopic networks of calcium carbonate crystals that form ossicles.

Summary

• The endoskeleton of echinoderms is composed of plates, known as ossicles, that are formed from networks of calcium carbonate crystals called the stereom. • The water vascular system of echinoderms is essentially a system of fluid-filled canals that extends along each of the body regions and has many external projections called tube feet. • Although most species lack respiratory and excretory organs, echinoderms do have a circulatory system and a digestive system. They generally have an open circulatory system and a complete digestive system. • Most echinoderms reproduce by sexual reproduction through the fusion of sperm and eggs. They generally have separate sexes, and fertilization is usually external.

Practice

Use this resource to answer the questions that follow.

• Echinoderms at http://www.mesa.edu.au/echinoderms/default.asp .

1. Describe the digestive system of a typical echinoderm. How do sea stars use their stomachs during feeding? 2. What do echinoderms usually feed on?

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Practice Answers

1. Echinoderms have a simple digestive system composed of a mouth, stomach, intestines, and anus. Sea stars can push their stomachs out of their bodies during feeding, engulfing their prey. 2. Crinoids and some brittle stars are passive filter-feeders, whereas sea stars are predators that hunt or eat decomposing organisms. Sea urchins are grazing herbivores, and sea cucumbers are deposit feeders, removing food particles from sand or mud.

Review

1. How do echinoderms use their water vascular system to move? 2. What kind of functions does the water vascular system have other than movement? 3. Describe the nervous system of an echinoderm. 4. What role do pheromones play in the metamorphosis of certain echinoderms? 5. How well can echinoderms regenerate?

Review Answers

1. When the muscles of the ampullae contract, water is forced into the suckered podia, which then extend outward. As the podia stretch, they can use their suckers to attach to a location farther away from their previous point of attachment. This results in a slow but powerful form of movement. 2. Most echinoderms do not have respiratory or excretory organs, so the thin walls of the tube feet serve this purpose by allowing oxygen to diffuse in and waste to diffuse out. The water vascular system also assists in feeding. 3. The nervous system is not centralized and usually consists of a network of interconnected nerve cells dis- tributed throughout the body. There is a nerve ring surrounding the mouth in the central region and nerves that extend from the ring into each arm. Echinoderms do not have brains. 4. Pheromones secreted by a cluster of adult echinoderms may indicate to larvae swimming nearby that they have found a good place to settle. The larvae sense the pheromones and respond by settling and initiating the process of metamorphosis. 5. Most echinoderms can only undergo regeneration if a certain portion of the animal remains intact, usually including the central disk region. However, sea stars have quite extensive powers of regeneration, and they can often produce an entire organism from a small segment such as an arm.

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1.44 Echinoderm Classification - Advanced

• Understand the five classes that make up living echinoderms. • Explore the main differences between each class of echinoderm.

This is obviously a sea cucumber. So classifying echinoderms can’t be that difficult, can it? Actually, it is not. Sea cucumbers are marine animals with leathery skin and an elongated body containing a single branched gonad. Sea cucumbers are found on the sea floor worldwide. These are quite different from the other echinoderms. In fact, all echinoderms easily fall into one of five classes.

Classification of Echinoderms

Unlike arthropods, echinoderm classification is not currently undergoing any drastic changes, and essentially all existing species fall into five clearly defined classes. Classifications that include all of the fossil species are somewhat more complex and can include 25 different classes. The five classes of existing echinoderms are these:

• Crinoidea. • Asteroidea. • Ophiuroidea. • Echinoidea. • Holothuroidea.

Echinoderm species are generally classified into each of these groups based on their body forms and organization. Differences between the classes can also be found in various features of the endoskeleton and the organ systems. In this section we will consider the similarities and differences between each of these groups. Organisms in each of the five classes are described in the Table 1.12.

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TABLE 1.12:

Class (includes) Description Example Crinoidea Fewer than 100 species exist. Many feather star have more than five arms. They • Feathers stars are the earliest and most primitive • Sea lilies echinoderms. They live on the ocean floor in mainly deep water and are filter feeders.

Asteroidea Almost 2,000 species exist. Most sea star have five arms. Many are brightly • Sea stars colored. They live on the ocean floor in mainly shallow water and are predators or scavengers.

Ophiuroidea About 2,000 species exist. The cen- brittle star tral disk is distinct from the arms. • Brittle stars They move by flapping their arms, which lack suckers. They live on the ocean floor in shallow or deep water and are predators, scavengers, deposit feeders, or filter feeders. Echinoidea About 100 species exist. They do sea urchin not have arms but do have tube • Sea urchins feet. They have a specialized mouth • Sand dollars part with teeth to scrape food from • Sea biscuits rocks. They live on the ocean floor • Heart urchins in shallow or deep water and are predators, herbivores, or filter feed- ers. Holothuroidea About 1,000 species exist. They sea cucumber have long bodies without arms. Un- • Sea cucumbers like other echinoderms, they have a respiratory system. They live on the ocean floor in shallow or deep wa- ter and are deposit feeders or filter feeders.

Crinoidea

The crinoids, such as the sea lilies and feather stars, are considered to be the most primitive of the echinoderm species. They have five-fold radial symmetry, but they often have more than five arms. There are about 80 crinoid species, and many of them live in deep regions of the ocean, so they tend to be less familiar than echinoderms that are commonly found in shallow coastal regions. Although there are only a few species around today, crinoids make up a large fraction of the echinoderm fossil record. They were dominant in the Paleozoic era (540 - 250 million years ago) but experienced an almost complete extinction at the end of the Permian period (250 million years ago). An example of a feather star is shown in the Figure 1.150. The sea lilies are considered “stalked” crinoids because they have an extended stalk that attaches to solid substrates on the ocean floor. The sea lilies are the only existing echinoderms that live attached to a solid surface. Feather stars

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FIGURE 1.150 A crinoid feather star.

are unstalked crinoids that look much like sea stars of the class Asteroidea. The main difference between feather stars and sea stars is that the mouth of a feather star is situated on the “top” of the body, whereas the sea star mouth is generally on the “bottom” side. They are able to move by a slow walking motion, or they can swim by waving their arms to generate water currents. Both sea lilies and feather stars have a distinct body region called the calyx. The calyx is a cup-like structure that lies just below the radiating arms of the animal and contains the digestive system. Crinoids are filter feeders, and they trap food using a sticky mucus substance secreted by their tube feet.

