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Inver KEY .... Figure 33.1 What function do the red whorls of this organism have? CONCEPTS 33.1 Sponges are basal animals that lack true tissues 33.2 Cnidarians are an ancient phylum of eumetazoans 33.3 lophotrocholoans, a clade identified by the surrounding water. The tentacles emerge from a tube of calcium carbonate secreted by the worm that protects and supports its soft body. Light-sensitive structures on the tentacles can detect the shadow cast by a predator, triggering the worm to contract muscles that rapidly withdraw the tentacles into the tube. Christmas tree worms are invertebrates-animals that lack a backbone. Invertebrates account for 95% of known animal 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 a species consisting simply ofa flat bilayer of cells to other species with silk-spinning glands, pivoting spines, dozens of jointed legs, or tentacles covered with suction cups, just to list a few. In this chapter, we'll take a tour ofthe invertebrate world, using the phylogenetic tree in Figure 33.2 as a guide. Figure 33.3 on the next three pages, surveys 23 invertebrate phyla. As representatives of invertebrate diversity, many of those phyla are explored in more detail in the rest of this chapter. molecular data, have the widest range of animal body forms 33.4 Ecdysoloans are the most species-rich animal group 33.5 Echinoderms and chordates are deuterostomes t first glance, you might mistake the organism shown A in Figure 33.1 for some type of seaweed. But this colorful inhabitant of coral reefs is actually an animal, not an alga. Specifically, it is a species of segmented worm known as a Christmas tree worm (Spirobranchus giganteus). The two tree-shaped whorls are tentacles, which the worm uses for gas exchange and for filtering small food particles from ... Figure 33.2 Review of animal phylogeny. Except for sponges (basal animals in phyla Calcarea and Silicea) and a few other groups, all animals have tissues and are in the clade Eumetazoa Most animals are In the diverse clade Bilatena ANCESTRAL PROTIST Cnidaria LOPhWOChO"'~ m C Common ancestor of all animals 3 1\ •" Q •.• ••~ 666 JIJ. .... Calcarea andSiliceaW 'cd,,,,,,, • Deuterostomia ~ • Figure 33,3 Exploring Invertebrate Diversity Kingdom Animalia encompasses 1.3 million known species, and estimates oftotal species range far higher. Of the 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 called "sponges." Sponges are sessile animals that lack true tissues. They live as suspension feeders, trapping particles that pass through the internal channels of their bodies (see Concept 33.1). A sponge Cnidaria (10,000 species) Cnidarians include corals, jellies, and hydras. These animals have a diplobastic, radially symmetrical body plan that includes a gastrovascular cavity with a single opening that serves as both mouth and anus {see Concept 33.2). The single known species in this phylum, Trichoplax adhaercns, does not even look like an animal. It consists of a few thousand cells arranged in a double-layered plate. Trichoplax can reproduce by dividing into two individuals or by budding off many multicellular individuals. [-;;" ~. A plawzoan (lM) V( A jelly Acoel flatworms (lM) Acoel flam'orms have a simple nervous system and a saclike gut, and thus had been placed in ph}~um Platyhelminthes. Molecular analyses, however, indicate Acoela is a separate lineage that diverged before the three main bilaterian clades (see Concept 32.4). A denophore, or comb jelly Ctenophores (comb jellies) are diploblastic and radially symmetrical like cnidarians, suggesting that both phyla diverged from other animals very early. Comb jellies make up much ofthe ocean's plankton. They have many distinctive traits, including eight "combs" ofcilia that propel the animals through the water. \X'hen a small animal contacts the tentacles of some comb jellies, spedalized cells burst open, covering the prey with sticky threads. LOPHOTROCHOZOANS ~helminthes (20,000 species) --;... • . '" '.:~\ ~~ • •• .....,..-1 " - .~~ A marine flatworm Flatworms (including tapeworms, planarians, and flukes) have bilateral symmetry and a central nervous system that processes information from sensory structures. They have no body cavity or organs for circulation (see Concept 33.3). Despite their microscopic size, rotifers have specialized organ systems, including an alimentary canal {digestive tract). They feed on microorganisms suspended in water {see Concept 33.3). A rotifer (LM) Brachiopods, or lamp shells, may be easily mistaken for clams or other molluscs. However, most brachiopods have a unique stalk that anchors them to their substrate (see Concept 33.3). Ectoprocts {also known as bryozoans) live as sessile colonies and are covered by a tough exoskeleton (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 cycliophoran, Symbion pandora, was discovered in 1995 on the Greek acanthias, prickly, and cephalo, head) are called spinyheaded worms because of the curved hooks on the proboscis at the anterior end of their body. All species are parasites. Some acanthocephalans manipulate their intermediate hosts {generally An acanthocephalan (lM) arthropods) in ways that increase their chances of reaching their final hosts (generally vertebrates). For example, acanthocephalans that infect New Zealand mud (nibs force their hosts to move to more visible areas on the beach, where the crabs are more likely to be eaten by birds, the worms' final hosts. mouthparts ofa lobster. This tiny, vase-shaped creature has a A (ydiophoran (colorized SEM) unique body plan and a particularly bizarre life cycle. Males impregnate females that are still developing in their mothers' bodies. The fertilized females then escape, settle elsewhere on the lobster, and release their offspring. The offspring apparently leave that lobster and search for another one to which they attach. I I I Proboscis worms, or ribbon worms, swim through water or Molluscs (including snails, clams, squids, and octopuses) have a soft body that in burrow in sand, extending a unique proboscis to capture prey. Like flatworms, they lack a true coelom. However, unlike flatworms, nemerteans have an many species is protected by a hard shell (see Concept 33.3). alimentary canal and a closed circulatory system in which the blood is contained in vessels and hence is distinct from fluid •. ies) ~~J An octopus Annelids, or segmented worms, are distinguished from other worms by their body segmentation. Earthworms are the most familiar annelids, but the phylum also includes marine and freshwater species (see Concept 33.3). A ribbon worm in the body cavity. A marine annelid ECDYSOZOA Loriciferans (from the Latin lorica, corset, andferre, to bear) are tiny animals that inhabit the deep-sea bottom. A loriciferan can telescope it.s head, neck, and thorax: in and out of the lorica, a pocket formed by six plates surrounding the abdomen. Though the natural history ofloricifenms is mostly a mystery, at least some species likely eat bacteria. 668 UNIT fiVE A lorkiferan (LM) The Evolutionary History of Biological Diversity A priapulan Priapulans are worms with a large, rounded proboscis at the anterior end. (They are named after Priapos, the Greek god of fertility, who was symbolized by a giant penis.) Ranging from 0.5 mm to 20 em in length, most species burrow through seafloor sediments. Fossil evidence suggest.s that priapulans were among the major predators during the Cambrian period. Tardigrades (colorized SEM) Tardigrades (from the Latin tardus, slow, and grOOus, step) are sometimes caUed water bears for their rounded shape, stubby appendages, and lumbering, bearlike gait. Most tardigrades are less than 0.5 mm in length. Some live in oceans or fresh water, while others live on plants or animals. AJ;, many as 2 million tardigrades can be found on a square meter of Onychophorans, also called velvet worms, originated during the Cambrian explosion (see Chapter 32). Originally, they thrived in the ocean, but at some point they succeeded in colonizing land. Today they live only in humid forests. Onychophorans have fleshy antennae and several dozen pairs of saclike 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! I I I •. Roundworms are enormously abundant and diverse in the soil and in aquatic habitats; many species parasitize plants and animals. The most distinctive feature of roundworms is a tough cuticle that coats the body (see Concept 33.4). The vast majority of known animal species, including insects, crustaceans, and arachnids, are arthropods. All arthropods have a segmented exoskeleton and jointed appendages (see Concept 33.4). A roundworm A scorpion (an arachnid) DEUTER05TOMIA An acorn worm like echinoderms and chordates, hemichordates are members of the 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 in length. Chordata (52,000 species) A sea urchin Echinoderms, such as sand dollars, sea stars, and sea urchins, are aquatic animals in the deuterostome clade that are bilaterally symmetrical as larvae but not as adults. They move and feed by using a network of internal canals to pump water to different parts of their body (see Concept 33.5). More than 90% of all known chordate species have backbones (and thus are vertebrates). However, the phylum Chordata also includes three groups of invertebrates: lancelets, tunicates, and hagfishes. See Chapter 34 for a full discussion of this phylum. t A tunicate CHAPTER THIRTY·THREE Invertebrates 669 r;;:~:7 ?r=·~sal animals that lack true tissues Animals in the phyla Calcarea and Silicea are known infor· mally as "sponges:' (Previously, all sponges were placed in a lI..iIIiiii. Dl!IJterostomia single phylum, Porifera, now thought to be paraphyletic based on molecular data.) Among the simplest of animals, sponges are sedentary and were mistaken for plants by the ancient Greeks. They range in size from a few millimeters to a few meters and live in both fresh and maCalcarea and Silicea C",dafla Lophotrochozw Ecdywzw rine waters. Sponges are suspension fceders: They capture food particles suspended in the water that passes through their body, which in some species resembles a sac perforated with pores. Water is drawn through the pores into a central cavity, the spongococ1, and then flows out of the sponge through a larger opening called the osculum (Figure 33.4). More complex sponges have folded body walls, and many contain branched water canals and several oscula. Sponges are basal animals; that is, they represent a lineage that originates near the root of the phylogenetic tree of animals. Unlike nearly all other animals, sponges lack true tissues, groups of similar cells that act as a functional unit and are isolated from other tissues by membranous layers. However, the sponge body does contain several different cell types. For example, lining the interior of the spongocoel are flagellated choanocytes, or collar cells (named