<<

The : Putting a Backbone Into Spineless Note: These links do not work. Use the links within the outline to access the images in the popup windows. This text is the same as the scrolling text in the popup windows.

I. What is a ? (Page 1)

Phylum Chordata: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/phylum_chordata.html

The chordate animals form the second major group of the line. In this topic you will learn about the chordates and receive a brief introduction to the .

Characteristics: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/characteristics.html

This diagram of a generalized chordate animal illustrates the four chordate characteristics. These are the , dorsal, hollow cord, pharyngeal slits, and a post-anal tail. Note that the nerve cord is expanded to form a brain at the anterior end of the animal, and that the is the first part of the digestive tract. The tail is defined as “post anal” because it extends beyond the .

Notochord: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/notochord.html

The notochord is a skeletal structure located above the gut. It is stiff enough to provide support for the body, yet flexible enough to bend. The notochord is composed of large, fluid-filled cells that bulge due to internal water pressure. They are held in place by two sheaths of tissue. Thus, the stiffness of the notochord results from hydrostatic pressure, not from the presence of hard minerals as are found in shells or bone.

During Development: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/during_development.html

You have recently learned the basics of early development in deuterostome embryos. The process of gastrulation is shown in this diagram. The region colored green is the developing notochord. It begins to form during gastrulation and is in place on the dorsal side of the embryo by the end of the gastrulation process. Note that at this point, before a has formed, the notochord is within the roof of the developing gut. It will detach from the gut tube during coelom formation.

Swimming Motions: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/swimming_motions.html

The function of the notochord is to stiffen the body as the animal swims. Muscles along the sides of the body contract alternately on right and left sides, resulting in a sinuous body motion. Forward locomotion is much faster and more efficient than it would be in a flabby body, lacking a notochord. The ability to swim efficiently is probably one reason for the notable success of Chordate animals as early as the period.

Nerve Cord: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/nerve_cord.html

In this cross section through the body of a simple, chordate animal, the location of the notochord and nerve cord are clearly visible. The nerve cord lies just above the notochord on the dorsal side of the body.

Annelids and : http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/annelids_and_arthropods.html

These illustrations should serve as a reminder that the nerve cords of and arthropods are located along the ventral side of the body. Also, these nerve cords are solid, whereas the nerve cord of chordate animals has a hollow center. Water Flow: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/water_flow.html

This diagram illustrates water flow through the pharynx of a simple chordate animal. Water enters through the mouth and exits from slits within both sides of the pharynx. Without slits, water would continue through the digestive tract and emerge from the anus.

II. What are the different kinds of chordates? (Page 2-4)

Phylum Chordata: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_bigimages/phylum_chordata.html

This chart is very helpful in reviewing how the deuterostome line fits into the large division of bilaterally symmetrical animals. It also illustrates the relationship between the and chordates. Phylum Chordata contains all of the groups beneath the “Chordates” heading. The urochordates and are two subphyla of chordate animals. The vertebrates are the third , consisting of the jawless lampreys and the jawed vertebrates (, , , and ). We will return to this chart later to clarify the important changes that occurred during .

Tunicate Larvae: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/tunicate_larvae.html

Here we see a living and a diagram that illustrates the major chordate characteristics. Note that the notochord stiffens the tail, but does not extend into the region. Water flows into the pharynx through the mouth, then exits through slits into an excurrent that expels water from the body.

Metamorphosis: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/.html

Most are sessile as adults. When ready to change to the adult body form, the free-swimming larval stage attaches to a solid object by means of adhesive glands at the tip of its head. During metamorphosis, the notochord degenerates and the nerve cord is reduced to a few small ganglia. At the same time, the pharynx grows until it occupies most of the body cavity. The soft body is supported by a thick covering called a tunic, from which the animal’s common name is derived. The structure of the tunic is unusual in that it contains cellulose, a substance found in walls, but rarely in animal tissues.