Asteroidea

The class Asteroidea contains the most well known echinoderms: the sea stars that are also often called starfish. There are roughly 1800 living species of sea stars, and many are brightly and beautifully colored such as those shown in the Figure 1.151.

FIGURE 1.151 Colorful species of sea stars. (a) A sea star found in the Pacific Ocean. (b), (c), and (d) Various sea star species found in Tanzania.

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Most sea stars have five arms, or rays, that radiate out from a central disk, although some species have more than five and some have fewer. The mouth is located on the bottom surface of the animal. One unusual feature of sea stars is that they have two stomachs: a cardiac stomach and a pyloric stomach. Sea stars are generally predators that hunt and feed on small, shelled invertebrates, many of which are too large to be taken into the sea star’s mouth. The cardiac stomach allows them to get around this problem because it can be everted out of their bodies to surround prey and begin digestion externally. Once the food source is partially digested, the cardiac stomach is brought back into the organism, bringing the digested material with it. This material is then transferred to the pyloric stomach, which continues the digestion process internally. The internal anatomy of a sea star showing the relative position of the pyloric and cardiac stomachs is shown in the Figure 1.152.

FIGURE 1.152 The internal anatomy of an asteroid. 1 = pyloric stomach, 2 = anus, 3 = rectal gland, 4 = stone canal, 5 = madreporite, 6 and 7 = digestive system extensions into the ray (pyloric duct, pyloric cecum), 8 = cardiac stomach, 9 = gonad, 10 = ambulacral plates, 11 = ampullae.

How does an organism that moves extremely slowly and does not have a centralized nervous system hunt? Luckily, the shelled invertebrates, such as mollusks, that sea stars feed on are also not speedy movers. The water vascular system is also useful for predatory feeding. When a sea star encounters a mussel, for example, the powerful force of the water vascular system can actually enable the tube feet to pry open the bivalve exoskeleton. The Figure 1.153 shows two different photos of sea stars in the process of eating a mussel.

FIGURE 1.153 Sea stars feeding on mollusks. Notice the tube feet in the process of prying open the mussel shell in (b).

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The nervous system of asteroids is similar to that of most echinoderms that we examined earlier in this lesson. Sea stars also have simple eyes, called ocelli, that are located at the end of each arm and can sense light.

Ophiuroidea

Brittle stars make up the class Ophiuroidea. There are 2000 species of brittle stars, and they are closely related and very similar to the sea stars of the class Asteroidea. One difference is that the central disk region of their bodies is more defined and is distinctly separated from the arms. An example of a brittle star is shown in the Figure 1.154.

FIGURE 1.154 A brittle star. Notice the long, thin rays and the distinct separation between the rays and the central disk. Unlike the rays of sea stars, there are very few internal organs within the brittle star’s rays.

The central disk region houses all of the internal organs of the animal, freeing up the arms for use in locomotion. Unlike sea stars, the brittle stars do not move using their tube feet. In fact, their feet lack both the bulb-like ampullae and the suckers of the podia. Instead, brittle stars move by flapping their arms and using them to carry out a form of “walking.” The arms of brittle stars tend to be longer, thinner, and more solid than those of asteroids. The ossicles in the arms of brittle stars are large and take up most of the internal space. Another distinction between brittle stars and sea stars is their habitat. Most brittle star species are very unfamiliar to us because they predominantly inhabit deep regions of the ocean. Brittle stars also differ from most echinoderms in that they do not have a complete digestive system. They have a mouth, esophagus, and stomach but no anus. Waste is expelled through the mouth. The ossicles of brittle stars are often fused to form larger solid plates of endoskeleton. The endoskeleton as a whole is referred to as a “test.” This is a feature shared with the sea urchins of the Echinoidea class that we will discuss next.

Echinoidea

Echinoidea is made up of the roughly 900 species of sea urchins, sand dollars, sea biscuits, and heart urchins. Unlike the three classes discussed so far (crinoids, asteroids, and ophiuroids), echinoids do not have arms, or rays, projecting from a central region of the body. They still have pentameral symmetry with five rows of tube feet that extend from the top (aboral) to the bottom (oral) surface, surrounding a central axis. The endoskeletal ossicles of echinoids overlap and are fused to form an oval globe (sea urchins) or a flattened oval disk (sand dollars). Like the endoskeleton of brittle stars, the fused endoskeletal formations of echinoids are also called “tests.” An example of a sea urchin test is shown in the Figure 1.155.

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FIGURE 1.155 A sea urchin test. The endoskeleton of a sea urchin is composed of fused ossicles that form a solid ovoid called a test. The lighter, paired bands extending from the “top” to the “bottom” of the test are the rows of tube feet. There are five pairs of bands positioned around a central axis.

One of the most distinct features of echinoids is an organ found in the mouth region of sea urchins called Aristotle’s lantern. This modification of the endoskeleton is shown in the Figure 1.156. It consists of five teeth that function to chew and scrape food, such as algae, from solid rock surfaces. Another endoskeletal distinction is that the spines of echinoids are generally more prominent than in other echinoderms.

FIGURE 1.156 Sea urchin mouth regions showing Aris- totle’s lantern. A striking modification of the sea urchin endoskeleton is a jaw-like mouthpart, called Aristotle’s lantern, that contains five teeth. This allows the animal to consume algae by scraping it off of solid surfaces like rocks and reefs. (a) Aristotle’s lantern viewed from the inside of a sea urchin test. The teeth projecting into the mouth opening are underneath the lantern. (b) The mouth of a sea urchin, showing the small teeth projecting from the center.

The internal organs of echinoids are similar to those of other echinoderms, with the exception of the gonads. The gonads are particularly large and numerous in most echinoids. Sea urchins, for example, generally have five gonads that take up a large fraction of the internal volume of the organism, as shown in the Figure 1.157. This feature is shared with many holothurians.

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FIGURE 1.157 Sea urchin gonads. Notice how the large gonads take up a considerable amount of space within the body of the animal.