for the membranous collar around the base of the flagellum). The similarity between choanocytes and the cells of choanoflagellates supports molecular evidence suggesting that animals evolved from a choanoflagellate-like ancestor (see Figure 32.3). The body ofa sponge consists oftwo layers ofcells separated by a gelatinous region called the mesohyl. Wandering through the mesohyl are cells called amoebocytes, named for their use ofpseudopodia. Amoebocytes have many functions. They take up food from the water and from choanocytes, digest it, and carry nutrients to other cells. They also manufacture tough skeletal fibers within the mesohy!. In some groups of sponges, these fibers are sharp spicules made from calcium carbonate or silica. Other sponges produce more flexible fibers composed of a protein called spongin; you may have seen these pliant skeletons being sold as fluffy brown bath sponges. Most sponges are hermaphrodites, meaning that each individual functions as both male and female in sexual reproduction by producing sperm and eggs. Almost all sponges exhibit sequential hermaphroditism, functioning first as one sex and then as the other. o Choanocytes. The spongocoel is lined with flagellated cells called choanocytes. By beating flagella, the choanocytes create a current that draws water in through the pores and out through the osculum. L \ • Coli" /~' Spongocoel. Water ~ passing through pores enters a cavity called the spongocoel. oepidermis Pores. Water enters the through pores formed by doughnutshaped cells that span the body wall. olayerEpidermis. The outer consists of tightly packed epidermal cells. othisMesohyl. The wall of sponge consists of two layers of cells separated by a gelatinous matrix. the mesohyl ("middle matter r r--s: r Phagocytosts of food particles I A Water /" \ • flow /"i.~ /' ... Figure 33.4 Anatomy of a sponge. fiVE The movement of a choanocyte's flagellum also draws water through its collar of fingerlike projections. Food particles are trapped in the mucus coating the projections, engulfed by phagocytosis, and either digested or transferred to amoebocytes. ~ ) • UNtT Amoebocyte o W 670 Choanocyte Osculum Azure vase sponge (Callyspongia plicifera) o Flagellum The Evolutionary History of Biological Diversity 1 fI Amoebocytes. These cells can transport nutrients to other cells of the sponge body, produce materials for skeletal fibers (spicules), or become any type of sponge cell as needed. Sponge gametes arise from choanocytes or amoebocytes. Eggs reside in the mesohyl, but sperm are carried out of the 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 parent sponge. After settling on a suitable substrate, a larva develops into a sessile adult. Sponges produce a variety of antibiotics and other defensive compounds. Researchers are now isolating these compounds, which hold promise for fighting human diseases. For example, a compound called cribrostatin isolated from marine sponges can kill penicillin-resistant strains of the bacterium Streptococcus. Other sponge-derived compounds are being tested as possible anticancer agents. CONCEPT CHECK Polyp Body stalk .. Figure 33.5 Polyp and medusa forms of cnidarians. The body wall of a cnidarian has two layers of cells: an outer layer of epidermis (from ectoderm) and an inner layer of gastrodermiS (from endoderm), Digestion begins in the gastrovascular cavity and is completed inside focxl vacuoles in the gastrodermal cells, Flagella on the gastrodermal cells keep the contents of the gastrovascular cavity agitated and help distribute nutrients. Sandwiched between the epidermis and gastrodermis is a gelatinous layer, the mesoglea, 33.1 I. Describe how sponges feed. 2. -',1@il IA Some molecular evidence suggests that the sister group of animals is not the choanoflagellates. but rather a group of parasitic protists. Mesomycetozoa. Given that these parasites lack collar cells, can this hypothesis be correct? Explain. For suggested answers, see Appendix A. All animals except sponges and a few other groups belong Lophotrochozoa to the clade Eumetazoa, aniEcdysozw mals with true tissues (see Deuterostom'a Chapter 32). One of the oldest lineages in this clade is the phylum Cnidaria. Cnidarians have diversified into a wide range of sessile and motile forms, including hydras, corals, and jellies (commonly called "jellyfish"). Yet most cnidarians still exhibit the relatively simple, diploblastic, radial body plan that existed some 570 million years ago. The basic body plan ofa cnidarian is a sac with a central digestive compartment, the gastrovascular cavity. A single opening to this cavity functions as both mouth and anus. There are m'o variations on this body plan: the sessile polyp and the motile medusa (Figure 33.5). Polyps are cylindrical forms that adhere to the substrate by the aboral end of their body (the end opposite the mouth) and extend their tentacles, waiting for prey. Examples of the polyp form include hydras and sea anemones. A medusa is a flattened, mouth-down version of the polyp. It moves freely in the water by a combination of passive drifting and contractions of its bell-shaped body. Medusae include free-swimming jellies. The tentacles of a jelly dangle from the oral surface, which points downward. Some cnidarians exist only as polyps or only as medusae; others have both a polyp stage and a medusa stage in their life cycle. Cnidarians are carnivores that often use tentacles arranged in a ring around their mouth to capture prey and push the food into their gastrovascular cavity, where digestion begins. Any undigested remains are expelled through the mouth/anus. 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 exploding outward and that give phylum Cnidaria its name. Specialized cnidae called nematocysts contain a stinging thread Calcarea and Silicea Cnidaria ~ Tentacle Cuticle of prey Thread Thread discharges • I Cnldocyte Thread (coiled) .. Figure 33.6 A cnidocyte of a hydra. This type of cnidocyte contains a stinging capsule, the nematocyst, which contains a coiled thread, When a '"trigger" is stimulated by touch or by certain chemicals, the thread ShOOIS out, puncturing and injecting poison into prey, CHAPTER THIRTY·THREE Invertebrates 671 that can penetrate the body wall of the cnidarian's prey. Other kinds of cnidae have long threads that stick to or entangle small prey that bump into the cnidarian's tentacles. Contractile tissues and nerves occur in their simplest forms in cnidarians. Cells of the epidermis (outer layer) and gastro~ dermis (inner layer) have bundles of microfilaments arranged into contractile fibers (see Chapter 6). The gastrovascular cavity acts as a hydrostatic skeleton against which the contractile cells can work. When a cnidarian closes its mouth, the volume of the cavity is fixed, and contraction of selected cells causes the animal to change shape. Movements are coordinated by a nerve net. Cnidarians have no brain, and the noncentralized nerve net is associated with sensory structures that are distributed radially around the body. Thus, the animal can detect and respond to stimuli from all dire<tions. As summarized in Table 33.1, the phylum Cnidaria is di~ vided into four major classes: Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa (Figure 33.7). T"'33.1 Classes of Phylum Cnidaria Class and Examples Main Characteristics Hydrozoa (Portuguese man-of-wars, hydras, Obelia, some corals; see Figures 33.7a and 33.8) Scyphozoa (jellies, sea nettles; see Figure 33.7b) Most marine, a few freshwater; both polyp and medusa stages in most species; polyp stage often colonial All marine; polyp stage absent or reduced; free-swimming; medusae up to 2 m in diameter All marine; box-shaped medusae; complex eyes; potent venom Cubozoa (box jellies, sea wasps; see Figure 33.7c) Anthozoa (sea anemones, most corals, sea fans; see Figure 33.7d) (3) These .. All marine; medusa stage completely absent; most sessile; many colonial ~ colonial polyps are members of class Hydrozoa. UNIT fiVE Most hydrozoans alternate between polyp and medusa forms, as in the life cycle of Obelia (Figure 33.8). The polyp stage, a colony of interconnected polyps in the case of Obelia, is more conspicuous than the medusa. Hydras, among the fewcnidarians found in fresh water, are unusual hydrowans in that they exist only in polyp form. \Vhen environmental conditions are favorable, a hydra reproduces asexually by budding, fonning outgrowths that pinch off from the parent and live independently (see Figure 13.2). \Vhen conditions deteriorate, hydras can reproduce sexually, forming resistant zygotes that remain dormant until conditions improve. Scyphozoans The medusa generally is the predominant stage in the life cycle ofthe class Scyphozoa. The medusae of mostspe<ies live among the plankton as jellies. Most coastal scyphozoans go through a stage as small polyps during their life cycle, whereas those that live in the open ocean generally lack the polyp stage altogether. (ubozoans As their name (which means "cube animals") suggests, cubozoans have a box-shaped medusa stage. Cubozoans can be distinguished from scyphozoans in other significant ways, such as having complex eyes embedded in the fringe of their medusae. They are comparatively strong swimmers and as a result are less likely to be stranded on shore. Cubozoans, which generally live in tropical oceans, are often equipped with highly toxic cnidocytes. The sea wasp (Chironexfleckeri), a cubozoan that lives off the coast of northern Australia, is one of the deadliest organisms: Its sting causes intense pain and can lead to respiratory failure, cardiac arrest, and death within minutes. The poison of sea wasps isn't universally fatal, however; sea turtles have defenses against it, allowing them to eat the cubozoan in great quantities. (b) Many jellies (class Scyphozoa) Cc) The sea wasp (Chironex are bioluminescent. Food fleekefl) is a member of class captured by nematocyst(ubozoa. Its poison, which bearmg tentacles is trans· can subdue fish and other ferred to specialized oral arms large prey. is more potent (that lack nematocysts) for than cobra venom. transport to the mouth . .&. Figure 33.7 Cnidarians. 