Pharynx: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/pharynx.html

The large pharynx ,with its many small slits, fills most of the space within the tunicate’s body as can be seen in the specimen on the left. Water flowing through the pharynx carries tiny food particles into the pharyngeal cavity. Water then passes through the pharyngeal slits and exits via an excurrent siphon. Food particles are too large to pass through to slits, so are trapped within the pharynx and pass through the gut where digestion and absorption occur. Undigested particles are expelled through the anus into the excurrent siphon and leave the body in the outgoing water current. Thus the pharynx of tunicates provides a sophisticated method of filter-feeding.

Sea Squirts: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/sea_squirts.html

Most of tunicates are sea squirts. These, small sac-like animals are common in coastal waters and often wash up on the beach. The two (for water inflow and outflow) are clearly visible in this sea squirt. If you were to pick up the animal, its body would strongly contract, squirting sea water from both siphons. Hence the name sea squirt.

Soft-bodied Animals: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/soft- bodied_animals.html These sea squirts appear fragile and defenseless, but few predators eat them. Presumably they contain foul-tasting chemicals and some species are poisonous. In fact, it is said that the wife of Julius Caesar eliminated some of his enemies by feeding them poisonous sea squirts!

Hermaphroditic: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/hermaphroditic.html

This diagram shows the location of gonads within the sea squirt’s body. Both and sperm are shed from long tubes that enter the excurrent siphon. Although sea squirts are hermaphroditic, eggs are usually fertilized by sperm from another individual that is spawning nearby.

Colonies of Sea Squirts: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/colonies.html

Many species of sea squirt form colonies. Here are several examples.

Excurrent Siphon: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/excurrent_siphon.html

In this sea squirt colony, the individual bodies are arranged around a central excurrent siphon that is shared by all of the sea squirts (seven in this colony). Each sea squirt retains its incurrent siphon, which serves as a mouth to bring water into the pharynx.

Nuisance: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/nuisance.html

These colonial sea squirts have created a massive growth on an underwater rope.

Salps: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/.html

This was photographed in Monterey Bay. It is one of the larger species, reaching a length of 30 cm. Like the sea squirts, it filter-feeds by pumping water through a large pharynx. Salp populations are quite dense in some parts of the ocean. The fecal pellets that they produce are rich in the nutrients needed by marine algae. Thus, salps play an important role in the marine ecosystem.

Transparent Body: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/transparent_body.html

The body wall of salps is composed of a clear, gelatinous material similar to that of . Thus the body is transparent, which may provide the salp’s main line of defense. The dark structure seen within this salp is the gonad.

Chain-like Colonies: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/chain- like.html

Many salps form colonies. In some species the chain of salps can reach a length of 20 meters. The salps, in conjunction with other gelatinous animals such as jellyfish, siphonophores and comb jellies, form what is called a “jelly web” in deeper ocean waters. The significance of this web of in the ecology of oceans has only recently been recognized and is not yet well understood.

Larvaceans: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/larvaceans.html

These small tunicates are literally larvae that lost the ability to undergo metamorphosis. At the same time, they developed mature gonads and thus were able to reproduce. So the adult larvacean its entire life in a larval body form. Since they lack a large pharynx, the larvaceans have developed a different style of filter-feeding. They secrete thin strands of to form what is called a “house” in which they live. An empty, football-sized house is shown in this image. The larvacean beats its tail to generate a water current through the house. Food organisms stick to the mucus and are consumed. When the house becomes cluttered with inedible debris, as the one shown here, the larvacean leaves it and constructs a new house. These tunicates are difficult to see, since they are small as well as transparent. Some marine biologists think that they may more abundant than realized and constitute one of the major planktonic animals of the sea.

Head of a : http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/head.html

In this diagram of a cephalochordate animal, the main chordate characteristics are well illustrated. Note that the notochord extends all the way to the tip of the head. This feature is not found in any other group of chordates.

Lancelet: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/.html

Lancelets are small animals, rarely exceeding 3 cm in length. Their stiff notochord and strong body muscles provide an efficient swimming locomotion, while the thin, dorsal fin confers stability in the water.

In the Sand: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/in_the_sand.html

Lancelets feed while buried in the sand, with only the tip of their head extending into the water. They pull water through the pharynx and extract tiny food organisms, similar to the feeding style of the tunicates. Also like the tunicates, lancelets absorb oxygen from the water as it passes through the pharyngeal slits.