Holothuroidea

The holothurians, or the sea cucumbers (see http://www.tolweb.org/Holothuroidea/19240 for more info), are the most distinct class of echinoderms. Most echinoderms assume a position with the oral surface (the side having the mouth) facing down, (except in crinoids) while they remain in contact with the substrate on which they reside. The aboral surface is considered the surface opposite the one housing the mouth, and this is usually the top of the animal. Holothurians differ in this respect. As you can see from the Figure 1.158, sea cucumbers are elongated along the oral-aboral axis, and they lie on their sides with the oral surface at one end and the aboral surface at the other end of the cylindrical body.

FIGURE 1.158 A diagram of a holothurian body. a - Tentacles. b –Anal opening. c –Tube feet on the ventral side. d - Papillae on the back. Notice that the body of the animal is elongated through the axis between the oral (mouth) surface and the aboral (opposite the mouth) surface.

The result of this difference is that, superficially, they appear to have a bilaterally symmetrical, worm-like body. But if you examine them more closely, the radial symmetry becomes apparent. They still have five-fold symmetry with five rows of tube feet extending along the oral-aboral axis. There are around 1100 species of sea cucumbers, and they range in size from a few centimeters to one meter long. The madreporite of most echinoderms connects the water vascular system to the external aquatic environment, but in the holothurians it connects to the coelomic cavity and takes in coelomic fluid instead of seawater. Otherwise, the function of the water vascular system is similar. Holothurians are also unique among echinoderms in that they have a respiratory system. The respiratory trees are the center of the respiratory system. They branch from the anal region and exchange gases with water that they draw in through the anus. Another distinctive feature of some sea

246 www.ck12.org Chapter 1. Invertebrates - Advanced cucumber species, which is related to the respiratory tree, is a defensive mechanism based on structures that branch from the base of the respiratory tree in the anal region, called the Cuvierian tubules. This is a system of sticky, branched tubules that are extruded to entangle predators when the animal feels threatened. A sea cucumber releasing its Cuvierian tubules is shown in the Figure 1.159. When they are extruded, they are released from the sea cucumber along with part of the intestine. These organs are regenerated within a few weeks. Holothurians are mostly deposit feeders, and the mouth region is surrounded by tentacles that are used to sift through sediment at the bottom of the ocean to collect food particles.

FIGURE 1.159 Cuvierian tubules. This photograph shows a sea cucumber releasing sticky Cuvierian tubules as a defensive mecha- nism. Part of the intestine is released with these tubules and must be regenerated following this expulsion.

Vocabulary

• Aristotle’s lantern: The five-part jaws and accessory ossicles of certain sea urchins.

• calyx: The outer (usually green) whorl of the perianth of a flower, comprised of sepals; it often functions to protect the developing bud; a cup-like structure that lies just below the radiating arms of sea lilies and feather stars and contains the digestive system.

• cardiac stomach: One stomach of a sea star that is everted out of their bodies to surround prey and begin digestion externally.

• Cuvierian tubules: Clusters of sticky tubules, located at the base of the respiratory tree, which may be discharged by some sea cucumbers (holothurians) when mechanically stimulated, as for example, when being threatened by a predator.

• pyloric stomach: One stomach of a sea star that continues digestion after the cardiac stomach brings partially digested material inside.

• stalked: A description for sea lilies, which extend stalks that attach to solid substrates on the ocean floor.

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• test: Large solid plates of endoskeleton found on brittle stars and echinoids; the skeleton used to protect diatoms and amoeboid protists; it is made of minerals such as calcium carbonate or silica.

Summary

• Unlike arthropods, echinoderm classification is not currently undergoing any drastic changes, and essentially all existing species fall into five clearly defined classes. • The crinoids, such as the sea lilies and feather stars, are considered to be the most primitive (earliest to evolve) of the echinoderm species. • The class Asteroidea contains the most well known echinoderms: the sea stars that are also often called starfish. • Brittle stars make up the class Ophiuroidea. There are 2000 species of brittle stars, and they are closely related and very similar to the sea stars of the class Asteroidea. • Echinoidea is made up of the roughly 900 species of sea urchins, sand dollars, sea biscuits, and heart urchins. • Sea cucumbers are elongated along the oral-aboral axis, and they lie on their sides with the oral surface at one end and the aboral surface at the other end of the cylindrical body.

Practice

Use this resource to answer the questions that follow.

• Echinoderms: The Spiny Animals at http://www.oceanicresearch.org/education/wonders/echinoderm.html .

1. How do sea stars obtain oxygen? 2. Describe how sea stars are able to hunt bivalves. 3. What is the test of sea urchins? 4. How does the water vascular system of sea cucumbers differ from other echinoderms? 5. How do sea cucumbers feed?

Practice Answers

1. Sea stars have bumps on their surface called Dermal Branchiae, which are used to absorb oxygen from the water. 2. When a sea star tries to eat a mussel, it surrounds the mussel and applies outward force with its tube feet, pulling the shells apart. It then expels its cardiac stomach and starts digesting. 3. The test is an egg-like, spherical structure constructed of rows of radially arranged plates fused together. This is where the spines of a sea urchin join the rest of the skeleton. 4. The water vascular system of echinoderms is not filled with sea water like other echinoderms, so there is no direct interface between the outside water and the internal organs. 5. Some sea cucumbers stick out their tentacles to feed, then retract one tentacle at a time and lick them off. Other sea cucumbers use their tentacles to sift through sand for food.

Review

1. What group of echinoderms is the only one that lives attached to the sea floor? 2. A starfish actually has two stomachs. What is the purpose of having two stomachs? 3. What distinguishes brittle stars from starfish? How do they move? 4. What is the function of Aristotle’s lantern, and which class of echinoderms has this special organ? 5. How do sea cucumbers defend themselves against predators?

