Inver
.... Figure 33.1 What function do the red whorls of this KEY CONCEPTS organism have?
33.1 Sponges are basal animals that lack true tissues the surrounding water. The tentacles emerge from a tube of 33.2 Cnidarians are an ancient phylum of calcium carbonate secreted by the worm that protects and sup eumetazoans ports its soft body. Light-sensitive structures on the tentacles 33.3 lophotrocholoans, a clade identified by can detect the shadow cast by a predator, triggering the worm molecular data, have the widest range of to contract muscles that rapidly withdraw the tentacles into animal body forms the tube. 33.4 Ecdysoloans are the most species-rich animal Christmas tree worms are invertebrates-animals that group lack a backbone. Invertebrates account for 95% ofknown ani 33.5 Echinoderms and chordates are deuterostomes mal species. They occupy almost every habitat on Earth, from the scalding water released by deep-sea hydrothermal vents to the rocky, frozen ground of Antarctica. Adaptation to these varied environments has produced an immense diversity of forms, ranging from aspecies consisting simply ofa flat bilayer of cells to other species with silk-spinning glands, pivoting t first glance, you might mistake the organism shown spines, dozens ofjointed legs, or tentacles covered with suc in Figure 33.1 for some type ofseaweed. But this col tion cups, just to list a few. Aorful inhabitant of coral reefs is actually an animal, In this chapter, we'll take a tour ofthe invertebrate world, us not an alga. Specifically, it is a species of segmented worm ing the phylogenetic tree in Figure 33.2 as a guide. Figure 33.3 known as a Christmas tree worm (Spirobranchus giganteus). on the next three pages, surveys 23 invertebrate phyla. As rep The two tree-shaped whorls are tentacles, which the worm resentatives of invertebrate diversity, many of those phyla are uses for gas exchange and for filtering small food particles from explored in more detail in the rest ofthis chapter.
... Figure 33.2 Review of animal Calcarea JIJ. phylogeny. Except for sponges (basal andSiliceaW animals in phyla Calcarea and Silicea) and a .... few other groups, all animals have tissues and ANCESTRAL Cnidaria are in the clade Eumetazoa Most animals are PROTIST In the diverse clade Bilatena m C Common 3 LOPhWOChO"'~ ancestor of 1\ all animals •Q" •.-• 'cd,,,,,,, • ••~ Deuterostomia ~
666 • Figure 33,3 Exploring Invertebrate Diversity
Kingdom Animalia encompasses 1.3 million known species, and estimates oftotal species range far higher. Ofthe 23 phyla surveyed here, those illustrated with smaller-sized "preview" photographs are discussed more fully in this chapter or another.
Animals in these phyla are informally The single known species in called "sponges." Sponges are sessile this phylum, Trichoplax animals that lack true tissues. They live as adhaercns, does not even look suspension feeders, trapping particles that like an animal. It consists ofa pass through the internal channels oftheir few thousand cells arranged bodies (see Concept 33.1). in a double-layered plate. A sponge Trichoplax can reproduce by dividing into two individuals Cnidaria (10,000 species) or by budding off many multicellular individuals. Cnidarians include corals, jellies, and ~. A plawzoan (lM) hydras. These animals have a diplobastic, [-;;" radially symmetrical body plan that V( includes a gastrovascular cavity with a single opening that serves as both mouth Ctenophores (comb jellies) are and anus {see Concept 33.2). diploblastic and radially symmetrical like cnidarians, A jelly suggesting that both phyla diverged from other animals very early. Comb jellies make up much ofthe ocean's plankton. Acoel flam'orms have a simple nervous They have many distinctive system and a saclike gut, and thus had traits, including eight "combs" been placed in ph}~um Platyhelminthes. ofcilia that propel the animals Molecular analyses, however, indicate A denophore, or comb jelly through the water. \X'hen a small Acoela is a separate lineage that diverged animal contacts the tentacles of before the three main bilaterian clades (see some comb jellies, spedalized cells burst open, covering the prey Acoel flatworms (lM) Concept 32.4). with sticky threads.
LOPHOTROCHOZOANS
~helminthes (20,000 species) ...--;- • Flatworms (including tapeworms, Despite their microscopic size, rotifers . '.:~\ planarians, and flukes) have bilateral have specialized organ systems, including '" symmetry and a central nervous system an alimentary canal {digestive tract). They ~~ • •• .....,..-1 that processes information from sensory feed on microorganisms suspended in " -.~~ structures. They have no body cavity or water {see Concept 33.3). A marine flatworm organs for circulation (see Concept 33.3). A rotifer (LM)
Ectoprocts {also known as bryozoans) live Brachiopods, or lamp shells, may be easily as sessile colonies and are covered by a mistaken for clams or other molluscs. tough exoskeleton (see Concept 33.3). However, most brachiopods have a unique stalk that anchors them to their substrate (see Concept 33.3).
Ectoprocts A brachiopod Continued on next page
CHAPTER THIRTY·THREE Invertebrates 667 • Figure 33.. (continued) •• • Invertebrate Diversity
Acanthocephalans (from the The only known species of Greek acanthias, prickly, and cycliophoran, Symbion pandora, cephalo, head) are called spiny was discovered in 1995 on the headed worms because ofthe mouthparts ofa lobster. This curved hooks on the proboscis at tiny, vase-shaped creature has a the anterior end oftheir body. All unique body plan and a species are parasites. Some particularly bizarre life cycle. acanthocephalans manipulate Males impregnate females that their intermediate hosts {generally are still developing in their An acanthocephalan (lM) arthropods) in ways that increase mothers' bodies. The fertilized their chances of reaching their final hosts (generally vertebrates). A (ydiophoran (colorized SEM) females then escape, settle For example, acanthocephalans that infect New Zealand mud elsewhere on the lobster, and release their offspring. The offspring (nibs force their hosts to move to more visible areas on the beach, apparently leave that lobster and search for another one to which where the crabs are more likely to be eaten by birds, the worms' they attach. final hosts.
III •. ies)
Proboscis worms, or ribbon Molluscs (including snails, clams, squids, worms, swim through water or and octopuses) have a soft body that in burrow in sand, extending a many species is protected by a hard shell unique proboscis to capture (see Concept 33.3). ~~J prey. Like flatworms, they lack a true coelom. However, unlike flatworms, nemerteans have an An octopus alimentary canal and a closed circulatory system in which the blood is contained in vessels and hence is distinct from fluid A ribbon worm Annelids, or segmented worms, are distinguished from other worms by their in the body cavity. body segmentation. Earthworms are the most familiar annelids, but the phylum also includes marine and freshwater species (see Concept 33.3). A marine annelid
ECDYSOZOA
Loriciferans (from the Latin lorica, Priapulans are worms with a corset, andferre, to bear) are tiny large, rounded proboscis at the animals that inhabit the deep-sea anterior end. (They are named bottom. A loriciferan can telescope after Priapos, the Greek god of it.s head, neck, and thorax: in and fertility, who was symbolized by out ofthe lorica, a pocket formed a giant penis.) Ranging from by six plates surrounding the 0.5 mm to 20 em in length, most abdomen. Though the natural species burrow through seafloor history ofloricifenms is mostly a sediments. Fossil evidence mystery, at least some species suggest.s that priapulans were likely eat bacteria. A lorkiferan (LM) A priapulan among the major predators during the Cambrian period.
668 UNIT fiVE The Evolutionary History of Biological Diversity Tardigrades (from the Latin Onychophorans, also called tardus, slow, andgrOOus, step) are velvet worms, originated during sometimes caUed water bears for the Cambrian explosion (see their rounded shape, stubby Chapter 32). Originally, they appendages, and lumbering, thrived in the ocean, but at bearlike gait. Most tardigrades are some point they succeeded in less than 0.5 mm in length. Some colonizing land. Today they live live in oceans or fresh water, while only in humid forests. others live on plants or animals. Onychophorans have fleshy AJ;, many as 2 million tardigrades antennae and several dozen Tardigrades (colorized SEM) can be found on a square meter of pairs ofsaclike legs. An onychophoran moss. Harsh conditions may cause tardigmdes to enter a state of dormancy; while dormant, they can survive temperatures as low as - 272'C, dose to absolute zero!
III •.