672 Hydrozoans The Evolutionary History of Biological Diversity Cd) Sea anemones and other members of class Anthozoa exist only as polyps. epolyps, Some of the colony's equipped with OMedusae swim off, grow, and reproduce sexually. Gather polyps, specialized for reproduction, lack tentacles and produce tiny medusae by asexual budding. tentacles, are specialized for feeding. o Acolony of interconnected polyps (inset, LM) results from asexual reproduction by budding. j; 1--------1>. Medusa :r~erm SEXUAL Egg REPRODUCTION Portion of a colony of polyps ASEXUAL REPRODUCTION (BUDDING) De~eloping polyp I~ K,y oandThedevelops planula eventually settles into a new polyp. "The zygote de~elops into a solid ciliated larva called a planula. • Haploid (nJ 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 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? -'moil. Anthozoans Sea anemones (see Figure 33.7d) and corals belong to the class Anthozoa (meaning "flower animals n ). These cnidarians occur only as polyps. Corals live as solitary or colonial forms, and many species secrete a hard external skeleton of calcium carbonate. Each polyp generation builds on the skeletal remains of earlier generations, constructing "rocksn with shapes characteristic of their species. It is these skeletons that we usually think of as coral. Coral reefs are to tropical seas what rain forests are to tropical land areas: They provide habitat for a wealth ofother species. Unfortunately, like rain forests, coral reefs are being destroyed at an alarming rate by human activity. Pollution and overfishing are major threats, and global warming (see Chapter 55) may also be contributing to their demise by raising seawater temperatures above the narrow range in which corals thrive. CONCEPT CHECK 33.2 1. Compare and contrast the polyp and medusa forms of cnidarians. 2. Describe the structure and function of the stinging cells for which cnidarians are named. 3. -','!lO"g If the common ancestor of cnidarians were an open-ocean jelly, what might you infer about evolutionary trends in the relative importance of the polyp and medusa stages? For suggested answers, see Appendix A. CHAPTER TH'RTY·THREE Invertebrates 673 r~:;:::;c~:Z~~ns, a clade identified by molecular data, have the widest range of animal body forms C~k~re~ ~nd Slll(e~ Crud~ri~ lophotro<hozoa Ecdys.olO<l Dl'uterostomia The vast majority of animal species belong to the dade Bilateria, whose members ex- T." 33.2 Classes of Phylum Platyhelminthes Class and Examples Main Characteristics Turbellaria (mostly free· living flatworms, such as Dugesia: see Figures 33.9 and 33.10) Most marine, some freshwater, a few terrestrial; predators and scavengers; body surface ciliated Monogenea (monogeneans) Marine and freshwater para· sites; most infect external SurfdCes of fishes; life history simple; ciliated larva starts infection on host Trematoda [trematodes, also calle<! flukes; see Figure 33.11) Parasites, mostly of verte· brates; two suckers attach to host; most life cycles include intermediate and final hosts Cestoda [tapeworms; see Figure 33.12) Parasites of vertebrates; scolex attaches to host; proglottids produce eggs and break off after fertilization; no head or digestive system; life cycle with one or more intermediate hosts hibit bilateral symmetry and triploblastic development (see Chapter 32). Most bilaterians are also coelomates. \Vhile the se· quence ofbilaterian evolution is a subject of active investigation, most researchers think that the most recent common ancestor of Hving bilaterians probably existed in the late Proterozoic eon (about 575 million years ago). Most major groups of bilaterians first appeared in the fossil record during the Cambrian explosion. As you read in Chapter 32, molecular evidence suggests that there are three major clades of bilaterally symmetrical animals: Lophotrochozoa, Ecdysozoa, and Deuterostomia. This section will focus on the first of these clades, the lophotrochozoans. Concepts 33.4 and 33.5 will explore the other two clades. Although the clade Lophotrochozoa was identified by molecular data, its name comes from features found in some ofits mem· bers. Some lophotrochozoans develop a structure called a lophophore, acrown ofciliated tentacles that functions in feeding, while others go through a distinctive stage called the trochophore larva (see Figure 3213). Other members ofthe group have neither of these features. Few other unique morphological features are widely shared within the group-in fact, the lophotrochozoans are the most diverse bilaterian clade in terms of body plan. This diversity in form is reflected in the number of phyla classified in the group: Lophotrochozoo includes about 18 phyla, more than twice the number in any other clade ofbilaterians. Well now introduce six lophotrochozoan phyla: the flatworms, rotifers, ectoprocts, brachiopods, moUuscs, and annelids. exchange and the elimination of nitrogenous waste (ammonia) can occur by diffusion across the body surface. Flatworms have no organs spedalized for gas exchange, and their relatively sim· pIe excretory apparatus functions mainly to maintain osmotic balance with their surroundings. This apparatus consists of protoncphridia, networks of tubules with ciliated structures called flame bulbs that pull fluid through branched ducts opening to the outside (see Figure 44.11). Most flarn'orms have a gastrovascular cavity with only one opening. Though flatworms lack a circulatory system, the fine branches of the gastrovascular cavity distribute food directly to the animal's cells. Flahvorms are divided into four classes (Table 33.2); Turbellaria (mostly free-living flarn'OTms), Monogenea (monogeneans), Trematoda (trematodes, or flukes), and Cestoda (tapeworms). Flatworms Turbellarians Flatworms (phylum Platyhelminthes) live in marine, freshwater, and damp terrestrial habitats. In addition to free· living forms, flarn'orms include many parasitic species, such as flukes and tapeworms. Flatworms are so named because they have thin bodies that are flattened dorsoventrally (between the dorsal and ventral surfaces); platyhelminth means "flat worm." (Note that worm is not a formal taxonomic name but a general term for animals with long, thin bodies.) The smallest flatworms are nearly microscopic free-living species, while some tapeworms are more than 20 m long. Although flatworms undergo triploblastic development, they are acoelomates (animals that lack a body cavity). Their flat shape places all their cells close to water in the surrounding environmentor in their gut. Because ofthis proximity to water, gas Turbellarians are nearly all free-living and mostly marine (Figure 33.9). The best-known freshwater turbellarians are members ofthe genus Dugesia, commonly called planarians. Abundant in unpolluted ponds and streams, planarians prey on smaller animals or feed on dead animals. They move by using cilia on their ventral surface, gliding along a film of mucus they secrete. Some other turbellarians also use their muscles to swim through water with an undulating motion. A planarian's head is equipped with a pair of light·sensitive eyespots and lateral flaps that function mainly to detect specific chemicals. The planarian nervous system is more complex and centralized than the nerve nets of cnidarians (Figure 33.10). Experiments have shown that planarians can learn to modify their responses to stimuli. 674 UNIT fiVE The Evolutionary History of Biological Diversity ... Figure 33.10 Anatomy of a planarian. a turbellarian. Pharynx. The mouth is at the tip of a muscular pharynK. Digestive juices are spilled onto prey, and the pharym sucks small pieces of food into the gastrovascular cavity, where digestion continues. Digestion is completed within the cells lining the gastrovascular cavity, which has many fine subbranches that provide an e>rt.ensive surface area. Undigested wastes are egested 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, separating into a head end and a tail end; each end then regenerates the missing parts. Sexual reproduction also occurs. Planarians are hermaphrodites, and copulating mates typically crossfertilize each other. Ventral nerve cords. From the ganglia, a pair of ventral nerve cords runs the length of the body. o Mature flukes live in the blood vessels of the human intestine A female fluke fits into a groove running Monogeneans and Trematodes Monogeneansand trematodes live as parasites in or on other animals. Many have 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 complex life cycles with alternating sexual and asexual stages. Manytrematodes require an intermediate host in which larvae develop before infecting the final host (usuallya vertebrate), where the adult worms live. For example, trematodes that parasitize humans spend part of their lives in snail hosts (Figure 33.11). Around the world, some 200 million people are infected with blood. flukes (Schistosoma) and suffer from schistosomiasis, a disease whose symptoms include pain, anemia, and dysentery. Living within different hosts puts demands on trematodes that free-living animals don't face. Ablood. fluke, for instance, must evade the immune systems of both snails and humans. By mimicking the surface proteins of its hosts, the blood fluke creates a partial immunological camou- Ganglia. At the anterior end of the worm, near the main sources of sensory input, is a pair of ganglia, dense clusters of nerve cells. the length of the larger male's body, as shown in the lM at right. o These larvae penetrate ~ the skin and blood vessels of humans working in fields irrigated with water contaminated with infected human feces, AI" >----< 1 mm e Blood flukes reproduce sexually in the human host. The fertilized eggs exit the host in feces. o Ifpond the feces reach a or other source of water, the eggs develop into ciliated larvae. These larvae infect snails, the intermediate hosts, o within AseKual reproduction a snail results In another type of motile larva, which escapes from the snail host. ... Figure 33.11 The life cycle of a blood fluke (Schistosoma manson;), a trematode. Snails eat algae, whose growth is stimulated by nutrients found in fertilizer. How would the contamination of irrigation water with fertilizer likely affect the occurrence of schistosomiasis? Explain, _I,J:t.iI!. CHAPTER THIRTY·THREE Invertebrates 675 flage for itself. It also releases molecules that manipulate the hosts' immune systems into tolerating the parasite's existence. These defenses are so effective that individual blood flukes can survive in humans for more than 40 years. Most monogeneans, however, are external parasites of fish. The monogenean life cycle is relatively simple; a ciliated, freeswimming larva initiates the infection of a host fish. Although monogeneans have been traditionally aligned with the trematodes, some structural and chemical evidence suggests they are more closely related to tapeworms. Tapeworms Tapeworms (class Cestoda) are also parasitic (figure 33.12). The adults live mostly inside vertebrates, including humans. In many tapeworms, the anterior end, or scolex, is armed with suckers and often hooks that the worm uses to attach itself to the intestinal lining of its host. Tapeworms lack a mouth and gas· trovascular cavity; they absorb nutrients released by digestion in the host's intestine. Absorption occurs across the tapeworm's body surface. Posterior to the scolex is a long ribbon of units called proglottids, which are little more than sacs of sex organs. After sexual reproduction, proglottids loaded with thousands of fertilized eggs are released from the posterior end of a tapeworm and leave the host's body in feces. In one type