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Review Answers

1. The sea lilies are the only existing echinoderms that live attached to a solid surface. All other echinoderms are mobile. 2. Sea stars are generally predators that hunt and feed on small shelled invertebrates, many of which are too large to be taken into the sea star’s mouth. The cardiac stomach allows them to get around this problem because it can be everted out of their bodies to surround prey and begin digestion externally. Once the food source is partially digested, the cardiac stomach is brought back into the organism, bringing the digested material with it. This material is then transferred to the pyloric stomach, which continues the digestion process internally. 3. One difference is that the central disk region of brittle stars is more defined and is distinctly separated from the arms. Unlike sea stars, the brittle stars do not move using their tube feet. In fact, their feet lack both the bulb-like ampullae and the suckers of the podia. Instead, brittle stars move by flapping their arms and using them to carry out a form of “walking.” 4. Echinoids have a special organ found in the mouth region of sea urchins called Aristotle’s lantern. It consists of five teeth that function to chew and scrape food, such as algae, from solid rock surfaces. 5. A distinctive feature of some sea cucumber species is a defensive mechanism based on structures that branch from the base of the respiratory tree in the anal region, called the Cuvierian tubules. This is a system of sticky, branched tubules that are extruded to entangle predators when the animal feels threatened.

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1.45 Echinoderm Ecology - Advanced

• Understand the ecological niches that echinoderms occupy. • Learn about the impact of echinoderms on human life.

What do echinoderms eat? All sorts of sea life. Here, a Crown-of-thorns seastar (Acanthaster planci) feeds on live table coral polyps on a shallow reef near Fiji. When population numbers are high, these echinoderms can destroy reefs. The Crown-of- thorns seastar is one of the largest sea stars in the world, ranging in size from 10 to 14 inches (25 to 35 cm) and having up to 21 arms.

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Ecology of Echinoderms

All echinoderm species are marine organisms. Echinoderms can be found in most regions of all oceans from the poles to the equator. For a video of sea stars that inhabit the waters near Antarctica, see http://www.youtube.com/w atch?v=HG17TsgV_qI (2:53).

MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/139358

Echinoderms inhabit both shallow, coastal regions and deep-sea regions. Most species are fairly slow moving bottom-dwellers (benthic) as adults, although the larval stages are usually free-swimming. Echinoderm feeding depends on the class and species, but it can include filter feeders that collect food particles filtered from seawater, deposit feeders that sift through sediments at the bottom of the ocean to collect food particles, predators, and scavengers. Generally, crinoids are filter feeders, asteroids and ophiuroids are predators or scavengers (although ophiuroids can also be deposit or filter feeders), echinoids are predators, herbivores, or filter feeders, and holothurians are either filter or deposit feeders. A video of sea urchins feeding can be found at http://www.youtube.com/watch ?v=D3W4OCnHyCs (3:50).

MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/139359

Some echinoderms also engage in symbiotic relationships. For example, there are some polychaete species (annelids, Polychaetae) that live on asteroids and feed on stray food particles that do not make it into the sea star’s mouth. Other polychaetes and some crab species live inside the anal region of sea cucumbers and feed on food carried along with the water that is drawn in for respiration by the sea cucumber. The various feeding habits of species within each echinoderm class are listed in Table 1. TABLE 1.13: Feeding Habits of Species Found in Echinoderm Classes

Class Feeding Crinoids Filter feeders Asteroids

Predators

Scavengers

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TABLE 1.13: (continued)

Class Feeding Ophiuroids

Predators

Scavengers

Deposit feeders

Filter feeders

Echinoids

Predators

Herbivores

Filter Feeders

Holothurians

Deposit Feeders

Filter Feeders

Echinoderms play an important role in the ecosystems they inhabit by providing a source of food for many species. Sea urchins, for example, are the primary food source of otters.

Economic and Biological Importance

The role of echinoderms in our economy is mainly as a food source. There is also a small market for the calcified endoskeletons that are used by farmers as a source of lime. In several countries, including Japan, France, China, and Indonesia, sea urchins and sea cucumbers are eaten as food. The eggs and gonads of sea urchins are considered by some to be quite delicious. The field of embryology, the study of how embryos develop, has relied heavily on the contributions of sea urchins. Large pools of eggs and sperm can be collected fairly easily from a living sea urchin, and they can be fertilized in vitro, or outside of the living organism, under a microscope. This allows scientists to observe the entire process of fertilization and embryonic development in great detail. Several molecular advances in sea urchin research have been very helpful to these efforts. One such advance is the discovery of a tool called a morpholino. Morpholinos are synthetic molecules that can be designed to block the function of a specific gene in a cell. The morpholinos can be injected into the eggs, and then the effect of losing that particular gene function on embryonic development can be observed. Another important advance was determining the complete genomic DNA sequence of a species of purple sea urchin. This work was completed in 2006, and it showed that 70% of the genes in a sea urchin have a corresponding human gene. Therefore, the knowledge gained about sea urchin embryonic development is often directly applicable to the development of human embryos.

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Vocabulary

• embryology: A branch of comparative anatomy that studies the development of vertebrate animals before birth or hatching.

• morpholino: A type of molecule used in altering the development of genes by preventing the access of other molecules.

• polychaetae: A class of annelids characterized by their bristles.

Summary

• All echinoderm species are marine organisms. • Echinoderms can be found in most regions of all oceans from the poles to the equator. They inhabit both shallow, coastal regions and deep-sea regions. • The role of echinoderms in our economy is mainly as a food source. • The field of embryology, the study of how embryos develop, has relied heavily on the contributions of sea urchins.

Practice

Use this resource to answer the questions that follow.

• Echinoderms at http://animal.discovery.com/marine-life/echinoderm-info.htm .

1. Where do many echinoderms live? 2. How has human activity impacted echinoderms? 3. Which echinoderms are usually eaten?

Practice Answers

1. Many echinoderms live in the shallow intertidal zone close to shore. Others live at deeper depths in the ocean. 2. Echinoderms need clean water in order to survive. Pollutants released by humans kill echinoderms. 3. People eat sea urchins and sea cucumbers. There is a threat of overharvesting these species of echinoderms.

Review

1. What kinds of environments do echinoderms inhabit? 2. How do echinoderms feed? 3. Why are sea urchins particularly useful in the field of embryology? 4. What are morpholinos? 5. What organisms form symbiotic relationships with echinoderms?

Review Answers

1. Echinoderms can be found in most regions of all oceans from the poles to the equator. They inhabit both shallow, coastal regions and deep-sea regions.