Roundworms are enormously abundant The vast majority ofknown animal species, and diverse in the soil and in aquatic including insects, crustaceans, and habitats; many species parasitize plants arachnids, are arthropods. All arthropods and animals. The most distinctive feature have a segmented exoskeleton and jointed of roundworms is a tough cuticle that appendages (see Concept 33.4). coats the body (see Concept 33.4). A roundworm A scorpion (an arachnid)
DEUTER05TOMIA
like echinoderms and chordates, hemichordates are members ofthe deuterostome clade (see Chapter 32). Hemichordates share some traits with other chordates, such as gill slits and a dorsal nerve cord. The largest group of hemichordates are the enteropneusts, or acorn worms. Acorn worms are marine and generally live buried in mud or under rocks; they may grow to more than 2 m An acorn worm in length.
Chordata (52,000 species)
Echinoderms, such as sand dollars, sea stars, More than 90% ofall known chordate species and sea urchins, are aquatic animals in the have backbones (and thus are vertebrates). deuterostome clade that are bilaterally However, the phylum Chordata also includes symmetrical as larvae but not as adults. three groups ofinvertebrates: lancelets, They move and feed by using a network of tunicates, and hagfishes. See Chapter 34 for a internal canals to pump water to different full discussion ofthis phylum. t A sea urchin parts oftheir body (see Concept 33.5). A tunicate
CHAPTER THIRTY·THREE Invertebrates 669 groups ofsimilar cells that act as a functional unit and are iso r;;:~:7 ?r=·~sal lated from other tissues by membranous layers. However, the animals that sponge body does contain several different cell types. For ex lack true tissues ample, lining the interior of the spongocoel are flagellated choanocytes, or collar cells (named for the membranous col
Calcarea and Silicea Animals in the phyla Calcarea lar around the base of the flagellum). The similarity between C",dafla and Silicea are known infor· choanocytes and the cells of choanoflagellates supports mo Lophotrochozw mally as "sponges:' (Previously, lecular evidence suggesting that animals evolved from a Ecdywzw all sponges were placed in a choanoflagellate-like ancestor (see Figure 32.3). lI..iIIiiii. Dl!IJterostomia single phylum, Porifera, now The body ofa sponge consists oftwo layers ofcells separated thought to be paraphyletic based on molecular data.) Among by a gelatinous region called the mesohyl. Wandering through the simplest ofanimals, sponges are sedentary and were mis the mesohyl are cells called amoebocytes, named for their use taken for plants by the ancient Greeks. They range in size from ofpseudopodia. Amoebocytes have many functions. They take a few millimeters to a few meters and live in both fresh and ma up food from the water and from choanocytes, digest it, and rine waters. Sponges are suspension fceders: They capture carry nutrients to other cells. They also manufacture tough food particles suspended in the water that passes through their skeletal fibers within the mesohy!. In some groups of sponges, body, which in some species resembles a sac perforated with these fibers are sharp spicules made from calcium carbonate or pores. Water is drawn through the pores into a central cavity, silica. Other sponges produce more flexible fibers composed of the spongococ1, and then flows out of the sponge through a a protein called spongin; you may have seen these pliant skele larger opening called the osculum (Figure 33.4). More com tons being sold as fluffy brown bath sponges. plex sponges have folded body walls, and many contain Most sponges are hermaphrodites, meaning that each in branched water canals and several oscula. dividual functions as both male and female in sexual repro Sponges are basal animals; that is, they represent a lineage duction by producing sperm and eggs. Almost all sponges that originates near the root of the phylogenetic tree of ani exhibit sequential hermaphroditism, functioning first as one mals. Unlike nearly all other animals, sponges lack true tissues, sex and then as the other.
o Choanocytes. The spongocoel is lined with flagellated cells called choanocytes. By beating flagella, the choanocytes create Flagellum a current that draws water in through the Choanocyte pores and out through the osculum. L \ • Coli" Osculum Azure vase sponge (Callyspongia plicifera)
o Spongocoel. Water ~ /~' passing through pores enters a cavity called the Phagocytosts of spongocoel. food particles Amoebocyte
o Pores. Water enters the epidermis through pores r oThe movement of a choanocyte's formed by doughnut flagellum also draws water through its shaped cells that span collar of fingerlike projections. Food particles are trapped in the mucus the body wall. I ~ coating the projections, engulfed by phagocytosis, and either digested or o Epidermis. The outer r--s: A transferred to amoebocytes. layer consists of tightly Water /" \ • /"i.~ packed epidermal cells. flow fI Amoebocytes. These cells can /' transport nutrients to other cells of o Mesohyl. The wall of the sponge body, produce materials this sponge consists of r for skeletal fibers (spicules), or two layers of cells separated become any type of sponge cell by a gelatinous matrix. the as needed. W mesohyl ("middle matter ) • 1 ... Figure 33.4 Anatomy of a sponge.
670 UNtT fiVE The Evolutionary History of Biological Diversity Sponge gametes arise from choanocytes or amoebocytes. Eggs reside in the mesohyl, but sperm are carried out of the Polyp sponge by the water current. Cross-fertilization results from some of the sperm being drawn into neighboring individuals. Fertilization occurs in the mesohyl, where the zygotes develop into flagellated, swimming larvae that disperse from the par Body ent sponge. After settling on a suitable substrate, a larva de stalk velops into a sessile adult. Sponges produce a variety of antibiotics and other defen sive compounds. Researchers are now isolating these com .. Figure 33.5 Polyp and medusa forms of cnidarians. The pounds, which hold promise for fighting human diseases. For body wall of a cnidarian has two layers of cells: an outer layer of example, a compound called cribrostatin isolated from marine epidermis (from ectoderm) and an inner layer of gastrodermiS (from endoderm), Digestion begins in the gastrovascular cavity and is sponges can kill penicillin-resistant strains of the bacterium completed inside focxl vacuoles in the gastrodermal cells, Flagella on Streptococcus. Other sponge-derived compounds are being the gastrodermal cells keep the contents of the gastrovascular cavity tested as possible anticancer agents. agitated and help distribute nutrients. Sandwiched between the epidermis and gastrodermis is a gelatinous layer, the mesoglea, CONCEPT CHECK 33.1 from the oral surface, which points downward. Some cnidari I. Describe how sponges feed. 2. -',1@ilIA Some molecular evidence suggests that ans exist only as polyps or only as medusae; others have both a polyp stage and a medusa stage in their life cycle. the sister group ofanimals is not the choanoflagel Cnidarians are carnivores that often use tentacles arranged lates. but rather a group ofparasitic protists. Me in a ring around their mouth to capture prey and push the food somycetozoa. Given that these parasites lack collar cells, can this hypothesis be correct? Explain. into their gastrovascular cavity, where digestion begins. Any undigested remains are expelled through the mouth/anus. For suggested answers, see Appendix A. The tentacles are armed with batteries of cnidocytcs, cells unique to cnidarians that function in defense and prey capture (Figure 33,6). Cnidocytes contain cnidae (from the Greek cnide, nettle), capsule-like organelles that are capable of ex ploding outward and that give phylum Cnidaria its name. Spe cialized cnidae called nematocysts contain a stinging thread
Calcarea and Silicea All animals except sponges Cnidaria and a few other groups belong Lophotrochozoa to the clade Eumetazoa, ani ~ Ecdysozw mals with true tissues (see Tentacle Deuterostom'a Cuticle Chapter 32). One ofthe oldest of prey lineages in this clade is the phylum Cnidaria. Cnidarians have diversified into a wide range of sessile and motile forms, in Thread cluding hydras, corals, and jellies (commonly called "jellyfish"). Yet most cnidarians still exhibit the relatively simple, diploblas tic, radial body plan that existed some 570 million years ago. The basic body plan ofa cnidarian is a sac with a central di gestive compartment, the gastrovascular cavity. A single opening to this cavity functions as both mouth and anus. There Thread are m'o variations on this body plan: the sessile polyp and the discharges motile medusa (Figure 33.5). Polyps are cylindrical forms that • adhere to the substrate by the aboral end oftheir body (the end Thread opposite the mouth) and extend their tentacles, waiting for I (coiled) prey. Examples of the polyp form include hydras and sea Cnldocyte anemones. A medusa is a flattened, mouth-down version of the polyp. It moves freely in the water by a combination of pas .. Figure 33.6 A cnidocyte of a hydra. This type of cnidocyte contains a stinging capsule, the nematocyst, which contains a coiled sive drifting and contractions ofits bell-shaped body. Medusae thread, When a '"trigger" is stimulated by touch or by certain chemicals, include free-swimming jellies. The tentacles of a jelly dangle the thread ShOOIS out, puncturing and injecting poison into prey,