of life cycle, infected feces contaminate the food or water of intermediate hosts, such as pigs or cattle, and the tapeworm eggs develop into larvae that encyst in muscles of these animals. A human acquires the larvae by eating undercooked meat contaminated with cysts, and the worms develop into mature adults within the human. Large tapeworms can block the intestines and rob enough nutrients from the human host to cause nutritional deficiencies. Doctors use an oraUy administered drug, niclosamide, to kill the adult worms. Roti!ers Rotifers (phylum Rotifera) are tiny animals that inhabit freshwater, marine, and damp soil habitats. Ranging in size from about SO Ilm to 2 mm, rotifers are smaller than many protists but nevertheless are multicellular and have specialized organ systems (Figure 33,13). in contrast to cnidarians and flatworms, which have a gastrovascular cavity, rotifers have an alimentary canal, a digestive tube with a separate mouth and anus. Internal organs lie within the pseudocoelom, a body cavitythat is not completely lined by mesoderm (see Figure 32.8b). Fluid in the pseudocoelom serves as a hydrostatic skeleton (see Chapter SO). Movement of a rotifer's body distributes the fluid throughout the body, circulating nutrients. The word rotifer, derived from Latin, means "wheel-bearer," a reference to the crown of cilia that draws a vortex of water into the mouth. Posterior to the mouth, a region of the digestive tract called the pharynx bears jaws called trophi that grind up food, mostly microorganisms suspended in the water. Rotifers exhibit some unusual fonns of reproduction. Some species consist only of females that produce more females from wU'ertilized eggs, a type ofreproduction caUed parthenogenesis. Other species produce two types of eggs that develop by parthenogenesis. One type forms females while the other type (produced when conditions deteriorate) develops into simplified males that cannot even feed themselves. These males survive only long enough to fertilize eggs, which form resistant 'I---'7Proglottids with reproductive strUdures Scolex .... figure 33.12 Anatomy of a tapeworm. The inset shows a close-up of the scolex (colorized SEM), 676 UNIT fiVE The Evolutionary History of Biological Diversity .... Figure 33.13 A rotifer. These pseudocoelomates, smaller than many protists, are generally more anatomically complex than flatworms (lM). zygotes that can survive when a pond dries up. When conditions are favorable, the zygotes break dormancy and develop into a new female generation that reproduces by parthenogenesis until conditions become unfavorable again. It is puzzling that so many rotifer species survive without males. The vast majority of animals and plants reproduce sexually 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 of extinction and lower rates of speciation. Seeking to understand this unusual group, Nobel Prizewinning biologist Matthew Meselson, of Harvard Vniversity, has been studying a class of asexual rotifers named Bdel1oidea. Some 360 species ofbdel1oid rotifers are known, and all ofthem reproduce by parthenogenesis without any males. Paleontologists have discovered bdel10id rotifers preserved in 35-mil1ionyear-old amber, and the morphology of these fossils resembles only the female form, with no evidence of males. By comparing the DNA ofbdelloids with that of their closest sexually reproducing rotifer relatives, Meselson and his colleagues concluded that bdel10ids have likely been asexual for much longer than 35 million years. Howthese animals manage to flout the general rule against long-lived asexuality remains a puzzle. Lophophorates: Ectoprocts and Brachiopods Bilaterians in the phyla Ectoprocta and Brachiopoda are among those known as lophophorates. These animals have a /ophophore, a crown of ciliated tentacles that surround the mouth (see Figure 32. 13a). As the cilia draw water toward the mouth, these tentacles trap suspended food particles. Other similarities, such as a V-shaped alimentary canal and the absence of a distinct head, reflect these organisms' sessile existence. In contrast to flatworms, which lack a body cavity, and rotifers, which have a pseudocoelom, lophophorates have a true coelom that is completely lined by mesoderm (see Figure 32.8a). Ectoprocts (from the Greek ectv, outside, and procta, anus) are colonial animals that superficially resemble clumps of moss. (In fact, their common name, bryozoans, means ~moss animals.~) In most species, the colony is encased in a hard exoskeleton (external skeleton) studded with pores through which the lophophores extend (Figure 33.14a). Most ectoproct species live in the sea, where they are among the most widespread and numerous sessile animals. Several species are important reef builders. Ectoprocts also live in lakes and rivers. Colonies of the freshwater ectoproct Pectinate/ILl magnifica grow on submerged sticks or rocks and can grow into a gelatinous, ball-shaped mass more than 10 em across. Brachiopods, or lamp shells, superficially resemble clams and other hinge-shelled molluscs, but the two halves ofthe bra- (a) Ectoprocts, such as this sea mat (b) Brachiopods have a hinged (Membranipora membranacea), shell. The two parts of the are coloniallophophorates. shell are dorsal and ventral. ... Figure 33.14 Lophophorates. chiopod shell are dorsal and ventral rather than lateral, as in clams (Figure 33.14b). All brachiopods are marine. Most live attached to the seafloor by a stalk, opening their shell slightly to allow water to flow through the lophophore. The living brachiopods are remnants of a much richer past that included 30,000 species in the Paleozoic and Mesozoic eras. Some living brachiopods, such as those in the genus Lingula, are nearly identical to fossils of species that lived 400 million years ago. Molluscs Snails and slugs, oysters and clams, and octopuses and squids are all molluscs (phylum Mollusca). Most molluscs are marine, though some inhabit fresh water, and some snails and slugs live on land. Molluscs are soft-bodied animals (from the Latin mol/uscus, soft), but most secrete a hard protective shell made ofcalcium carbonate. Slugs, squids, and octopuses have a reduced internal shell or have lost their shell completely during their evolution. Despite their apparent differences, all molluscs have a similar body plan (Figure 33.15, on the next page). Molluscs are coelomates, and their bodies have three main parts: a muscular foot, usually used for movement; a visceral mass containing most of the internal organs; and a mantle, a fold of tissue that drapes over the visceral mass and secretes a shell (ifone is present). In many molluscs, the mantle extends beyond the visceral mass, producing a water-filled chamber, the mantle cavity, which houses the gills, anus, and excretory pores. Many molluscs feed by using a straplike rasping organ called a radula to scrape up food. Most molluscs have separate sexes, and their gonads (ovaries or testes) are located in the visceral mass. Many snails, however, are hermaphrodites. The life cycle of many marine molluscs includes 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 circulatory fluid called hemolymph through arteries into sinuses (body spaces). The organs of the mollusc are thus continually bathed in hemolymph. Nephridium. Excretory organs called nephridia remove metabolic wastes from the hemolymph. Visteral mass. Coelom The long digestive tract is coiled in the visceral mass. Intestine Gonads Mantle cavity (..."....~~!r:H Anus The nervous system consists of a nerve ring aroundfrom the esophagus, which nerve cords extend. Gill I~-==i:::::.,.-_":::!~!!~=E~~~;:'\ Mouth Foot Nerve cords Esophagus Radula. The mouth region in many mollusc species contains a rasp-like feeding organ called a radula. This belt of backwardcurved teeth repeatedly thrusts outward and then retracts into the mouth, scraping 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; see Figure 33.16) Marine; shell with eight plates; foot used for locomotion; radula; no head Gastropoda (snails, slugs; see Figures 33.17 and 33.18) Marine, freshwater, or terrestrial; head present; a symmetrical body, usually with a coiled shell; shell reduced or absent; foot for locomotion; radula Bivalvia (dams, mussels, scallops, oysters; see Figures 33.19 and 33.20) Marine and freshwater; flattened shell with two valves; head reduced; paired gills; no radula; most are suspension feeders; mantle forms siphons Cephalopoda (squids, octopuses, cuttlefishes, chambered nautiluses; see Figure 33.21) Marine; head surrounded by grasping tentddes, usually with suckers; shell external, internal, or absent; mouth with or without radula; locomotion by jet propulsion using siphon formed from foot which is also characteristic of marine annelids (segmented worms) and some other 10photrochozoans. The basic body plan of molluscs has evolved in various ways in the phylum's eight classes. We'll examine four ofthose classes here (Table 33.3): Polyplacophora (chitons), Gastropoda (snails and slugs), Bivalvia (clams, oysters, and other bivalves), and Cephalopoda (squids, octopuses, cuttlefishes, and cham~ bered nautiluses). 678 UNIT fiVE The Evolutionary History of Biological Diversity .... Figure 33.16 A chiton. Note the eight-plate shell charaderistic of molluscs in the class Polyplacophora. Chitons Chitons have an oval-shaped body and a shell divided into eight dorsal plates (Figure 33.16). The chiton's body itself, however, is unsegmented. You can find these marine animals clinging to rocks along the shore during low tide. If you try to 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 use its foot to creep slowly over the rock surface. Chitons use their radula to scrape algae off the rock surface. Gastropods About three-quarters of all living species of molluscs are gastropods (Figure 33.17). Most gastropods are marine, but there are also many freshwater species. Some gastropods have adapted to life on land, including garden snails and slugs. A distinctive characteristic of class Gastropoda is a developmental process known as torsion. As a gastropod embryo develops, its visceral mass rotates up to 180', causing the animal's anus and mantle cavity to wind up above its head (Figure 33.18). After torsion, some organs that were bilateral may be reduced in size, while others may be lost on one side of the body. Torsion should not be confused with the formation of a coiled shell, which is an independent developmental process. Most gastropods have a single, spiraled shell into which the animal can retreat when threatened. The shell is often conical but is somewhat flattened in abalones and limpets. Many gastropods have a distinct head with eyes at the tips of tentacles. Gastropods move literally at a snail's pace by a rippling motion of their foot or by means of cilia, often leaving a trail of slime in their wake. Most gastropods use their radula to graze on algae or plants. Several groups, however, are predators, and their radula has become modified for boring holes in the shells ofother molluscs 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 gastropods. Instead, the lining of their mantle cavity functions as a lung, exchanging respiratory gases with the air. Bivalves The moUuscs of class Bivalvia include many species of clams, oysters, mussels, and scallops. Bivalves have a shell divided into two halves (Figure 33.19). The halves are hinged at the middorsal line, and powerful adductor muscles draw them tightly together to protect the animal's soft body. Bivalves have no distinct head, and the radula has been lost. Some bivalves have eyes and sensory tentacles along the outer edge oftheir mantle. The mantle cavity of a bivalve contains gills that are used for gas exchange as well as feeding in most species (Figure 33.20). Most bivalves are suspension feeders. They trap fine food particles in mucus that coats their gills, and cilia then convey those particles to the mouth. Water enters the mantle cavity through an incurrent siphon, passes over the gills, and then exits the mantle cavity through an excurrent siphon. ... 