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2. Generally, crinoids are filter feeders, asteroids and ophiuroids are predators or scavengers (although ophiuroids can also be deposit or filter feeders), echinoids are predators, herbivores, or filter feeders, and holothurians are either filter or deposit feeders. 3. Large pools of eggs and sperm can be collected fairly easily from a living sea urchin, and they can be fertilized in vitro, or outside of the living organism, under a microscope. This allows scientists to observe the entire process of fertilization and embryonic development in great detail. 4. Morpholinos are synthetic molecules that can be designed to block the function of a specific gene in a cell. The morpholinos can be injected into the eggs, and then the effect of losing that particular gene function on embryonic development can be observed. 5. There are some polychaete species that live on asteroids and feed on stray food particles that do not make it into the sea star’s mouth. Other polychaetes and some crab species live inside the anal region of sea cucumbers and feed on food carried along with the water that is drawn in for respiration by the sea cucumber.

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1.46 Nonvertebrate Chordates - Advanced

• Describe the characteristics of nonvertebrate chordates.

Would you believe this animal eats its own brain? This is a sea squirt, which is a tunicate. Many physical changes to the tunicate’s body occur during metamorphosis into an adult; one of the most interesting changes involves the digestion of the cerebral ganglion, which controls movement and is the equivalent of the human brain. From this, the common saying that the sea squirt "eats its own brain" arises.

Characteristics and Classification of Nonvertebrate Chordates

The phylum Chordata includes all vertebrates. These organisms are named chordates because they have a notochord, a stiff but flexible rod of cells that runs the length of the body. The notochord provides resistance against which

255 1.46. Nonvertebrate Chordates - Advanced www.ck12.org muscles can contract, allowing the body to bend. In most chordates, the notochord is replaced by the backbone - but not in all chordates. This phylum also includes two groups that don’t have backbones. The nonvertebrate species in the phylum Chordata ( nonvertebrate chordates) are exclusively marine organisms. They include the tunicates and the lancelets of the subphyla Urochordata and Cephalochordata respectively. Uro- chordates and cephalochordates are small organisms that are usually only a few centimeters in length, or about the size of your hand (or smaller). In addition to these two subphyla, there is also a class of Vertebrata, Myxini, that includes the hagfish (mentioned in additional concepts - see the Fish Evolution concept), which lacks a backbone and is therefore technically a nonvertebrate class of chordates. This class represents an important bridge between nonvertebrate and vertebrate chordate species.

Vocabulary

• hagfish: The class Myxini; they are eel-shaped, slime-producing marine animals.

• lancelets: Cephalochordates; they are fish-like, marine chordates with a global distribution in shallow, tem- perate, and tropical seas, usually found half-buried in sand.

• nonvertebrate chordates: Chordates without bones or teeth; examples include the tunicates (urochordates) and lancelets (hemichordates).

• notochord: A rod-shaped, semi-rigid support structure that forms between the dorsal nerve cord and the gut of an animal.

• tunicates: Various marine chordates of the subphylum Tunicata, or Urochordata; they have a cylindrical or globular body enclosed in a tough outer covering; examples include the sea squirts.

Summary

• The tunicates and the lancelets, of the subphyla Urochordata and Cephalochordata respectively, are the two main nonvertebrate chordates.

Practice

Use this resource to answer the questions that follow.

• Invertebrate Chordates at http://www.oceanclassrooms.com/ms101_u9_c2_se_1- .

1. What three types of organisms comprise the subphylum Urochordata? 2. Compare a tunicate to a sponge. 3. What may be the fastest growing multicellular animal? 4. Describe characteristics of larvaceans. 5. Where are lancelets likely to be found?

Practice Answers

1. Tunicates (sea squirts), colonial drifting salps, and larvaceans comprise the subphylum Urochordata. 2. Like sponges, tunicates pump water to filter feed, but tunicates are very complex organisms. Sponges are very simple multicellular animals.

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3. Salps can grow faster than any other multicellular animal. 4. Larvaceans drift in mucous structures they secrete, which provide protection and trap food. They must rebuild these structures every few hours because they become heavy and sink when they’ve trapped too much. 5. Lancelets are likely to be found in sand and gravel.

Review

1. What is the notochord? 2. What is the purpose of the notochord? 3. What are nonvertebrate chordates? 4. What are the two main types of nonvertebrate chordates?

Review Answers

1. The notochord is a rod-shaped, semi-rigid support structure that forms between the dorsal nerve cord and the gut of an animal. 2. The notochord provides resistance against which muscles can contract, allowing the body to bend. 3. Nonvertebrate chordates are chordates without bones or teeth. 4. The tunicates (urochordates) and lancelets (hemichordates) are the two main types of nonvertebrate chordates.

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1.47 Tunicates - Advanced

• Describe the characteristics of tunicates and the differences between their larval and adult forms.

What’s a tunicate? That’s a good question. How would a scientist explain tunicates? Probably by their role as one of the last links (along with the lancelets) in the evolutionary development from invertebrates to chordates.

Tunicates

The subphylum Urochordata includes around 3,000 species, commonly called tunicates, that reside in shallow marine waters. Adult tunicates, such as the ones shown in the Figure 1.160, do not bear much resemblance to other chordates. Tunicates, however, are actually chordates and played an important part in the evolution of vertebrates. For more information, visit Tunicates and not cephalochordates are the closest living relatives of vertebrates at http://www.nature.com/nature/journal/v439/n7079/abs/nature04336.html and Sea Squirt DNA Sheds Light On Vertebrate Evolution at http://www.tunicates.com/ . As adults, most tunicates are sessile (they do not move around) filter feeders that lack a notochord and a post-anal tail. They also lack the body segmentation that is found in other chordates. However, tunicate larvae, such as the one shown in the Figure 1.161, possess all four of the major distinguishing characteristics of chordates: a post-anal tail, a notochord, a dorsal nerve cord, and pharyngeal slits. The adult tunicate body is shaped like a small barrel with two large openings called siphons. The inhalent siphon brings water into a saclike, central cavity, called the atrial cavity (distinct from the coelom), and then through the pharyngeal slits where food is filtered into the digestive system. The water then exits the cavity through an exhalent siphon. The tunicate coelom is reduced to a small compartment surrounding the heart. The majority of the organs lie within a body cavity below the atrial cavity called the epicardium, or visceral cavity. The visceral cavity contains the digestive tract (except for the pharynx, which is situated in the atrial cavity), the heart, and the reproductive organs. The organization of the tunicate body is shown in the Figure 1.162.