CHAPTER THIRTY·THREE Invertebrates 671 that can penetrate the body wall ofthe cnidarian's prey. Other Hydrozoans kinds of cnidae have long threads that stick to or entangle small prey that bump into the cnidarian's tentacles. Most hydrozoans alternate between polyp and medusa forms, Contractile tissues and nerves occur in their simplest forms as in the life cycle of Obelia (Figure 33.8). The polyp stage, a in cnidarians. Cells ofthe epidermis (outer layer) and gastro~ colony ofinterconnected polyps in the case ofObelia, is more dermis (inner layer) have bundles of microfilaments arranged conspicuous than the medusa. Hydras, among the fewcnidar into contractile fibers (see Chapter 6). The gastrovascular cav ians found in fresh water, are unusual hydrowans in that they ex ity acts as a hydrostatic skeleton against which the contractile ist only in polyp form. \Vhen environmental conditions are cells can work. When a cnidarian closes its mouth, the volume favorable, a hydra reproduces asexually by budding, fonning out of the cavity is fixed, and contraction ofselected cells causes growths that pinch off from the parent and live independently the animal to change shape. Movements are coordinated by a (see Figure 13.2). \Vhen conditions deteriorate, hydras can re nerve net. Cnidarians have no brain, and the noncentralized produce sexually, forming resistant zygotes that remain dor nerve net is associated with sensory structures that are dis mant until conditions improve. tributed radially around the body. Thus, the animal can detect Scyphozoans and respond to stimuli from all dire (b) Many jellies (class Scyphozoa) Cc) The sea wasp (Chironex Cd) Sea anemones and other are bioluminescent. Food fleekefl) is a member of class members of class Anthozoa captured by nematocyst (ubozoa. Its poison, which exist only as polyps. bearmg tentacles is trans· can subdue fish and other .. ~ ferred to specialized oral arms large prey. is more potent (3) These colonial polyps are members of (that lack nematocysts) for than cobra venom. class Hydrozoa. transport to the mouth . .&. Figure 33.7 Cnidarians. 672 UNIT fiVE The Evolutionary History of Biological Diversity Gather polyps, specialized OMedusae swim for reproduction, lack off, grow, and tentacles and produce tiny reproduce sexually. medusae by asexual budding. e Some of the colony's polyps, equipped with tentacles, are specialized for feeding. o Acolony of interconnected polyps (inset, LM) results 1------1>. from asexual j; reproduction Medusa by budding. :r~erm SEXUAL Egg REPRODUCTION ASEXUAL REPRODUCTION Portion of (BUDDING) a colony of polyps De~eloping polyp K,y I~ oThe planula eventually settles "The zygote de~elops into a Haploid (nJ and develops into a new polyp. solid ciliated larva called a planula. • Diploid (2n) ... Figure :n.8 The life cycle of the hydrozoan Obelia. The polyp stage is asewal, and the medusa stage is sexual; these two stages alternate, one producing the other, Do not confuse this with the alternation of generations that occurs in plants and some algae: In Obelia, both the polyp and the medusa are diploid organisms, Typical of animals. only the single-celled gametes are haploid By contrast. plants ha~e a multicellular haploid generation and a multicellular diploid generation -'moil. Suppose that Obelia medusae and gametes were haploid. but all other stages were diploid. What aspects of its actual life cycle would have to change for this to occur? Anthozoans be contributing to their demise by raising seawater tempera Sea anemones (see Figure 33.7d) and corals belong to the class n tures above the narrow range in which corals thrive. Anthozoa (meaning "flower animals ). These cnidarians occur only as polyps. Corals live as solitary or colonial forms, and CONCEPT CHECK 33.2 many species secrete a hard external skeleton of calcium car 1. Compare and contrast the polyp and medusa forms bonate. Each polyp generation builds on the skeletal remains of ofcnidarians. n earlier generations, constructing "rocks with shapes charac 2. Describe the structure and function ofthe stinging teristic of their species. It is these skeletons that we usually cells for which cnidarians are named. think ofas coral. 3. -','!lO"g If the common ancestor ofcnidarians Coral reefs are to tropical seas what rain forests are to tropi were an open-ocean jelly, what might you infer about cal land areas: They provide habitat for a wealth ofotherspecies. evolutionary trends in the relative importance ofthe Unfortunately, like rain forests, coral reefs are being destroyed polyp and medusa stages? at an alarming rate by human activity. Pollution and overfishing For suggested answers, see Appendix A. are major threats, and global warming (see Chapter 55) may also CHAPTER TH'RTY·THREE Invertebrates 673 T." 33.2 Classes of Phylum Platyhelminthes r~:;:::;c~:Z~~ns, a clade identified Class and Examples Main Characteristics by molecular data, have the widest Turbellaria (mostly free·living Most marine, some fresh flatworms, such as Dugesia: water, a few terrestrial; range of animal body forms see Figures 33.9 and 33.10) predators and scavengers; body surface ciliated The vast majority of animal C~k~re~ ~nd Slll(e~ Monogenea (monogeneans) Marine and freshwater para· Crud~ri~ species belong to the dade sites; most infect external lophotro Flatworms Turbellarians Flatworms (phylum Platyhelminthes) live in marine, freshwa Turbellarians are nearly all free-living and mostly marine ter, and damp terrestrial habitats. In addition to free· living (Figure 33.9). The best-known freshwater turbellarians are forms, flarn'orms include many parasitic species, such as members ofthe genus Dugesia, commonly called planarians. flukes and tapeworms. Flatworms are so named because they Abundant in unpolluted ponds and streams, planarians prey have thin bodies that are flattened dorsoventrally (between on smaller animals or feed on dead animals. They move by us the dorsal and ventral surfaces); platyhelminth means "flat ing cilia on their ventral surface, gliding along a film ofmucus worm." (Note that worm is not a formal taxonomic name but they secrete. Some other turbellarians also use their muscles a general term for animals with long, thin bodies.) The small to swim through water with an undulating motion. est flatworms are nearly microscopic free-living species, while A planarian's head is equipped with a pair of light·sensitive some tapeworms are more than 20 m long. eyespots and lateral flaps that function mainly to detect specific Although flatworms undergo triploblastic development, chemicals. The planarian nervous system is more complex and they are acoelomates (animals that lack a body cavity). Their flat centralized than the nerve nets of cnidarians (Figure 33.10). shape places all their cells close to water in the surrounding en Experiments have shown that planarians can learn to modify vironmentor in their gut. Because ofthis proximity to water, gas their responses to stimuli. 674 UNIT fiVE The Evolutionary History of Biological Diversity ... Figure 33.10 Anatomy of Digestion is completed within a planarian. a turbellarian. the cells lining the gastrovas cular cavity, which has many Pharynx. The mouth is at the fine subbranches that provide tip of a muscular pharynK. an e>rt.ensive surface area. Digestive juices are spilled onto prey, and the pharym sucks small pieces of food Undigested wastes into the gastrovascular cavity, are egested where digestion continues. through the mouth. Gastrovascular caVity ... Figure 33.9 A marine flatworm (class Turbellarial. ';;"';Io,-Eyespots Some planarians can reproduce asexually through fission. The parent constricts roughly in the middle of its body, sepa rating into a head end and a tail end; each end then regenerates the missing parts. Sexual reproduction also occurs. Planarians Ganglia. At the anterior end Ventral nerve cords. From are hermaphrodites, and copulating mates typically cross of the worm, near the main sources the ganglia, a pair of of sensory input, is a pair of ganglia, ventral nerve cords runs fertilize each other. dense clusters of nerve cells. the length of the body. Monogeneans and Trematodes o Mature flukes live in the blood vessels of the human intestine A female fluke fits into a groove running Monogeneansand trematodes live as para the length of the larger male's body, as shown in sites in or on other animals. Many have the lM at right. suckers thatattach to the internal organs or outer surfaces of the host animal. A tough covering helps protect the parasites within their hosts. Reproductive organs occupy nearly the entire interior ofthese worms. As agroup, trematodes