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 Hinge area Coelom ,~;;ti::~G~",~ ~ Heart Adductor muscle ArluS ... Figure 33.17 Gastropods. (b) A sea slug. Nudibranchs. or sea slugs, lost their shell during their evolution. 11~~~~r1~~ .~ Stomach Excurrent iphon Intestine Mouth Foot ... Figure 33.18 The results of torsion in a gastropod. Because of torsion (twisting of the visceral mass) durirlg embryonic development. the digestive tract is coiled and the anus is near the anterior end of the animal. ~;~:~=:t::;~ Mantle cavity Gonad Gill Water flow Incurrent siphon ... Figure 33.20 Anatomy of a clam. Food particles suspended in water that enters through the incurrent siphon are collected by the gills and passed via cilia to the mouth. CHAPTER THIRTY·THREE Invertebrates 679 Most bivalves lead sedentary lives, a characteristic suited to suspension feeding. Sessile mussels secrete strong threads that tether them to rocks, docks, boats, and the shells of other ani~ mals. However, clams can pull themselves into the sand or mud, using their muscular foot for an anchor, and scallops can skitter along the seafloor by flapping their shells, rather like the mechanical false teeth sold in novelty shops. Cephalopods Cephalopods are active predators (Figure 32.21). They use their tentacles to grasp prey, which they then bite with beaklike jaws and immobilize with a poison found in their saliva. The foot of a cephalopod has become modified into a muscular excurrent siphon and part of the tentacles. Squids dart about by drawing water into their mantle cavity and then firing a jet of water through the excurrent siphon; they steer by pointing the siphon in different directions. Octopuses use a similar mechanism to escape predators. A mantle covers the visceral mass of cephalopods, but the shell is reduced and internal (in squids and cuttlefishes) or missing altogether (in many octopuses). One small group of shelled cephalopods, the chambered nautiluses, survives today. Cephalopods are the only molluscs with a closed circulatory system. They also have well-developed sense organs and .. Octopuses are considered among the most Intelligent invertebrates. • Squids are speedy carnivores with beak-like Jaws and well·developed eyes. a complex brain. The ability to learn and behave in a complex manner is probably more critical to fast-moving predators than to sedentary animals such as clams. The ancestors of octopuses and squids were probably shelled molluscs that took up a predatory lifestyle; the shell was lost in later evolution. Shelled cephalopods called ammonites, some of them as large as truck tires, were the dominant invertebrate predators of the seas for hundreds of millions of years until their disappearance during the mass extinction at the end of the Cretaceous period, 65.5 million years ago (see Chapter 25). Most species ofsquid are less than 75 cm long, but some are considerably larger. The giant squid (Architeulhis dux) was for a long time the largest squid known, with a mantle up to 2.25 m long and a total length of 18 m. In 2003, however, a specimen of the rare species Mesvnychoteuthis hami/toni was caught near Antarctica; its mantle was 2.5 m long. Some biologists think that this specimen was a juvenile and estimate that adults of its species could be ty,'ice as large! Unlike A. dux, which has large suckers and small teeth on its tentacles, M. hamilton! has two rows ofsharp hooks at the ends ofits tentacles that can deliver deadly lacerations. It is likely that A. dux and M. hamilton! spend most of their time in the deep ocean, where they may feed on large fishes. Remains of both giant squid species have been found in the stomachs of sperm whales, which are probably their only natural predator. In 2005, scientists reported the first observations 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 among the great mysteries of invertebrate life. 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 .. Chambered nautiluses are the only living cephalopods with an external shell ... Figure 33.21 Cephalopods. 680 UNIT fiVE The Evolutionary History of Biological Diversity Class and Examples Main Characteristics Oligochaeta (freshwater, marine, and terrestrial segmented worms; see Figure 33.22) Reduced head; no parapodia, but chaetae present Polychaeta (mostly marine segmented worms; see Figure 33.23) Many have a well-developed head; each segment usually has parapodia with many chaetae; free-living Hirudinea (leeches; see Figure 33.24) Body usually flattened, with reduced coelom and segmentation; chaetae usually absent; suckers at anterior and posterior ends; parasites, predators, and SCAvengers worms that live in the sea, in most freshwater habitats, and in damp soil. Annelids are coelomates, and they range in length from less than 1 mm to more than 3 m, the length ofa giant Australian earthworm. The phylum Annelida can be divided into three classes (Table 33,4, on the facing page): Oligochaeta (the earthworms and their relatives), Polychaeta (the polychaetes), and Hirudinea (the leeches), Each segment is surrounded by longitudinal muscle, which in turn is surrounded by circular muscle. Earthworms coordinate the contraction of these two sets of muscles to move (see Figure 50.33). These muscles work against the noncompressible coelomic fluid, which acts as a hydrostatic skeleton. Epidermis Oligochaetes Oligochaetes (from the Greek oligos, few, and cllaite, long hair) are named for their relatively sparse chaetae, or bristles made of chitin. TIlis class of segmented worms includes tlle earthworms and a variety of aquatic species. Figure 33.22 provides a guided tour ofthe anatomy of an earthwonn, which is representative of annelids. Earthworms eat their way through the soil, extracting Coelom. The coelom of the earthworm is partitioned by septa. Septum (partition between segments) Many of the internal structures are repeated within each segment of the earthworm. Metanephridium. Each segment of the worm contains a pair of excretory tubes, called meta nephridia, with ciliated funnel-shaped openings called nephrostomes. The meta nephridia remove wastes from the blood and coelomic fluid through exterior pores. longitudinal muscle Dorsal--f!'lII Chaetae. Each segment vessel has four pairs of chaetae, bristles that I-~===::: provide traction for burrowing. Fused nerve cords Nephrostome Venlral vessel Tiny blood vessels are abundant in the earthworm's --ii---Iskin, which functions as its respiratory organ. The blood contains oxygencarrying hemoglobin. Clitellum Metanephridium Intestine Giant Australian earthworm Cerebral ganglia. The earthworm nervous system features a brainlike pair of cerebral ganglia above and in front of the pharynx. A ring of nerves around the pharynx connects to a subpharyngeal ganglion, from which a fused pair of nerve cords runs posteriorly. L---"""""":~~~ Mouth The circulatory system, a network of vessels, is closed. The dorsal and ventral vessels are linked by segmental pairs of vessels. The dorsal vessel and five pairs of vessels that circle the esophagus are muscular and pump blood through the circulatory system. Ventral nerve cords with segmental ganglia. The nerve cords penetrate the septa and run the length of the anima!, as do the digestive tract and longitudinal blood vessels. ... Figure 33.22 Anatomy of an earthworm, an oligochaete. CHAPTER THIRTY·THREE Invertebrates 681 nutrients as the soil passes through the alimentary canal Undigested material, mixed with mucus secreted into the canal, is eliminated as fecal castings through the anus. Farmers value earthworms because the animals till and aerate the earth, and their castings improve the texture of the soil. (Charles Darwin 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 direclions in such a way that they exchange sperm (see figure 46.1), and then they separate. The received sperm are stored temporarily 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 worm's head and remains in the soil while the embryos develop. Some earthworms can also reproduce asexually by fragmentation followed by regeneration. Polychaeles Each segment of a polychaete has a pair of paddle-like or ridge-like structures called parapodia rnear feet") that function in locomotion (Figure 33.23). Each parapodium has numerous chaetae, so polychaetes usually have many more chaetae per segment than do oligochaetes. In many polychaetes, the parapodia are richly supplied with blood vessels and also function as gills. Polychaetes make up a large and diverse class, most of whose members are marine. A few species drift and swim among the plankton, many crawl on or burrow in the seafloor, and many others live in tubes. Some tube-d.....ellers, such as the fan worms, build their tubes by mixing mucus with bits ofsand and broken shells. Others, such as Christmas tree worms (see Figure 33.1 l, construct tubes using only their own secretions. Leeches The majority of leeches inhabit fresh water, but there are also marine species as well as terrestrial leeches found in moist vegetation. Leeches range in length from about 1 to 30 cm. .. Figure 33.24 A leech. A nurse applied thIS medJonalleech (Hirudo l'nf'didnaJisl to a patient's SCH"e thumb 10 drain blood from a hematoma (an abnormal accumulation of blood around an II'Iternal inJury). Many are predators that feed on other invertebrates, but some are parasites that suck blood by attaching temporarily to other animals, including humans (Figure 33.24). Some parasitic species use bladelike jaws to slit the skin oftheir host, whereas others secrete enzymes that digest a hole through the skin. The host is usually oblivious to this attack because the leech secretes an anesthetic. After making the incision, the leech secretes another chemical, hirudin, which keeps the blood ofthe host from coagulating near the incision. The parasite then sucks as much blood as it can hold, often more than ten times its own weight. After this gorging, a leech can last for months without another meal. Until this century, leeches were frequently used for bloodletting. Today they are used to drain blood that accumulates in tissues following certain injuries or surgeries. Researchers have also investigated the potential use of hirudin to dissolve unwanted blood dots that form during surgery or as a result of heart disease. Several recombinant forms of hirudin have been developed, two ofwhich were recently approved fordinical use. As a group, Lophotrocholoa encompasses a remarkable range ofbody plans, as illustrated by members ofsuch phyla as Rotifera, Ectoprocta, Mollusca, and Annelida. Next we'll explore the diversity of Ecdysoloa, a dominant presence on Earth in terms of sheer number of speCies. CONCEPT CHECI( n.) I. Explain how tapeworms can survive without a .. Figure 33.23 A polychaete. HesioIyra berg; lives on the seafloor around deep-sea hydrothermal vents. coelom, a mouth, a digestive system, or an excretory system. 