Features Similar to Vertebrates: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/features.html

In addition to the four characteristics common to all chordates, the lancelets have several structures similar to those found in vertebrate animals. Both lancelets and vertebrates have segmental body wall muscles which are controlled by nerve roots from the nerve cord. Lancelets have an outgrowth of the digestive tract that resembles a rudimentary vertebrate . They also have a pulsating ventral blood vessel that may be the forerunner of the vertebrate , and their entire is much like that of young vertebrate embryos. Thus the lancelet, often called amphioxus, is routinely used in classes to illustrate the essential features of vertebrate form and function.

Backbone: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/backbone.html

The vertebrate animals are defined as possessing a , or backbone. This structure is composed of many small vertebrae that are connected by joints along the dorsal side of the body. Unlike the notochord, the vertebral column is hardened by minerals and composed of or bone. The initial function of the vertebral column was to protect the nerve cord by providing solid arches above it. In the single shown here, you can see where the nerve cord was located in life. The bases of vertebrae replace the notochord during the of land-dwelling vertebrates, thus providing a stronger support and a place to anchor the limbs.

Vertebrates: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/vertebrates.html

Here you can see representatives of the 7 groups of living vertebrates. You are not required to memorize the names, but you will need to know the common names of each group (such as cartilaginous fish).

Embryo: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/embryos.html

In vertebrate embryos, a prominent notochord is present. Likewise, there is a pharynx with slits, although the slits do not completely open in mammalian embryos. As development proceeds in land-dwelling vertebrates, the notochord is lost, and the pharyngeal region changes, giving rise to other structures as the slits close. In fish, the pharyngeal slits become part of the apparatus, whereas pieces of the notochord are retained and incorporated into the vertebral column. In all vertebrates, the anterior end of the nerve cord becomes greatly expanded to from a brain.

First Vertebrates: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/first_vertebrates.html

Here is a drawing based on from 500 million ago. This animal looks much like a lancelet, but had a brain and large , in addition to a vertebral column. As far as we know, this is the first vertebrate animal.

Ancestor: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/ancestor.html

The likely ancestor of the first vertebrates is this ancient cephalochordate, shown both as the imprint and an artist’s reconstruction. Another drawing of the same cephalochordate was used earlier to illustrate the swimming motion of early chordate animals.

Ostracoderms: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/ostracoderms.html

These drawings are based on fossils of the first known fish, named Ostracoderms, that inhabited the oceans prior to the evolution of land-dwelling vertebrates. These fish were small, 3-10 cm long. Their bodies were covered with bony plates and usually flattened in the dorsal-ventral plane, suggesting that they spent most of their time on the sea bottom. They lacked , and were probably filter-feeders like the cephalochordate ancestors. Midway through their history, most Ostracoderm species moved into fresh water habitats. Before becoming extinct, they gave rise to other groups of fish that had jaws and paired appendages.

Lamprey: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/.html

Almost all modern fish have jaws. However two small groups, the and the lampreys are jawless and apparently evolved from a line of Ostracoderms that retained the primitive, jawless condition. Like their ancestors, they also lack paired appendages. You will learn more about these jawless fish later in the course.

Ancient Fish: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_popups/ancient_fish.html

Here is an artist’s reconstruction of one of the early jawed fish. Hinged jaws allowed these fish to eat a much wider variety of food, including tough plant material and animal prey. The presence of paired fins gave them greater stability in the water and more ability to maneuver. Thus, these fish were stronger swimmers than the Ostracoderms, and many become predators of other aquatic animals.

Evolution of the Vertebrates: http://courses.ncsu.edu/zo495x/common/zo155_site/wrap/chordates/chordate_bigimages/vertebrate_evolution.html

This chart serves to bring together several concepts of vertebrate evolution, as well as indicating the relationship between groups of vertebrates. You do not need to know the names written in black at the top of the chart, but you should remember the types of vertebrates that have jaws, four limbs, and embryos with an amnion. The orange boxes indicate points in evolutionary history were key structures appeared. You have already learned about the importance of the vertebral column, jaws and paired appendages. As the course progresses, the other important events depicted here will be described.