parasitize awide range ofhosts, and most species have com >----< o These larvae penetrate ~ 1 mm plex life cycles with alternating sexual and the skin and blood asexual stages. Manytrematodes require an vessels of humans e Blood flukes reproduce intermediate host in which larvae develop working in fields AI" sexually in the human irrigated with water host. The fertilized eggs before infecting the final host (usuallya ver contaminated with exit the host in feces. tebrate), where the adult worms live. For ex infected ample, trematodes that parasitize humans human feces, spend part of their lives in snail hosts o If the feces reach a (Figure 33.11). Around the world, some pond or other source of water, the eggs 200 million people are infected with blood. develop into ciliated flukes (Schistosoma) and suffer from schis larvae. These larvae infect snails, the tosomiasis, a disease whose symptoms in intermediate hosts, clude pain, anemia, and dysentery. o AseKual reproduction Living within different hosts puts de within a snail results In another type of motile mands on trematodes that free-living ani larva, which escapes mals don't face. Ablood. fluke, for instance, from the snail host. must evade the immune systems of both ... Figure 33.11 The life cycle of a blood fluke (Schistosoma manson;), a trematode. snails and humans. By mimicking the sur _I,J:t.iI!. Snails eat algae, whose growth is stimulated by nutrients found in fertilizer. How face proteins of its hosts, the blood fluke would the contamination of irrigation water with fertilizer likely affect the occurrence of creates a partial immunological camou- schistosomiasis? Explain, CHAPTER THIRTY·THREE Invertebrates 675 flage for itself. It also releases molecules that manipulate the that encyst in muscles of these animals. A human acquires the hosts' immune systems into tolerating the parasite's existence. larvae by eating undercooked meat contaminated with cysts, These defenses are so effective that individual blood flukes can and the worms develop into mature adults within the human. survive in humans for more than 40 years. Large tapeworms can block the intestines and rob enough nu Most monogeneans, however, are external parasites of fish. trients from the human host to cause nutritional deficiencies. The monogenean life cycle is relatively simple; a ciliated, free Doctors use an oraUy administered drug, niclosamide, to kill the swimming larva initiates the infection ofa host fish. Although adult worms. monogeneans have been traditionally aligned with the trematodes, some structural and chemical ev Roti!ers idence suggests they are more closely related to Rotifers (phylum Rotifera) are tiny animals that inhabit fresh tapeworms. water, marine, and damp soil habitats. Ranging in size from about SO Ilm to 2 mm, rotifers are smaller than many protists Tapeworms but nevertheless are multicellular and have specialized organ Tapeworms (class Cestoda) are also parasitic systems (Figure 33,13). in contrast to cnidarians and flat (figure 33.12). The adults live mostly inside worms, which have a gastrovascular cavity, rotifers have an vertebrates, including humans. In many tape alimentary canal, a digestive tube with a separate mouth and worms, the anterior end, or scolex, is armed anus. Internal organs lie within the pseudocoelom, a body cav with suckers and often hooks that the worm itythat is notcompletely lined by mesoderm (see Figure 32.8b). uses to attach itself to the intestinal lining of Fluid in the pseudocoelom serves as a hydrostatic skeleton its host. Tapeworms lack a mouth and gas· (see Chapter SO). Movement ofa rotifer's body distributes the trovascular cavity; they absorb nutrients re fluid throughout the body, circulating nutrients. leased by digestion in the host's intestine. The word rotifer, derived from Latin, means "wheel-bearer," Absorption occurs across the tapeworm's a reference to the crown of cilia that draws a vortex of water body surface. into the mouth. Posterior to the mouth, a region ofthe diges Posterior to the scolex is a long ribbon of tive tract called the pharynx bears jaws called trophi that grind units called proglottids, which are little more up food, mostly microorganisms suspended in the water. than sacs of sex organs. After sexual repro Rotifers exhibit some unusual fonns of reproduction. Some duction, proglottids loaded with thousands species consist only of females that produce more females from offertilized eggs are released from the poste wU'ertilized eggs, a type ofreproduction caUed parthenogenesis. rior end of a tapeworm and leave the host's Other species produce two types of eggs that develop by body in feces. In one type of life cycle, in parthenogenesis. One type forms females while the other type fected feces contaminate the food or water of (produced when conditions deteriorate) develops into simpli intermediate hosts, such as pigs or cattle, fied males that cannot even feed themselves. These males sur and the tapeworm eggs develop into larvae vive only long enough to fertilize eggs, which form resistant 'I---'7Proglottids with reproductive strUdures Scolex .... Figure 33.13 A rotifer. These pseudocoelomates, smaller than .... figure 33.12 Anatomy of a tapeworm. The inset shows a many protists, are generally more anatomically complex than close-up of the scolex (colorized SEM), flatworms (lM). 676 UNIT fiVE The Evolutionary History of Biological Diversity zygotes that can survive when a pond dries up. When condi tions are favorable, the zygotes break dormancy and develop into a new female generation that reproduces by parthenogen esis until conditions become unfavorable again. It is puzzling that so many rotifer species survive without males. The vast majority ofanimals and plants reproduce sex ually at least some of the time, and sexual reproduction has certain advantages over asexual reproduction. For example, species that reproduce asexually tend to accumulate harmful mutations in their genomes faster than sexually reproducing species. As a result, asexual species should experience higher rates ofextinction and lower rates ofspeciation. Seeking to understand this unusual group, Nobel Prize winning biologist Matthew Meselson, of Harvard Vniversity, (a) Ectoprocts, such as this sea mat (b) Brachiopods have a hinged has been studying a class ofasexual rotifers named Bdel1oidea. (Membranipora membranacea), shell. The two parts of the Some 360 species ofbdel1oid rotifers are known, and all ofthem are coloniallophophorates. shell are dorsal and ventral. reproduce by parthenogenesis without any males. Paleontolo ... Figure 33.14 Lophophorates. gists have discovered bdel10id rotifers preserved in 35-mil1ion year-old amber, and the morphology ofthese fossils resembles only the female form, with no evidence of males. By comparing chiopod shell are dorsal and ventral rather than lateral, as in the DNA ofbdelloids with that oftheir closest sexually repro clams (Figure 33.14b). All brachiopods are marine. Most live ducing rotifer relatives, Meselson and his colleagues concluded attached to the seafloor by a stalk, opening their shell slightly to that bdel10ids have likely been asexual for much longer than allow water to flow through the lophophore. The living bra 35 million years. Howthese animals manage to flout thegeneral chiopods are remnants of a much richer past that included rule against long-lived asexuality remains a puzzle. 30,000 species in the Paleozoic and Mesozoic eras. Some living brachiopods, such as those in the genus Lingula, are nearly Lophophorates: Ectoprocts and Brachiopods identical to fossils ofspecies that lived 400 million years ago. Bilaterians in the phyla Ectoprocta and Brachiopoda are among Molluscs those known as lophophorates. These animals have a /ophophore, a crown of ciliated tentacles that surround the Snails and slugs, oysters and clams, and octopuses and squids mouth (see Figure 32. 13a). As the cilia draw water toward the are all molluscs (phylum Mollusca). Most molluscs are ma mouth, these tentacles trap suspended food particles. Other rine, though some inhabit fresh water, and some snails and similarities, such as a V-shaped alimentary canal and the ab slugs live on land. Molluscs are soft-bodied animals (from the sence ofa distinct head, reflect these organisms' sessile exis Latin mol/uscus, soft), but most secrete a hard protective shell tence. In contrast to flatworms, which lack a body cavity, and made ofcalcium carbonate. Slugs, squids, and octopuses have rotifers, which have a pseudocoelom, lophophorates have a a reduced internal shell or have lost their shell completely dur true coelom that is completely lined by mesoderm (see ing their evolution. Figure 32.8a). Despite their apparent differences, all molluscs have a sim Ectoprocts (from the Greek ectv, outside, and procta, anus) ilar body plan (Figure 33.15, on the next page). Molluscs are are colonial animals that superficially resemble clumps of coelomates, and their bodies have three main parts: a muscu moss. (In fact, their common name, bryozoans, means ~moss lar foot, usually used for movement; a visceral mass contain animals.