2. How does the modification of the 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 lophotrcchozoans live on land, above the surface of the soil. Focusing on gravity, hypothesize why this is so. for suggested answers. see Appendix A. 682 UNIT fiVE The Evolutionary History of Biological Diversity r::;;:::o~~'~re the most species-rich animal group r;;==; Although defined primarily by molecular evidence, the dade Lopholnxhowa Ecdysozoa includes animals Ecdysozoa that shed a tough external coat Deuterostorma (cuticle) as they grow; in fact, the group derives its name from this process, which is called molting, or ecdysis. Ecdysozoa consists of about eight animal phyla and contains mOTe known species than all other protist, fungus, plant, and animal groups combined. Here we'll focus Calcarea and Silicea Cmdana on the two largest e<dysozoan phyla, the nematodes and arthropods, which are among the most successful and abun- dant of all animal groups. Nematodes Some of the most ubiquitous animals, nematodes (phylum Nematoda), or roundworms, are found in most aquatic habitats, in the soil, in the moist tissues of plants, and in the body fluids and tissues of animals. In contrast to annelids, nematodes do not have segmented bodies. The cylindrical bodies of nematodes range from less than 1 mm to more than a meter in length, often tapering to a fine tip at the posterior end and to a more blunt tip at the anterior end (Figure 33.25). A nematode's body is covered by a tough cuticle; as the worm grows, it periodically sheds its old cuticle and secretes a new, larger one. Nematodes have an alimentary canal, though they lack a circulatory system. Nutrients are transported throughout the body via fluid in the pseudocoelom. The body wall muscles are all longitudinal, and their contraction produces a thrashing motion. Nematodes usually reproduce sexually, by internal fertilization. In most species, the sexes are separate and females are larger than males. A female may deposit 100,000 or more fertilized eggs (zygotes) per day. The zygotes of most species are resistant cells that can survive harsh conditions. Multitudes ofnematodes live in moist soil and in decomposing organic matter on the bottoms of lakes and oceans. While 25,000 species are known, perhaps 20 times that number actually exist. It has been said that if nothing but nematodes remained on Earth, they would still preserve the outline of the planet and many onts features. These free-living worms play an important role in decomposition and nutrient cycling, but little is known about most species. One species of soil nematode, Caellorhabditis elegalls, however, is very well studied and has become a model research organism in biology (see Chapter 21). Ongoing studies on C. elegans are revealing some of the mechanisms involved in aging in humans, among other findings. Phylum Nematoda includes many significant agricultural pests that attack the roots ofplants. Other species of nematodes parasitize animals. Humans are hosts to at least 50 nematode species, including various pinworms and hookworms. One notorious nematode is Trichinella spiralis, the worm that causes trichinosis (Figure 33.26). Humans acquire this nematode by eating raw or undercooked pork or other meat (including wild game such as bear or walrus) that has juvenile worms encysted in the muscle tissue. \'(!ithin the human intestines, the juveniles develop into sexually mature adults. Females burrow into the intestinal muscles and produce more juveniles, which bore through the body or travel in lymphatic vessels to other organs, including skeletal muscles, where they encyst. Parasitic nematodes have an extraordinary molecular toolkit that enables them to redirect some of the cellular functions of their hosts and thus evade their immune systems. Plant-parasitic nematodes inject molecules that induce the development of root cells, which then supply nutrients to the parasites. Trichinella controls the expression of specific muscle-cell genes that code for proteins that make the cell elastic enough to house the nematode. Additionally, the infected muscle cell releases signals that attract blood vessels, which Encysted juveniles Muscle tissue ... 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 parasites have been dubbed "animals that act like viruses." • FI Arthropods Hox genes? 33.28 • Did the arthropod body plan result from new 18 Zoologists estimate that there are about a billion billion (10 ) arthropods living on Earth. More than 1 million arthropod species have been described, most ofwhich are insects. In fact, two out of every three known species are arthropods, and members ofthe phylum Arthropoda can be found in nearly all habitats of the biosphere. By the criteria of species diversity, distribution, and sheer numbers, arthropods must be reo garded as the most successful of all animal phyla. Arthropod Origins Biologists hypothesize that the diversity and success of arthropods is related to their body plan-their segmented bodies, hard exoskeleton, and jointed appendages (arthropod means "jointed feet"). The earliest fossils with this body plan are from the Cambrian explosion (535-525 million years ago), indicating that the arthropods are at least that old. Along with arthropods, the fossil record of the Cambrian explosion contains many species of lvbopods, an extinct group from which arthropods may have evolved. Lobopods such as Hallucigenia (see Figure 25.4) had segmented bodies, but most of their body segments were identical to one another. Early arthropods, such as the trilobites, also showed little variation from segment to segment (Figure 33.27). As arthropods continued to evolve, the segments tended to fuse and become fewer, and the appendages became specialized for a variety of functions. These evolutionary changes resulted not only in great diversification but also in an efficient body plan that permits the division oflabor among different body regions. What genetic changes led to the increasing complexity of the arthropod body plan? Arthropods today have two unusual Hox genes, both of which influence segmentation. To test whether these genes could have driven the evolution of increased body segment diversity in arthropods, Sean Carroll (see pages 534-535) and colleagues studied Hox genes in onychophorans, dose relatives of arthropods (Figure 33.28). EXPERIMENT How did the highly successful arthropod body plan arise? One hypothesis suggests that it resulted from the origin (by a gene duplication event) of two unusual Hox genes found in arthropods: Ultrabithorax (Vbx) and abdominal-A (abd-A). To test this hypothesis, Sean Carroll, of the University of Wisconsin, Madison, and colleagues turned to the onychophorans, a group of invertebrates closely related to arthropods. Unlike many living arthropods, onychophorans have a body plan in which most body segments are identical to one another, Thus, Carroll and colleagues reasoned that if the origin of the Ubx and abd-A Hox genes drove the evolution of body segment diversity in arthropods, these genes probably arose on the arthropod branch of the evolutionary tree: - Other ecdysoloans Origin of ubx and abd-A Hoxg~? r--"'I-.- Arthropods Common ancestor Of/ ...- - - - - Onychophorans onychophorans and arthropods As the hypothesis depided above suggests, Ubx and abd-A would not have been present in the common ancestor of arthropods and onychophorans. and hence. onychophorans should not have these genes. To find out whether this was the case. Carroll and colleagues eKamined the Hox genes of the onychophoran Acanrhokara kaputensis. RESULTS The onychophoran A. kaputensis has all arthropod Hox genes, including Ubx and abd·A, Red indicates the body regions of this onychophoran embryo in which Ubx or abd~A genes were expressed. (The inset shows this area enlarged.) 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 disappeared with the great Permian extindions about 250 million years ago. Paleontologists have described about 4,000 trilobIte species. CONCLUSION Since A. kaputensis, an onychophoran, has the arthropod Hox genes, the evolution of increased body segment diversity in arthropods must not have been related to the origin of neY\' Hax genes, SOURCE 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 not arise from the acquisition of new Hox genes. Instead, the evolution of body segment diversity in arthropods may have been driven by changes in the sequence or regulation of existing Hox genes. (See Chapter 25 for a discussion of how changes in form can result from changes in the sequence or regulation of developmental genes such as Hox genes.) ter. Arthropods began to diversify on land following the colonization of land by plants in the early Paleozoic. Evidence includes a 428-million-year-old fossil of a millipede found in 2004 by an amateur fossil hunter in Scotland. Fossilized tracks of other terrestrial arthropods date from about 450 million years ago. 