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FIGURE 1.160 A tunicate species of the subphylum Uro- chordata. It is difficult to imagine that these sessile animals, which almost looks like plants on the bottom of the ocean, are actually chordates. However, tuni- cates develop from larvae that have a notochord, a dorsal nerve cord, a post- anal tail, and pharyngeal slits, which are all features characteristic of chordates.

FIGURE 1.161 The larva of a tunicate species. This larva has a notochord, a dorsal nerve cord, pharyngeal slits, and a post-anal tail.

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FIGURE 1.162 The organization of an adult tunicate body. The digestive system is outlined in green, the gonad is in purple, and the circulatory system is in red. Water is brought into the pharynx within the atrial cavity and through the pharyngeal slits via the inhalent siphon. Food particles captured by the pharyngeal slits are then passed into the remainder of the digestive system, situated in the lower cavity called the epicardium. The digestive system ends with an anus that opens back into the atrial cavity, and waste exits the body through the exhalent siphon.

Tunicates are generally hermaphrodites that sexually reproduce by cross-fertilization. That means that an individual does not use its own sperm to fertilize its eggs. Instead, they obtain sperm released into the open ocean by other individuals. Fertilization is usually internal. Tunicates can also undergo asexual reproduction by budding. This method is frequently carried out by colonial species, with the newly budded organisms remaining attached to the colony. Tunicates have indirect development involving a larval stage. It is the tunicate larva that has most of the features commonly associated with chordates, including a notochord and a post-anal tail. The transition from free- swimming larvae to sedentary adults involves major anatomical changes in the animal. One very odd change is that the cerebral ganglion, located in the head region of the larval stage, is essentially consumed during metamorphosis. The sessile adult forms do not have a need for the same level of neuronal processing as the motile larval stages, and the head region is lost during metamorphosis. Tunicates derive their name from a thick extracellular body covering, called a tunic, that is made up of cellulose-like, complex sugars. Another interesting feature of tunicates is their circulatory system. Instead of blood vessels, the heart pumps blood through a number of small pockets called sinuses. In addition, the heart periodically changes the direction that it pumps blood. There are four different classes within the urochordate subphylum:

• Ascidiacea. • Thaliacea. • Larvacea. • Sorberacea.

The class Ascidiacea is the largest class in the urochordate subphylum. They are generally sedentary organisms with all of the standard characteristics of tunicates described above. A member of the ascidiacean class is shown in the Figure 1.163. Thaliaceans, shown in the Figure 1.164, are free-floating, colonial urochordates commonly called salps. They propel themselves through the ocean using force generated by expelling water through the excurrent siphon, which is usually one opening shared by many members of a colony. Giant Pyrosome and Salps, a video of giant salps, can be found at http://www.youtube.com/watch?v=5EQGA_4BZ 5s (1:17).

260 www.ck12.org Chapter 1. Invertebrates - Advanced

FIGURE 1.163 Tunicates of the class Ascidiacea. This is the largest class in the urochordate subphylum. It is comprised of sessile, filter-feeding animals that have pharyngeal slits as adults.

FIGURE 1.164 A colony of urochordate members of the class Thaliacea. Unlike the ascidians, thaliaceans are free-swimming organisms that often form colonies like the one shown in this picture. They are propelled by the force of water expelled from their siphons and muscle contractions.

MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/54211

Species within the class Larvacea are also free-swimming urochordates, with a long tail that contains the notochord and dorsal nerve cord. A member of the Larvacea class is shown in the Figure 1.165. In part because they retain the notochord and dorsal nerve cord as adults, and because they have a long tail, they look similar to the larval stage of Ascidiacea species like the one shown in the Figure 1.161. Sorberacea is a sedentary class of urochordates that differs from the other classes in that it includes predatory species that feed on other invertebrates such as crustaceans.

Vocabulary

• atrial cavity: A saclike, central cavity that houses the pharynx.

• epicardium: A body cavity below the atrial cavity that contains the digestive system, heart, and reproductive

261 1.47. Tunicates - Advanced www.ck12.org

FIGURE 1.165 A member of the urochordate class Larvacea. Notice the long tail that houses the notochord and a number of muscles. Compare the overall appearance of this organism to the larvae of the class Ascidiacea. The organism in this photograph contains a fluorescent protein within certain tissues, and the photograph was taken using special filters that show the locations of that protein.

organs; it is also known as the visceral cavity.

• sinuses: A number of small pockets that function as blood vessels.

• siphons: Two large openings that control the flow of water through a tunicate.

• tunic: A thick extracellular body covering found on tunicates that is made up of cellulose-like, complex sugars.

• tunicates: Various marine chordates of the subphylum Tunicata, or Urochordata; they have a cylindrical or globular body enclosed in a tough outer covering; examples includes the sea squirts.

Summary

• As adults, most tunicates are sessile (they do not move around) filter feeders that lack a notochord and a post-anal tail. • Tunicate larvae have the characteristics that define chordates. • Tunicates are generally hermaphrodites that sexually reproduce by cross-fertilization. • Tunicates make up four different classes: Ascidiacea, Thaliacea, Larvacea, and Sorberacea.

Practice

Use this resource to answer the questions that follow.

• Introduction to the Urochordata at http://www.ucmp.berkeley.edu/chordata/urochordata.html .

1. How do the larvae of tunicates eventually become sessile? 2. What fossil record do we have of tunicates?

262 www.ck12.org Chapter 1. Invertebrates - Advanced

Practice Answers

1. The larvae will eventually attach to a substrate and lose their tails, ability to move, and the majority of their nervous system in order to become sessile. 2. Urochordates have a sparse fossil record. A Precambrian fossil known as Yarnemia might have been a species of Urochordata, but there is still controversy over this classification. Complete body fossils of tunicates are rare, but tunicates in some families generate microscopic spicules that may be preserved as microfossils.