~) In most species, the colony is encased in a hard ing most of the internal organs; and a mantle, a fold of tissue exoskeleton (external skeleton) studded with pores through that drapes over the visceral mass and secretes a shell (ifone is which the lophophores extend (Figure 33.14a). Most ecto present). In many molluscs, the mantle extends beyond the proct species live in the sea, where they are among the most visceral mass, producing a water-filled chamber, the mantle widespread and numerous sessile animals. Several species are cavity, which houses the gills, anus, and excretory pores. important reef builders. Ectoprocts also live in lakes and rivers. Many molluscs feed by using a straplike rasping organ called a Colonies of the freshwater ectoproct Pectinate/ILl magnifica radula to scrape up food. grow on submerged sticks or rocks and can grow into a gelati Most molluscs have separate sexes, and their gonads (ovaries nous, ball-shaped mass more than 10 em across. or testes) are located in the visceral mass. Many snails, however, Brachiopods, or lamp shells, superficially resemble clams are hermaphrodites. The life cycle of many marine molluscs in and other hinge-shelled molluscs, butthe two halves ofthe bra- cludes aciliated larval stage, the trochophore (see Figure 32.13b), CHAPTER THIRTY·THREE Invertebrates 677 Heart. Most molluscs have an open circulatory system. The dorsally located heart pumps Nephridium. Excretory organs circulatory fluid called hemolymph through arteries called nephridia remove metabolic into sinuses (body spaces). The organs of the wastes from the hemolymph. mollusc are thus continually bathed in hemolymph. Visteral mass. The long digestive tract is coiled in the visceral mass. Coelom Intestine Gonads Radula. The mouth region in many Mantle mollusc species cavity contains a rasp-like feeding organ Anus ..."....~~!r:H called a radula. This The nervous system ( belt of backward- consists of a nerve Gill curved teeth ~-==i:::::.,.-_":::!~!!~=E~~~;:'\ repeatedly thrusts esophagus,ring aroundfromthe I outward and then which nerve cords Mouth retracts into the mouth, scraping extend. Foot Nerve Esophagus cords and scooping like a backhoe. ... Figure 33.15 The basic body plan of a mollusc. T."J:U Major Classes of Phylum Mollusca Class and Examples Main Characteristics Polyplacophora (chitons; Marine; shell with eight plates; see Figure 33.16) foot used for locomotion; radula; no head Gastropoda (snails, slugs; Marine, freshwater, or terres see Figures 33.17 and 33.18) trial; head present; a symmetri cal body, usually with a coiled shell; shell reduced or absent; foot for locomotion; radula Bivalvia (dams, mussels, Marine and freshwater; flat scallops, oysters; see tened shell with two valves; Figures 33.19 and 33.20) head reduced; paired gills; no .... Figure 33.16 A chiton. Note the eight-plate shell charaderistic radula; most are suspension of molluscs in the class Polyplacophora. feeders; mantle forms siphons Cephalopoda (squids, Marine; head surrounded by Chitons octopuses, cuttlefishes, grasping tentddes, usually chambered nautiluses; with suckers; shell external, Chitons have an oval-shaped body and a shell divided into see Figure 33.21) internal, or absent; mouth with eight dorsal plates (Figure 33.16). The chiton's body itself, or without radula; locomotion however, is unsegmented. You can find these marine animals by jet propulsion using siphon clinging to rocks along the shore during low tide. If you try to formed from foot dislodge a chiton by hand, you will be surprised at how well its foot, acting as a suction cup, grips the rock. A chiton can also which is also characteristic of marine annelids (segmented use its foot to creep slowly over the rock surface. Chitons use worms) and some other 10photrochozoans. their radula to scrape algae offthe rock surface. The basic body plan ofmolluscs has evolved in various ways Gastropods in the phylum's eight classes. We'll examine four ofthose clas ses here (Table 33.3): Polyplacophora (chitons), Gastropoda About three-quarters ofall living species of molluscs are gas (snails and slugs), Bivalvia (clams, oysters, and other bivalves), tropods (Figure 33.17). Most gastropods are marine, but and Cephalopoda (squids, octopuses, cuttlefishes, and cham~ there are also many freshwater species. Some gastropods have bered nautiluses). adapted to life on land, including garden snails and slugs. 678 UNIT fiVE The Evolutionary History of Biological Diversity A distinctive characteristic ofclass Gastropoda is a develop Bivalves mental process known as torsion. As a gastropod embryo de The moUuscs ofclass Bivalvia include many species of clams, velops, its visceral mass rotates up to 180', causing the animal's oysters, mussels, and scallops. Bivalves have a shell divided into anus and mantle cavity to wind up above its head (Figure 33.18). two halves (Figure 33.19). The halves are hinged at the mid After torsion, some organs that were bilateral may be reduced in dorsal line, and powerful adductor muscles draw them tightly size, while others may be lost on one side of the body. Torsion together to protect the animal's soft body. Bivalves have no dis should not be confused with the formation of a coiled shell, tinct head, and the radula has been lost. Some bivalves have which is an independent developmental process. eyes and sensory tentacles along the outer edge oftheir mantle. Most gastropods have a single, spiraled shell into which the The mantle cavity ofa bivalve contains gills that are used for animal can retreat when threatened. The shell is often conical gas exchange as well as feeding in most species (Figure 33.20). but is somewhat flattened in abalones and limpets. Many gas Most bivalves are suspension feeders. They trap fine food par tropods have a distinct head with eyes at the tips of tentacles. ticles in mucus that coats their gills, and cilia then convey those Gastropods move literally at a snail's pace by a rippling motion particles to the mouth. Water enters the mantle cavity through oftheir foot or by means ofcilia, often leaving a trail ofslime in an incurrent siphon, passes over the gills, and then exits the their wake. Most gastropods use their radula to graze on algae or mantle cavity through an excurrent siphon. plants. Several groups, however, are predators, and their radula has become modified for boring holes in the shells ofother mol luscs or for tearing apart prey. In the cone snails, the teeth ofthe radula act as poison darts that are used to subdue prey. Terrestrial snails lack the gills typical of most aquatic gas tropods. Instead, the lining oftheir mantle cavity functions as a lung, exchanging respiratory gases with the air. ... Figure 33.19 A bivalve. This scallop has many eyes (dark blue spots) peering out from each half of its hirlged shell. (a) A land snail Coelom Hinge area ~;;ti::~G~",~ Heart Adductor ,~ muscle ArluS ... Figure 33.17 (b) A sea slug. Nudibranchs. or sea slugs, lost Gastropods. their shell during their evolution. 11~~~~r1~~ .~ Excurrentiphon Stomach Intestine Mouth ;~:~=:t::;~ Waterflow Foot Mantle~ Incurrent cavity Gonad Gill siphon ... Figure 33.18 The results of torsion in a gastropod. Because ... Figure 33.20 Anatomy of a clam. Food particles suspended of torsion (twisting of the visceral mass) durirlg embryonic development. the in water that enters through the incurrent siphon are collected by the digestive tract is coiled and the anus is near the anterior end of the animal. gills and passed via cilia to the mouth. CHAPTER THIRTY·THREE Invertebrates 679 Most bivalves lead sedentary lives, a characteristic suited to a complex brain. The ability to learn and behave in a complex suspension feeding. Sessile mussels secrete strong threads that manner is probably more critical to fast-moving predators tether them to rocks, docks, boats, and the shells ofother ani~ than to sedentary animals such as clams. mals. However, clams can pull themselves into the sand or The ancestors of octopuses and squids were probably mud, using their muscular foot for an anchor, and scallops can shelled molluscs that took up a predatory lifestyle; the shell skitter along the seafloor by flapping their shells, rather like the was lost in later evolution. Shelled cephalopods called mechanical false teeth sold in novelty shops. ammonites, some of them as large as truck tires, were the dominant invertebrate predators of the seas for hundreds of Cephalopods millions of years until their disappearance during the mass extinction at the end of the Cretaceous period, 65.5 million Cephalopods are active predators (Figure 32.21). They use years ago (see Chapter 25). their tentacles to grasp prey, which they then bite with beak Most species ofsquid are less than 75 cm long, but some are like jaws and immobilize with a poison found in their saliva. considerably larger. The giant squid (Architeulhis dux) was for The foot ofa cephalopod has become modified into a muscu a long time the largest squid known, with a mantle up to 2.25 m lar excurrent siphon and part of the tentacles. Squids dart long and a total length of 18 m. In 2003, however, a specimen about by drawing water into their mantle cavity and then fir of the rare species Mesvnychoteuthis hami/toni was caught ing a jet ofwater through the excurrent siphon; they steer by near Antarctica; its mantle was 2.5 m long. Some biologists pointing the siphon in different directions. Octopuses use a think that this specimen was a juvenile and estimate that similar mechanism to escape predators. adults of its species could be ty,'ice as large! Unlike A. dux, A mantle covers the visceral mass of cephalopods, but the which has large suckers and small teeth on its tentacles, M. shell is reduced and internal (in squids and cuttlefishes) or hamilton! has two rows ofsharp hooks at the ends ofits tenta missing altogether (in many octopuses). One small group of cles that can deliver deadly lacerations. shelled cephalopods, the chambered nautiluses, survives today. It is likely that A. dux and M. hamilton! spend most oftheir Cephalopods are the only molluscs with a closed circula time in the deep ocean, where they may feed on large fishes. tory system. They also have well-developed sense organs and Remains of both giant squid species have been found in the stomachs ofsperm whales, which are probably their only nat .. Octopuses are considered among the most ural predator. In 2005, scientists reported the first observa Intelligent invertebrates. tions of A. dux in the wild, photographed while attacking baited hooks at a depth of 900 m. M. hamiltvni has yet to be observed in nature. Overall, these marine giants remain • Squids are speedy carnivores with among the great mysteries of invertebrate life. beak-like Jaws and well·developed eyes. Annelids Annelida means "little rings; referring to the annelid body's re~ semblance to a series of fused rings. Annelids are segmented Table lJ.4 Classes of Phylum Annelida Class and Examples Main Characteristics Oligochaeta (freshwater, marine, Reduced head; no parapodia, and terrestrial segmented but chaetae present worms; see Figure 33.22) Polychaeta (mostly marine Many have a well-developed segmented worms; see head; each segment usually Figure 33.23) has parapodia with many chaetae; free-living Hirudinea (leeches; see Body usually flattened, with .. Chambered Figure 33.24) reduced coelom and segmen nautiluses are the only living tation; chaetae usually absent; cephalopods with suckers at anterior and pos an external shell terior ends; parasites, preda tors, and SCAvengers ... Figure 33.21 Cephalopods. 680 UNIT fiVE The Evolutionary History of Biological Diversity worms that live in the sea, in most freshwater habitats, and in Oligochaetes damp soil. Annelids are coelomates, and they range in length Oligochaetes (from the Greek oligos, few, and cllaite, long hair) from less than 1 mm to more than 3 m, the length ofa giant Aus tralian earthworm. are named for their relatively sparse chaetae, or bristles made of The phylum Annelida can be divided into three classes chitin. TIlis class ofsegmented worms includes tlle earthworms (Table 33,4, on the facing page): Oligochaeta (the earth and a variety ofaquatic species. Figure 33.22 provides a guided worms and their relatives), Polychaeta (the polychaetes), and tour ofthe anatomy ofan earthwonn, which is representative of Hirudinea (the leeches), annelids. Earthworms eat their way through the soil, extracting Each segment is surrounded by longitudinal muscle, which in Coelom. The coelom Metanephridium. Each turn is surrounded by circular muscle. Earthworms coordinate of the earthworm is segment of the worm the contraction of these two sets of muscles to move partitioned by septa. contains a pair of (see Figure 50.33). These muscles work against the non excretory tubes, called compressible coelomic fluid, which acts as a hydrostatic skeleton. meta nephridia, with ciliated funnel-shaped openings called Epidermis nephrostomes. The Many of the internal Septum meta nephridia remove structures are repeated (partition wastes from the blood within each segment of between and coelomic fluid the earthworm. segments) through exterior pores. longitudinal muscle Dorsal--f!'lII Chaetae. Each segment vessel has four pairs of chaetae, bristles that I-~===::: Tiny blood provide traction for vessels are burrowing. abundant in the earthworm's --ii---Iskin, which functions as its Fused Venlral respiratory or nerve vessel gan. The blood cords contains oxygen carrying Nephrostome hemoglobin. Clitellum Metanephridium Intestine Giant Australian earthworm Cerebral ganglia. The earthworm L---"""""":~~~ nervous system features a brain like pair of cerebral ganglia above and in front of the pharynx. A Mouth ring of nerves around the pharynx connects to a subpharyngeal ganglion, from which a fused pair of nerve cords runs posteriorly. Ventral nerve cords with segmental The circulatory system, a network of vessels, ganglia. The nerve cords penetrate the is closed. The dorsal and ventral vessels are septa and run the length of the anima!, linked by segmental pairs of vessels. The as do the digestive tract and dorsal vessel and five pairs of vessels that longitudinal blood vessels. circle the esophagus are muscular and pump blood through the circulatory system. ... Figure 33.22 Anatomy of an earthworm, an oligochaete. CHAPTER THIRTY·THREE Invertebrates 681 nutrients as the soil passes through the alimentary canal Undi .. Figure 33.24 Aleech. A gested material, mixed with mucus secreted into the canal, is nurse applied thIS medJonalleech (Hirudo l'nf'didnaJisl to a patient's eliminated as fecal castings through the anus. Farmers value SCH"e thumb 10 drain blood from a earthworms because the animals till and aerate the earth, and hematoma (an abnormal accumulation of blood around an their castings improve the texture ofthe soil. (Charles Darwin II'Iternal inJury). estimated that a single acre of British farmland contains about SO,coo earth....,orms, producing 18 tons ofcastings per year.) Earthworms are hermaphrodites, but they cross-fertilize. Two earthworms mate by aligning themselves in opposite direc lions in such a way that they exchange sperm (see figure 46.1), and then they separate. The received sperm are stored tem porarily while an organ called the c1itellum secretes a cocoon of mucus. The cocoon slides along the worm, picking up the eggs and then the stored sperm. The cocoon then slips off the Many are predators that feed on other invertebrates, but some worm's head and remains in the soil while the embryos de are parasites that suck blood by attaching temporarily to other velop. Some earthworms can also reproduce asexually by frag animals, including humans (Figure 33.24). Some parasitic mentation followed by regeneration. species use bladelike jaws to slit the skin oftheir host, whereas others secrete enzymes that digest a hole through the skin. Polychaeles The host is usually oblivious to this attack because the leech Each segment of a polychaete has a pair of paddle-like or secretes an anesthetic. After making the incision, the leech se ridge-like structures called parapodia rnear feet") that func cretes another chemical, hirudin, which keeps the blood ofthe tion in locomotion (Figure 33.23). Each parapodium has nu host from coagulating near the incision. The parasite then merous chaetae, so polychaetes usually have many more sucks as much blood as it can hold, often more than ten times chaetae per segment than do oligochaetes. In many poly its own weight. After this gorging, a leech can last for months chaetes, the parapodia are richly supplied with blood vessels without another meal. and also function as gills. Until this century, leeches were frequently used for bloodlet Polychaetes make up a large and diverse class, most of ting. Today they are used to drain blood that accumulates in tis whose members are marine. A few species drift and swim sues following certain injuries or surgeries. Researchers have among the plankton, many crawl on or burrow in the seafloor, also investigated the potential use of hirudin to dissolve un and many others live in tubes. Some tube-d .....ellers, such as the wanted blood dots that form during surgery or as a result of fan worms, build their tubes by mixing mucus with bits ofsand heart disease. Several recombinant forms of hirudin have been and broken shells. Others, such as Christmas tree worms (see developed, two ofwhich were recently approved fordinical use. Figure 33.1 l, construct tubes using only their own secretions. As a group, Lophotrocholoa encompasses a remarkable range ofbody plans, as illustrated by members ofsuch phyla as Leeches Rotifera, Ectoprocta, Mollusca, and Annelida. Next we'll ex plore the diversity of Ecdysoloa, a dominant presence on The majority of leeches inhabit fresh water, but there are also Earth in terms ofsheer number ofspeCies. marine species as well as terrestrial leeches found in moist vegetation. Leeches range in length from about 1 to 30 cm. CONCEPT CHECI( n.) I. Explain how tapeworms can survive without a .. Figure 33.23 A coelom, a mouth, a digestive system, or an excretory polychaete. HesioIyra berg; system. lives on the seafloor around deep-sea hydrothermal vents. 