Review

1. Adult tunicates don’t look like other chordates. What characteristics do they lack? 2. How do tunicates reproduce? 3. What dramatic changes occur to tunicate larvae during metamorphosis? 4. How is the circulatory system of tunicates different from other animals? 5. Briefly describe the classes that are classified as tunicates.

Review Answers

1. As adults, most tunicates are sessile filter feeders that lack a notochord and a post-anal tail. They also lack the body segmentation that is found in other chordates. 2. Tunicates are generally hermaphrodites that sexually reproduce by cross-fertilization, which means they can- not self-fertilize. Fertilization is usually internal. Tunicates can also undergo asexual reproduction by budding. This method is frequently carried out by colonial species, with the newly budded organisms remaining attached to the colony. 3. One very odd change is that the cerebral ganglion, located in the head region of the larval stage, is essentially consumed during metamorphosis. The sessile adult forms do not have a need for the same level of neuronal processing as the motile larval stages, and the head region is lost during metamorphosis. 4. Instead of blood vessels, a tunicate’s heart pumps blood through a number of small pockets called sinuses. In addition, the heart periodically changes the direction that it pumps blood. 5. The class Ascidiacea is the largest class in the urochordate subphylum. They are generally sedentary organisms with all of the standard characteristics of tunicates described above. Thaliaceans are free-floating, colonial urochordates commonly called salps. The class Larvacea are also free-swimming urochordates, with a long tail that contains the notochord and dorsal nerve cord. Sorberacea is a sedentary class of urochordates that differs from the other classes in that it includes predatory species that feed on other invertebrates such as crustaceans.

263 1.48. Lancelets - Advanced www.ck12.org

1.48 Lancelets - Advanced

• Understand the main characteristics of lancelets and how they differ from tunicates.

What is almost a fish but not quite? A species of lancelet in the subphylum Cephalochordata. If you look closely, you can see the repeating units of muscle tissue, called myomeres, that run the length of the lancelet’s body. Why is it not a fish? Notice there are no fins and no true skeleton.

Lancelets

Lancelets include about 25 species of marine animals that make up the subphylum Cephalochordata. Lancelets are also called amphioxus, which translates to “both ends pointed,” because of the shape of their elongated bodies, as shown in the Figure 1.166. Unlike many of the tunicates, the lancelets are capable of swimming, however, they spend most of their time buried in sandy, shallow regions of the ocean. Adult lancelets retain the pharyngeal slits, notochord, dorsal nerve cord, and post-anal tail, which are all characteristic of chordates. Although lancelets have a brain-like swell at the end of the notochord in the head region, it is not very highly developed. Like most tunicates, lancelets are filter feeders with the pharynx situated in an atrial cavity where it functions to filter food particles from the water currents that flow through the cavity. Water is taken in through the mouth and expelled through an opening called the atriopore. The mouth has several specializations including a filtering structure, called a wheel organ, that is made up of ridges covered with cilia that help draw in water. Lancelets have a closed circulatory system with a heart-like, pumping organ located on the ventral side, and they reproduce sexually. Unlike other aquatic chordates, lancelets do not use the pharyngeal slits for respiration. Gas exchange occurs through the

264 www.ck12.org Chapter 1. Invertebrates - Advanced

FIGURE 1.166 Lancelet (Cephalochordata). Unlike tu- nicates, lancelets retain all four defining chordate traits in the adult stage. Can you find them?

body wall. The sexes are separate, and fertilization is external. They undergo indirect development involving a free-swimming larval stage. A significant feature of lancelets, which is shared with vertebrates but not tunicates, is the presence of myomeres. Myomeres are repeating units, or blocks, of muscle that partially overlap to form a row along the body of the animal, as shown in the Figure 1.167. In addition to segmented muscle tissue, both cephalochordates and vertebrates have segmented bodies.

FIGURE 1.167 A diagram showing the organization of myomeres. Myomeres are repeating muscle segments that run the length of a lancelet’s body.

Classification of lancelets is quite simple due to the small number of species. There is only one class of cephalochor- dates, called Leptocardii, that contains two families: Asymmetronidae and Branchiostomidae. Despite the small size of the cephalochordate subphylum, these animals hold an important place in the evolutionary history of vertebrates, a place that is still being characterized and defined by scientists today. This will be discussed in the next section.

Vocabulary

• atriopore: The hole in a lancelet through which water exits the body.

• lancelets: Cephalochordates; fish-like, marine chordates with a global distribution in shallow, temperate, and tropical seas, usually found half-buried in sand.

• myomeres: Repeating blocks of skeletal muscle tissue found commonly in chordates.

• wheel organ: A filtering structure found in lancelets that is made up of ridges covered with cilia that help draw in water.

Summary

• Lancelets include about 25 species of marine animals that make up the subphylum Cephalochordata. • Adult lancelets retain the four characteristics common to all chordates. • Like most tunicates, lancelets are filter feeders.

265 1.48. Lancelets - Advanced www.ck12.org

Practice

Use this resource to answer the questions that follow.

• Cephalochordata at http://eol.org/pages/1585/overview .

1. Where are lancelets mainly found? 2. How do lancelet larvae feed? 3. What ecological role do lancelet’s serve?

Practice Answers

1. Lancelets are mainly found in shallow, subtidal tropical, subtropical, and temperate sand flats, where they burrow in clean gravel or sand. 2. Lancelet larvae feed by swimming upward, then hovering or slowly sinking passively with their mouths directed downward, feeding on plankton and other organisms. 3. Lancelets are harvested by humans for food in some areas and are an important food source for bottom-feeding fish and small crustaceans.

Review

1. What are some major differences between adult tunicates and lancelets? 2. How do lancelets feed? 3. How do lancelets obtain oxygen? 4. What significant feature do lancelets share with vertebrates?

Review Answers

1. Adult lancelets retain the pharyngeal slits, notochord, dorsal nerve cord, and post-anal tail, which are all characteristic of chordates. Adult tunicates do not retain these characteristics. 2. Like most tunicates, lancelets are filter feeders with the pharynx situated in an atrial cavity where it functions to filter food particles from the water currents that flow through the cavity. 3. Unlike other aquatic chordates, lancelets do not use the pharyngeal slits for respiration. Gas exchange occurs through the body wall. 4. A significant feature of lancelets that is shared with vertebrates is the presence of myomeres. Myomeres are repeating units, or blocks, of muscle that partially overlap to form a row along the body of the animal.