2. How does the modification ofthe molluscan foot in gastropods and cephalopods relate to their respecti\'e lifestyles? 3. Annelid anatomy can be described as -a tube within a tube:" Explain. 4. _lili','Ii* Relatively few free-living lophotrc chozoans live on land, above the surface ofthe soil. Focusing on gravity, hypothesize why this is so. for suggested answers. see Appendix A. 682 UNIT fiVE The Evolutionary History of Biological Diversity Multitudes ofnematodes live in moist soil and in decompos ing organic matter on the bottoms of lakes and oceans. While r::;;:::o~~'~re the most 25,000 species are known, perhaps 20 times that number actu species-rich animal group ally exist. It has been said that if nothing but nematodes re mained on Earth, they would still preserve the outline of the r;;==; Calcarea and Silicea Although defined primarily by planet and many ontsfeatures. These free-living worms play an Cmdana molecular evidence, the dade important role in decomposition and nutrient cycling, but little Lopholnxhowa Ecdysozoa includes animals is known about most species. One species of soil nematode, Ecdysozoa Deuterostorma that shed a tough external coat Caellorhabditis elegalls, however, is very well studied and has (cuticle) as they grow; in fact, become a model research organism in biology (see Chapter 21). the group derives its name from this process, which is called Ongoing studies on C. elegans are revealing some ofthe mech molting, or ecdysis. Ecdysozoa consists ofabout eight animal anisms involved in aging in humans, among other findings. phyla and contains mOTe known species than all other protist, Phylum Nematoda includes many significant agricultural fungus, plant, and animal groups combined. Here we'll focus pests thatattack the roots ofplants. Other species ofnematodes on the two largest e ... Figure 33.26 Juveniles of the parasitic nematode .. Figure 33.25 Afree-living nematode (colorized SEM), Trichinella spiralis encysted in human muscle tissue (LM). CHAPTER THIRTY·THREE Invertebrates 683 then supply the nematode with nutrients. These extraordinary • FI 33.28 • parasites have been dubbed "animals that act like viruses." Did the arthropod body plan result from new Arthropods Hox genes? 18 Zoologists estimate that there are about a billion billion (10 ) EXPERIMENT How did the highly successful arthropod body arthropods living on Earth. More than 1 million arthropod plan arise? One hypothesis suggests that it resulted from the ori species have been described, mostofwhich are insects. In fact, gin (by a gene duplication event) of two unusual Hox genes found in arthropods: Ultrabithorax (Vbx) and abdominal-A (abd-A). To two out of every three known species are arthropods, and test this hypothesis, Sean Carroll, of the University of Wisconsin, members ofthe phylum Arthropoda can be found in nearly all Madison, and colleagues turned to the onychophorans, a group of habitats of the biosphere. By the criteria ofspecies diversity, invertebrates closely related to arthropods. Unlike many living arthropods, onychophorans have a body plan in which most body distribution, and sheer numbers, arthropods must be reo segments are identical to one another, Thus, Carroll and col garded as the most successful ofall animal phyla. leagues reasoned that if the origin of the Ubx and abd-A Hox genes drove the evolution of body segment diversity in arthro Arthropod Origins pods, these genes probably arose on the arthropod branch of the evolutionary tree: Biologists hypothesize that the diversity and success of arthropods is related to their body plan-their segmented Other ecdysoloans bodies, hard exoskeleton, and jointed appendages (arthropod Origin of ubx and means "jointed feet"). The earliest fossils with this body plan abd-A Hoxg~? - Arthropods are from the Cambrian explosion (535-525 million years r--"'I-.- ago), indicating that the arthropods are at least that old. Along with arthropods, the fossil record of the Cambrian explosion contains many species oflvbopods, an extinct group Common ancestor Of/...---- Onychophorans from which arthropods may have evolved. Lobopods such as onychophorans and arthropods Hallucigenia (see Figure 25.4) had segmented bodies, but most of their body segments were identical to one another. Early As the hypothesis depided above suggests, Ubx and abd-A would not have been present in the common ancestor of arthro arthropods, such as the trilobites, also showed little variation pods and onychophorans. and hence. onychophorans should not from segment to segment (Figure 33.27). As arthropods con have these genes. To find out whether this was the case. Carroll tinued to evolve, the segments tended to fuse and become and colleagues eKamined the Hox genes of the onychophoran Acanrhokara kaputensis. fewer, and the appendages became specialized for a variety of functions. These evolutionary changes resulted not only in RESULTS The onychophoran A. kaputensis has all great diversification but also in an efficient body plan that per arthropod Hox genes, including Ubx and abd·A, mits the division oflabor among different body regions. What genetic changes led to the increasing complexity of Red indicates the body regions of this the arthropod body plan? Arthropods today have two unusual onychophoran embryo Hox genes, both of which influence segmentation. To test in which Ubx or abd~A genes were whether these genes could have driven the evolution of in expressed. (The creased body segment diversity in arthropods, Sean Carroll inset shows this (see pages 534-535) and colleagues studied Hox genes in ony area enlarged.) chophorans, dose relatives of arthropods (Figure 33.28). Ant", antenna J '" jaws L1-L15 '" body segments .. Figure 33.27 A trilobite fossil. Trilobites were common denlzens of the shallow seas throughout the PaleozoIC era but CONCLUSION Since A. kaputensis, an onychophoran, has the disappeared with the great arthropod Hox genes, the evolution of increased body segment di Permian extindions about 250 versity in arthropods must not have been related to the origin of neY\' million years ago. Paleontologists Hax genes, have described about 4,000 SOURCE trilobIte species. J K. Grenier. 5 C~rroll et ~I, Evolution 01 tfle entire ~rthropod Hox gene >et predated the ongln and r~diatlon 01 the onjl(hophor~rv'~rthropod d~de. CUflf'flt Biology 7547-553 (1997), .i,lI:tU!£W If Carroll and colleagues had found that A kaputen· sis did nor have the Ubx and abd-A Hox genes, how would their conclusion have been affected? Explain. 684 UNIT fiVE The Evolutionary History of Biological Diversity Their results indicate that arthropod body plan diversity did ter. Arthropods began to diversify on land following the colo not arise from the acquisition of new Hox genes. Instead, the nization of land by plants in the early Paleozoic. Evidence in evolution of body segment diversity in arthropods may have cludes a 428-million-year-old fossil of a millipede found in been driven by changes in the sequence or regulation ofexist 2004 by an amateur fossil hunter in Scotland. Fossilized tracks ing Hox genes. (See Chapter 25 for a discussion of how of other terrestrial arthropods date from about 450 million changes in form can result from changes in the sequence or years ago. regulation of developmental genes such as Hox genes.) Arthropods have well-developed sensory organs, including eyes, olfactory (smell) receptors, and antennae that function in both touch and smell. Most sensory organs are concentrated General Characteristics ofArthropods at the anterior end of the animal. Over the course ofevolution, the appendages ofsome arthro Like many molluscs, arthropods have an open circulatory pods have become modified, specializing in functions such as system, in which fluid called hemolymph is propelled by a walking, feeding, sensory reception, reproduction, and de heart through short arteries and then into spaces called si fense. Figure 33.29 illustrates the diverse appendages and nuses surrounding the tissues and organs. (The term blood is other arthropod characteristics ofa lobster. generally reserved for fluid in adosed circulatory system.) He The body of an arthropod is completely covered by the cuti molymph reenters the arthropod heart through pores that are cle, an exoskeleton constructed from layers of protein and the usually equipped with valves. The hemolymph-filled body si polysaccharide chitin. The cuticle can be thick and hard over nuses are collectively called the hemocoel, which is not part of some parts of the body and paper-thin and flexible over others, the coelom. Although arthropods are coelomates, in most such as the joints. The rigid exoskeleton protects the animal and species the coelom that forms in the embryo becomes much provides points ofattachment for the muscles that move the ap reduced as development progresses, and the hemocoel be pendages. But it also means that an arthropodcannotgrowwith comes the main body cavity in adults. Despite their similarity, out occasionally shedding its exoskeleton and producing a larger the open circulatory systems of molluscs and arthropods one. This molting process is energetically expensive. A molting probably arose independently. or recently molted arthropod is also vulnerable to predation and A variety ofspecialized gas-exchange organs have evolved in other dangers until its new, soft exoskeleton hardens. arthropods. These organs allow the diffusion ofrespiratory gases \Vhen the arthropod exoskeleton first evolved in the sea, its in spite of the exoskeleton. Most aquatic species have gills with main functions were probably protection and anchorage for thin, feathery extensions that place an extensive surface area in muscles, but it later additionally enabled certain arthropods to contact with the surrounding water. Terrestrial arthropods gen live on land. The exoskeleton's relative impermeability to wa erally have internal surfaces specialized for gas exchange. Most ter helped prevent desiccation, and its strength solved the insects, for instance, have tracheal systems, branched air ducts problem ofsupport when arthropods left the buoyancy ofwa- leading into the interior from pores in the cuticle. ... Figure 33.29 External anatomy of an arthropod. Many of the distlndive Cephalothorax Abdomen features of arthropods are apparent in this dorsal view of a lobster. along with some uniquely crustacean characteristics. The body is segmented. but this charaderistic is obvious only In the abdomen The appendages (including antennae. pincers. mouthparts, walking legs, and swimming appendages) are Jointed, The head bears a pair of compound (multilens) eyes, each situated on a movable stalk, The whole body, including appendages. is covered by an exoskeleton Swimming appendages (one pair located under each abdominal segment) U"-. Walking legs Pincer (defense) Mouthparts (feeding) CHAPTER THIRTY·THREE Invertebrates 685 T.'.:n.5 Subphyla of Phylum Arthropoda Subphylum and Examples Main Characteristics Cheliceriformes Body having one or two main (horseshoe cnlbs, parts; six pairs ofappendages spiders, scorpions, (chelicerae, pedipalps, and four ticks, mites; see pairs ofwalking legs); mostly Figures 33.30-33.32) terrestrial or marine Myriapoda (millipedes Distinct head bearing antennae and centipedes; see and chewing mouthparts; Figures 33.33 and 33.34) terrestrial; millipedes are her bivorous and have two pairs of walking legs per trunk segment; centipedes are carnivorous and have one pair of walking legs per .... Figure 33.30 Horseshoe crabs (Umulus polyphemus). trunk segment and poison claws Common on the Atlantic and Gulf coasts of the United States, these "living fossils" have changed little in hundreds of millions of years. on first body segment They are surviving members of a rich diversity of cheliceriforms that Hexapoda (insects, Body divided into head, thorax, once filled the seas. springtails; see and abdomen; antennae present; Figures 33.35-33.37) mouthparts modified for chewing, sucking, or lapping; three pairs of legs and usually two pairs of wings; mostly terrestrial Crustacea (crabs, Body of two or three parts; lobsters. crayfishes, antennae present; chewing shrimps; see mouthparts; three or more Figures 33.29 and 33.38) pairs of legs; mostly marine and freshwater .. Scorpions have pedipalps Morphological and molecular evidence suggests that living that are pincers special arthropods consist offour major lineages that diverged early in ized for defense and the the evolution of the phylum (Table 33.5): cheliceriforrns (sea capture of food. The tip of the tail bears a poisonous spiders, horseshoe crabs, scorpions, ticks, mites, and spiders); stinger. myriapods (centipedes and millipedes); hexapods (insects and their wingless, six~legged relatives); and crustaceans (crabs, lobsters, shrimps, barnacles, and many others). Cheliceriforms Cheliceriforms (subphylum Cheliceriformes; from the Greek .. Dust mites are ubiquitous c1wilQs, lips, and cheir, arm) are named for clawlike feeding ap scavengers in human dwellings but are harmless pendages called chelicerae, which serve as pincers or fangs. except to those people who Cheliceriforms have an anterior cephalothorax and a poste~ are allergic to them rior abdomen. They lack antennae, and most have simple eyes (colorized SEM), (eyes with a single lens). The earliestcheliceriforms were curyptcrids, or water scor· pions. These marine and freshwater predators grew up to 3 m long; it is thought that some species could have walked on land, much as land crabs do today. Most ofthe marine cheliceriforms, including all of the eurypterids, are extinct. Among the marine cheliceriforms that survive today are the sea spiders (pycnogo 686 UNIT fiVE The Evolutionary History of Biological Diversity and many mites are among a large group of Intestine Stomach parasitic arthropods. Nearly all ticks are Heart Brain bloodsucking parasites that live on the Digestive bodysurfaces ofreptiles or mammals. Par gland asitic mites live on or in a wide variety of vertebrates, invertebrates, and plants. Ovary Arachnids have a cephalothorax that Poison has six pairs ofappendages: the chelicerae; gland a pair ofappendages calledpedipalps that function in sensing, feeding, or reproduc tion; and four pairs of walking legs (Figure 33.32). Spiders use their fang Anus like chelicerae, which are equipped with poison glands, to attack prey. As the che Spinnerets Chelicera Pedipalp licerae pierce the prey, the spider secretes Sperm Silk gland receptacle digestive juices onto the prey's torn tis sues. The food softens, and the spider ... Figure 33.32 Anatomy of a spider. sucks up the liquid meal. In most spiders, gas exchange is car- ried out by book lungs, stacked platelike structures contained in an internal chamber (see Figure 33.32). The extensive sur face area of these respiratory organs is a structural adaptation that enhances the exchange of O2 and CO2 between the he molymph and air. A unique adaptation of many spiders is the ability to catch insects by constructing webs ofsilk, a liquid protein produced by specialized abdominal glands. The silk is spun by organs called spinnerets into fibers that then solidify. Each spider en gineers a web characteristic ofits species and builds it perfectly on the first try. This complex behavior is apparently inherited. Various spiders also use silk in other ways: as droplines for rapid escape, as a cover for eggs, and even as ~gift wrap~ for food that males offer females during courtship. Many small spiders also extrude silk into the air and let themselves be transported by wind, a behavior known as uballooning~ ... Figure :n.:n A millipede. Myriapods Millipedes and centipedes belong to the subphylum Myriapoda, the myriapods. All living myriapods are terrestrial. The myria pod head has a pair of antennae and three pairs ofappendages modified as mouthparts, including the jaw-like mandibles. Millipedes (class Diplopoda) have a large number of legs, though fewer than the thousand their name implies (Figure 33.33). Each trunk segment is formed from two fused segments and bears two pairs of legs. Millipedes eat decaying leaves and other plant matter. They may have been among the earliest animals on land, living on mosses and primitive vascular plants. Unlike millipedes, centipedes (class Chilopoda) are carni vores. Each segment ofa centipede's trunk region has one pair oflegs (Figure 33.34). Centipedes have poison claws on their foremost trunk segment that paralyze prey and aid in defense. ... Figure 33.34 A centipede. CHAPTER THIRTY·THREE Invertebrates 687 The insect body has three regions: head, thorax, and abdomen. The segmentation of the thorax and abdomen are obvious, but the segments that form the head are fused. Cerebral ganglion. The two nerve Abdomen Thorax Head cords meet in the head, where the ...... Compound eye Heart. The insect heart ganglia of several anterior segments drives hemolymph are fused into a cerebral ganglion through an open (brain, colored white below). The ~::::::>Antennae circulatory system. antennae, eyes, and other sense ~~~ organs are concentrated on the head. Anus Malpighian tubules. Metabolic wastes are Vagina removed from the organshemolymphcalledbyMalpighianexcretorYf.--_====~~:::)~1!~~~7~~1"~'il~~~ tubules, which are out-pocketings of the ''JI:~~ digestive tract. Tracheal tubes. Gas exchange in insects is Insect mouthparts are formed from accomplished by a tracheal system of branched. Nerve cords. The insect several pairs of modified appendages. chitin-lined tubes that infiltrate the body and nervous system The mouthparts include mandibles, carry oxygen directly to cells. The tracheal consists of a pair of which grasshoppers use for chewing. system opens to the outside of the body ventral nerve cords In other insects, mouthparts are through spiracles, pores that can control air with several specialized for lapping, piercing, flow and water loss by opening or closing . segmental ganglia. or sucking. ... Figure 33.35 Anatomy of a grasshopper, an insect.