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1.49 Nonvertebrate Chordate Evolution - Ad- vanced

• Discuss the different theories regarding chordate evolution.

Tunicates. Why? Why would this organism evolve? Tunicates are marine filter feeders with a sac-like body structure. Though they may seem to be very simple animals, they were an important evolutionary stepping stone.

Evolution of Nonvertebrate Chordates

Although the vast majority of chordates are represented by the vertebrate species, the invertebrate subphyla Uro- chordata and Cephalochordata provide important clues to the early evolution of the chordate phylum. Traditionally, urochordates have been considered to be the earliest group of chordates despite their limited fossil record, which dates back only as far as the Jurassic period. Cephalochordates were considered to be more closely related to vertebrates. This is based on shared features between cephalochordates and vertebrates, including the presence of myomeres, and the fact that, unlike urochordates, they retain most of their chordate-specific characteristics in adulthood (notochord and tail). The fact that urochordates lack most chordate features as adults also lends support to the notion that they represent an earlier chordate group. Molecular phylogenetic studies have recently challenged this view. In the last several years, the complete genomes of several urochordate species, including Ciona intestinalis in the class Ascidiacea and Oikopleura dioica in the class Larvacea, have been sequenced. By comparing these sequences with the human genome, scientists have been able to gain a much better understand- ing of the evolutionary relationship between urochordates and vertebrates. These studies help us learn about the

267 1.49. Nonvertebrate Chordate Evolution - Advanced www.ck12.org evolutionary changes in the genome that contribute to the differences we see in the organism’s physical traits. For example, 80% of the genes in the C. intestinalis genome have counterparts in the human genome, reflecting a very close evolutionary relationship. One major difference between the two genomes is their size. The total number of genes in the genome of C. intestinalis is about half the number found in the human genome. What scientists have discovered in studying the two genomes is that a big part of this size difference is due to the duplication of many genes that are only present as one copy in the urochordate genome. How does that relate to evolutionary differences or changes that have taken place between the two organisms? Having multiple copies of a gene allows the organism to accommodate adaptive mutations in some copies of the gene, which can lead to new proteins with new functions, without losing the essential function of the original gene. Comparing genes between the two species can help scientists learn which copy of a duplicated gene is most like the ancestral copy. They can then look at the differences in the other copies to understand some of the evolutionary changes that have been selected for in the more recent species. One major finding of these analyses is that, at the molecular level, the subphylum Urochordata is actually more closely related to Vertebrata than the cephalochordates are. This represents a major shift in thinking about the phylogenetic relationships of chordate subphyla and may indicate that cephalochordates were the earliest chordates. Studies of lancelets (http://www.underwatertimes.com/news.php?article_id=83410560791 ) have also shed light on the origins of some vertebrate traits that are essential to our health and survival. A fascinating example is the beginning of the adaptive immune system. In the Immune System concepts, you will learn about the human adaptive immune system. This system is shared with many other vertebrates, allowing them to identify foreign, infectious organisms within the body and respond by generating protein molecules, called antibodies, that are specifically designed to attack the particular offending invaders. Researchers examining lancelets have found that they have a very primitive form of adaptive immune proteins with similarities to antibodies. Lancelets are the earliest group in the animal kingdom to show the beginning of an adaptive immune system. This finding will now help us understand how the immune system evolved over time from its rudimentary beginnings to its current, highly complex state. These examples highlight the importance of studying organisms at the level of their molecules, particularly their DNA and proteins, for making accurate assessments about their evolutionary histories. The revolution in molecular biology that has made DNA sequencing efficient and inexpensive has made a major contribution to phylogenetic studies. These studies are likely to continue and rapidly expand our understanding of the evolutionary changes that led to the many species of chordates and other organisms living on Earth today.

Vocabulary

• adaptive immune system: A system which creates immunological memory after an initial response to a specific pathogen, leading to an enhanced response to subsequent encounters with that same pathogen. It is also known as the acquired immune system.

• myomeres: Repeating blocks of skeletal muscle tissue commonly found in chordates.

Summary

• Traditionally, urochordates have been considered to be the earliest group of chordates despite their limited fossil record, which dates back only as far as the Jurassic period. Cephalochordates were considered to be more closely related to vertebrates. • One major finding of these analyses is that, at the molecular level, the subphylum Urochordata is actually more closely related to Vertebrata than the cephalochordates are. • Researchers examining lancelets have found that they have a very primitive form of adaptive immune proteins with similarities to antibodies.

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Practice

Use this resource to answer the questions that follow.

• Chordate: Evolution and Paleontology at http://www.britannica.com/EBchecked/topic/114462/chordate/49 559/Evolution-and-paleontology .

1. When did chordates first appear in the fossil record? 2. What classical theory of chordate evolution is explained in this article? 3. What is the difference between a clade and a grade? Is Vertebrata a clade or a grade?

Practice Answers

1. Fossils containing chordates date back to the Cambrian period, although some scientists claim there is evidence that chordates originated prior. 2. The classical theory postulates that cephalochordates were the original chordates and that some became sessile, evolving into tunicates. Other mobile cephalochordates eventually evolved into vertebrates. 3. A group containing a common ancestor and all of its decedents is a clade. A grade does not meet both these requirements. Vertebrata is a clade, whereas invertebrates compose a grade.

Review

1. Which group of chordates were traditionally regarded as the earliest group? 2. Scientists discovered that urochordates have many genes in common with humans, but urochordates usually only have one copy of each gene. Why does having multiple copies of the same gene benefit an organism? 3. How is the lancelet immune system similar to that of vertebrates?

Review Answers

1. Traditionally, urochordates have been considered to be the earliest group of chordates. 2. Having multiple copies of a gene allows the organism to accommodate adaptive mutations in some copies of the gene, which can lead to new proteins with new functions, without losing the essential function of the original gene. 3. Researchers examining lancelets have found that they have a very primitive form of adaptive immune proteins with similarities to antibodies. Lancelets are the earliest group in the animal kingdom to show the beginning of an adaptive immune system.

Summary

269 1.50. References www.ck12.org

1.50 References

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