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 at the anterior end of the animal. Like many molluscs, arthropods have an open circulatory system, in which fluid called hemolymph is propelled by a heart through short arteries and then into spaces called sinuses surrounding the tissues and organs. (The term blood is generally reserved for fluid in a dosed circulatory system.) Hemolymph reenters the arthropod heart through pores that are usually equipped with valves. The hemolymph-filled body sinuses are collectively called the hemocoel, which is not part of the coelom. Although arthropods are coelomates, in most species the coelom that forms in the embryo becomes much reduced as development progresses, and the hemocoel becomes the main body cavity in adults. Despite their similarity, the open circulatory systems of molluscs and arthropods probably arose independently. A variety of specialized gas-exchange organs have evolved in arthropods. These organs allow the diffusion ofrespiratory gases in spite of the exoskeleton. Most aquatic species have gills with thin, feathery extensions that place an extensive surface area in contact with the surrounding water. Terrestrial arthropods generally have internal surfaces specialized for gas exchange. Most insects, for instance, have tracheal systems, branched air ducts leading into the interior from pores in the cuticle. General Characteristics ofArthropods Over the course ofevolution, the appendages of some arthropods have become modified, specializing in functions such as walking, feeding, sensory reception, reproduction, and defense. Figure 33.29 illustrates the diverse appendages and other arthropod characteristics of a lobster. The body of an arthropod is completely covered by the cuticle, an exoskeleton constructed from layers of protein and the polysaccharide chitin. The cuticle can be thick and hard over some parts of the body and paper-thin and flexible over others, such as the joints. The rigid exoskeleton protects the animal and provides points ofattachment for the muscles that move the appendages. But it also means that an arthropodcannotgrowwithout occasionally shedding its exoskeleton and producing a larger one. This molting process is energetically expensive. A molting or recently molted arthropod is also vulnerable to predation and other dangers until its new, soft exoskeleton hardens. \Vhen the arthropod exoskeleton first evolved in the sea, its main functions were probably protection and anchorage for muscles, but it later additionally enabled certain arthropods to live on land. The exoskeleton's relative impermeability to water helped prevent desiccation, and its strength solved the problem of support when arthropods left the buoyancy ofwa- ... 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 (horseshoe cnlbs, spiders, scorpions, ticks, mites; see Figures 33.30-33.32) Myriapoda (millipedes and centipedes; see Figures 33.33 and 33.34) Body having one or two main parts; six pairs of appendages (chelicerae, pedipalps, and four pairs ofwalking legs); mostly terrestrial or marine Distinct head bearing antennae and chewing mouthparts; 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 trunk segment and poison claws on first body segment Hexapoda (insects, springtails; see Figures 33.35-33.37) Crustacea (crabs, lobsters. crayfishes, shrimps; see Figures 33.29 and 33.38) Body divided into head, thorax, and abdomen; antennae present; mouthparts modified for chewing, sucking, or lapping; three pairs of legs and usually two pairs of wings; mostly terrestrial .... Figure 33.30 Horseshoe crabs (Umulus polyphemus). Common on the Atlantic and Gulf coasts of the United States, these "living fossils" have changed little in hundreds of millions of years. They are surviving members of a rich diversity of cheliceriforms that once filled the seas. Body of two or three parts; antennae present; chewing mouthparts; three or more pairs of legs; mostly marine and freshwater Morphological and molecular evidence suggests that living arthropods consist offour major lineages that diverged early in the evolution of the phylum (Table 33.5): cheliceriforrns (sea spiders, horseshoe crabs, scorpions, ticks, mites, and spiders); myriapods (centipedes and millipedes); hexapods (insects and their wingless, six~legged relatives); and crustaceans (crabs, lobsters, shrimps, barnacles, and many others). .. Scorpions have pedipalps that are pincers specialized for defense and the capture of food. The tip of the tail bears a poisonous stinger. Cheliceriforms Cheliceriforms (subphylum Cheliceriformes; from the Greek c1wilQs, lips, and cheir, arm) are named for clawlike feeding appendages called chelicerae, which serve as pincers or fangs. Cheliceriforms have an anterior cephalothorax and a poste~ rior abdomen. They lack antennae, and most have simple eyes (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 (pycnogonids) and horseshoe crabs (Figure 33.30). The bulk ofmodem cheliceriforms are arachnids, a group that includes scorpions, spiders, ticks, and mites (Figure 33.31). TIcks 686 UNIT fiVE The Evolutionary History of Biological Diversity .. Dust mites are ubiquitous scavengers in human dwellings but are harmless except to those people who are allergic to them (colorized SEM), <Ill .... Figure 33.31 Arachnids. Web-building spiders are generally most active during the daytime. and many mites are among a large group of Stomach Intestine parasitic arthropods. Nearly all ticks are Heart bloodsucking parasites that live on the Digestive bodysurfaces ofreptiles or mammals. Pargland asitic mites live on or in a wide variety of vertebrates, invertebrates, and plants. Ovary Arachnids have a cephalothorax that has six pairs ofappendages: the chelicerae; a pair ofappendages calledpedipalps that function in sensing, feeding, or reproduction; and four pairs of walking legs (Figure 33.32). Spiders use their fangAnus like chelicerae, which are equipped with poison glands, to attack prey. As the cheSpinnerets Chelicera Sperm licerae pierce the prey, the spider secretes Silk gland receptacle digestive juices onto the prey's torn tis... Figure 33.32 Anatomy of a spider. sues. The food softens, and the spider sucks up the liquid meal. In most spiders, gas exchange is carried out by book lungs, stacked platelike structures contained in an internal chamber (see Figure 33.32). The extensive surface area of these respiratory organs is a structural adaptation that enhances the exchange of O2 and CO 2 between the hemolymph and air. A unique adaptation of many spiders is the ability to catch insects by constructing webs of silk, a liquid protein produced by specialized abdominal glands. The silk is spun by organs called spinnerets into fibers that then solidify. Each spider engineers a web characteristic of its 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 ... Figure :n.:n A millipede. by wind, a behavior known as uballooning~ Brain Poison gland Pedipalp Myriapods Millipedes and centipedes belong to the subphylum Myriapoda, the myriapods. All living myriapods are terrestrial. The myriapod head has a pair of antennae and three pairs of appendages 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 carnivores. 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. Abdomen .. Thorax Head ............... Compound eye ~::::::>Antennae Heart. The insect heart drives hemolymph through an open circulatory system. ~~~ Cerebral ganglion. The two nerve cords meet in the head, where the ganglia of several anterior segments are fused into a cerebral ganglion (brain, colored white below). The antennae, eyes, and other sense organs are concentrated on the head. Anus Malpighian tubules. Metabolic wastes are removed from the Vagina excretorYf.--_====~~:::)~1!~~~7~~1"~'il~~~ hemolymph organs called byMalpighian tubules, which are out-pocketings of the digestive tract. Tracheal tubes. Gas exchange in insects is accomplished by a tracheal system of branched. chitin-lined tubes that infiltrate the body and carry oxygen directly to cells. The tracheal system opens to the outside of the body through spiracles, pores that can control air flow and water loss by opening or closing . ''JI:~~ Nerve cords. The insect nervous system consists of a pair of ventral nerve cords with several segmental ganglia. Insect mouthparts are formed from several pairs of modified appendages. The mouthparts include mandibles, which grasshoppers use for chewing. In other insects, mouthparts are specialized for lapping, piercing, or sucking. ... Figure 33.35 Anatomy of a grasshopper, an insect. Insects Insects and their relatives (subphylum Hexapoda) are more species-rich than all other forms of life combined. They live in almost every terrestrial habitat and in fresh water, and flying insects fill the air. Insects are rare, though not absent, in marine habitats, where crustaceans are the dominant arthropods. The internal anatomy of an insect includes several complex organ systems, which are highlighted in Figure 33.35. The oldest insect fossils date from the Devonian period, which began about 416 million years ago. However, when flight evolved during the Carboniferous and Permian periods, it spurred an explosion in insect diversity. Afossil record ofdiverse insect mouthparts indicates that specialized feeding on gymnosperms and other Carboniferous plants also contributed to early adaptive radiations of insects. Later, a major increase in insect diversity appears to have been stimulated by the evolutionary expansion of flowering plants during the mid-Cretaceous period (about 90 million years ago). Although insect and plant diversity decreased during the Cretaceous mass extinction, both groups rebounded over the next 65 million years. Studies indicate that rebounds of particular insect groups often were associated with radiations of the flowering plants on which they fed. 688 UNIT fiVE The Evolutionary History of Biological Diversity Flight is obviously one key to the great success of insects. An animal that can fly can escape many predators, find food and mates, and disperse to new habitats much faster than an animal that must crawl about on the ground. Many insects have one or two pairs of wings that emerge from the dorsal side of the thorax. Because the wings are extensions ofthe cuticle and not true appendages, insects can fly without sacrificing any walking legs. By contrast, the flying vertebrates-birds and bats-have one of their two pairs of walking legs modified into wings, making some of these species clumsy on the ground. Inse<t wings may have first evolved as extensions of the cuticle that helped the inse<t body absorb heat, only later becoming organs for flight. Other hypotheses suggest that wings allowed terrestrial insects to glide from vegetation to the ground or that they served as gills in aquatic insects. Still another hypothesis is that insect wings functioned for swimming before they functioned for flight. Morphological and molecular data indicate that wings evolved only once in insects. Dragonflies, which have two similar pairs of wings, were among the first insects to fly. Several insect orders that evolved later than dragonflies have modified flight equipment. The wings of bees and wasps, for instance, are hooked together and move as a single pair. Butterfly wings operate in a similar fashion because the anterior pair overlaps the posterior wings. In beetles, the posterior wings function in flight, while the anterior ones are modified as covers that prote<t the flight wings when the beetle is walking on the ground or burrowing. Many insects undergo metamorphosis during their development. In the incomplete metamorphosis of grasshoppers and some other inse<t groups, the young (called nymphs) resemble adults but are smaller, have different body proportions, and lack wings. The nymph undergoes a series of molts, each time looking more like an adult. With the final molt, the insect reaches full size, acquires wings, and becomes sexually mature. Insects with complete metamorphosis have larval stages specialized for eating and growing that are known by such names as caterpillar, maggot, or grub. The larval stage looks entirely different from the adult stage, which is specialized for dispersal and reproduction. Metamorphosis from the larval stage to the adult occurs during a pupal stage (Figure 33.36). Reproduction in insects is usually sexual, with separate male and female individuals. Adults come together and re<ognize each other as members of the same species by advertising with bright colors (as in butterflies), sound (as in crickets), or odors (as in moths). Fertilization is generally internal. In most species, sperm are deposited directly into the female's vagina at the time ofcopulation, though insome species the male deposits a sperm packet outside the female, and the female picks it up. An internal structure in the female called the spermathe<a stores the sperm, usually enough to fertilize more than one batch of eggs. Many insects mate only once in a lifetime. After mating, a female often lays her eggs on an appropriate food source where the next generation can begin eating as soon as it hatches. Insects are classified in morethan 30 orders, IS ofwhich are presented in Figure 33.37, on the next two pages. Animals as numerous, diverse, and widespread as insects are bound to affect the lives of most other terrestrial organisms, including humans. We depend on bees, flies, and many other insects to pollinate our crops and orchards. On the other hand, insects are carriers for many diseases, including African sleeping sickness (spread by tsetse flies that carry the protist Trypanosoma; see Figure 28.6) and malaria (spread by mosquitoes that carry the protist Plasmodium; see Figure 28.10). Furthermore, inse<ts compete with humans for food. In parts of Africa, for instance, insects claim about 75% of the crops. In the United States, billions ofdollars are spent each year on pesticides, spraying crops with massive doses ofsome ofthe deadliest poisons ever invented. Try as they may, not even humans have challenged the preeminence of insects and their arthropod kin. As Cornell University entomologist Thomas Eisner puts it: "Bugs are not going to inherit the Earth. They own it now. So we might as well make peace with the landlord." (b) Pupa (d) Emerging adult ... Figure 33.36 Metamorphosis of a butterfly. (a) The larva (caterpillar) spends its time eating and growing. molting as it grows, (b) After several molts. the larva de~elops into a pupa, (c) Within the pupa. the larval tissues are broken down. and the adult is built by the di~ision and differentiation of cells that were quiescent in the larva, (d) Eventually, the adult begins to emerge from the pupal cuticle (e) Hemolymph is pumped into veins of the wings and then withdrawn. leaving the hardened ~eins as struts supporting the wings. The insect will fly off and reproduce, deri~ing much of its nourishment from the food reserves stored by the feeding lar~a. (e) Adult CHAPTER THIRTY·THREE Invertebrates 689 Exploring Insect Diversity , Approximate Order Main Characteristics Number of Examples Species 8lattodea 4,"" Cockroaches ha\'c a dorso\'entrally l1att~ body, with lqs modified for rapid running. Forewings hen present, are leath· ery, ",ilereas hind wings are fan6J.:e. F w than 40 cockroKh §Ilt'cies moe in houses; the rest exploit habitUs r.lnging from tro~ 350.000 Coleoptera German cockroach forest floors to caves and descn.s. BcrtIcs comprise the most specks-rich Ofdcr of ins«u. Thqha\'e ''''-0 pairs of ",;ngs, one of.....hich is thick and stiff, the other membranous. lbty h;l\'e an armomi nosktltton and mouthPMU adapled for biting and ehe",;ng. BertIes undergo complete metamorphosis. 1.200 DermaptcnII 151,000 Diptera 85,000 Hemiptera Hymenopten lsoptcra 690 UNIT 125,000 2,"" fiVE Earwigs are generally nocturnal Ka\'mgers. Sotmo species are 'o\ingless, while others ha...e Iwo p,airs of"'ings, one of ....hich is thick and koathery, the other membranous. EalVoigs have biting mouthparts and large posterior pincer" They undergo incomplete metamorphosis. Diptl"I'1UIS ha.\T one pair of ..... ings;!.he SCCOlld pair has be<:OITK' modined into balancing organs called haltcrtS. Their mouthpam are adapll'd for sucking, piercing. or lapping. Dipll."rans undergo complete metamorphosis. Flies and mosquitoes are among the best·known dipterans, which live as scavengers. predators. and parasites. Hemipterans are so-called "true bugs; including bed bugs, u· sassin bugs, and chinch bugs. ([nsects in other orders are some· times erroneously called bugs.) Hemipterans have t..... o pairs of wings, one pair partly leathery. the other membranous. They have piercing or sucking mouthparts and undergo incomplete metamorphosis. Ants, bees, and wasps are generally highly social insects. They have two pairs of membranous wings. a mobile head. and chewing or sucking mouthparts. The females of many spe<ies have a posterior stinging organ. Hymenopterans undergo complete metamorphosis. Termites are Widespread social ins«ts that produce enormous colonies. It has been estimated that there are 700 kg oftet'mites for every per50Il on Earth! Some termites ha,'e t"'·o pairs of membranous ....ings, .....hile others are ....ingless. They f«d on .....ood. ....ith the aid of microbial symbionts carried in spe<ial.izcd chambers in their hindgut.690 The Evolutionary History of Biological Diversity Earwig Horsefly Leaffooted bog Cicad<l-killer wasp Terrmte Order Approximate Number of Examples Main Characteristics Species Lepidoptera 120.000 Butterflies and moths arc amOllg the best-known insects. They have two J»irs of ""ings covered ....ith tiny SQlI('$. To feed, they uncoil ill long proboscis. Most ft'rd on n«ur, but $Orne species feed on other substan<:es. including animal blood or tears. Odonala 5.000 SwallowtaIl butterfly Dragonflies and damselflies han' two J»irs of large. mem~ nous '>lings. They N'''' an dongakd abdomen. Large. compound ~. and cheo.ooing mouthparts. They undcrgo incompktt metamorphosis and arc active predators. Dragoofty Orthopl~ra 13.000 Grasshoppers. crickcu. and their rdatin'$ alt' mostly herbr.'Orous. They have Large hind Iq;s adapted for jumping. '....0 pairs of.... ings (one leathery, one membnnous), and biting orcbewing mouthparts. Males commonly make courtship sounds by rubbing togelht-r body palU. such as ill ridge on their hind legs. Orthopterans undergo incomplete mrtamorphosis. Stick inserts and leaf insects are exquisite mimics of plants. Phasmalodea l"he eggs of some sjX'Cies e"en mimic seeds orthe plants on ....hich the insects live. Their body iscylindrkal or natlenro dorsoventrally. They lack foreWings but have fanlike hind \dngs. Their mouthparts arc adapted for biting or chewing. Phthiraplcr<i Siphonaplel'll Katydid 2.400 2.400 450 Commonly called sucking lice, these insects spend their entire life as an ectoparasite feeding on the hair or feathers of a single host. Their legs, equipped with clawlike tarsi, are adapted for clinging to their hosts. They lack wings and have reduced eyes. Sucking lice undergo incomplete metamorphosis. Stick insect Human body louse fleas are bloodsucking ectoparasites on birds and mammals. Their body is Wingless and laterally compressed. Their legs are modified for clinging to their hosts and for long-distance jumping. They undergo complete metamorphosis. f~ , ~. Flea Silverfish arc small, Wingless insects with a nauenM body and reduced eycs. They live inleaflittl'l" or under Inri:. Theyan also infest buildings, whl'l"e they can become pests. Silverfish Trichapten 7.100 The larvae of caddistlies lin' in streams, ,,'here they make house'S from sand gr;AiRS•..-ood fragments. or ot.hef material held tosether by silk. Adults haw f',Hl pairs of hairy wings and chOling or lapping mouthpalU. They undtrgo complete metamorphosis. CH.~lU lHIUl·TllIIU Caddisfty Invertebrates 691 Crustaceans \Vhjle arachnids and insects thrive on land, crustaceans, for the most part. have remained in marine and freshwater environ- ments. Crustlre.ms (subph~um Crustlce,) typically M" highly specialized appendages. Lobsters and crayfIShes, for instance. have a toolkit of 19 pairs of appendages (~Figure 33.29). The anteriormost appendages are antennae; crustaceans are the only arthropods with two pairs. Three or more pairs of appendages are modified as mouthparts. including the hard mandibles. Walking legs are present on the thorax, and, unlike insects, crustaceans also have appendages on their abdomen. A lost appendage can be regenerated at next molt. Small crustaceans exchange gases across thin areas of the cuticle; larger species have gills. Nitrogenous ....'astes also diffuse through thin areas of the cuticle. but a pair of glands regulates the salt balance of the hemolymph. Sexes are separate in most crustaceans. In the case of lobsters and crayfish. the male uses a specialized pair of abdominal appendages to transfer sperm to the reproductive pore of the female during copulation. Most aquatic crustaceans go through one or more swimming larval stages. One ofthe largest groups ofcrustaceans (numbering about IO,lXX') species) is the isopods, which include terrestrial. fresh· water, and marine species. Some isopod species are abundant in habitats at the bottom of the deep ocean. Among the ter· restrial isopods are the pill bugs. or wood lice. common on the undersides of moist logs and leaves. Lobsters. crayfishes. crabs. and shrimpsare all relati\'t~ly large crust:aceanscalleddecapods (Figure 33.38a). Thecuticleofdecapods is hardened by calcium carbonate; the portion that covers the dorsal side ofthe cephalothorax forms a shield called the carapace. Most decapod species are marine. Crayfishes, however, live in fresh water, and some tropical crabs live on land. Many small crustaceans are important members of marine and freshwater plankton communities. Planktonic crustaceans include many species ofcopcpods, which are among the most numerous of all animals, as well as the shrimplike krill, which grow to about 5 cm long (Figure 33.38b). A major food source for baleen whales (indudingblue whales. humpbacks, and right whales), krill are now being harvested in great numbers by humans for food and agricultural fertilizer. The larvae of many larger-bodied crustaceans are also planktonic. Barnacles are a group of mostly sessile austaceans ....t\ose cutide is hardened into a sheI.I containing ca.kium carbonate (FtgUre 33.38c). Most bamades anchor tIlernseh'eS to rocks, boat hulls, pilings, ,00 othe< sub"""ll"d...raa.. Thek nalur.l! _ . is as strong as synthetic glues. To feed. these barnacles extend appendages from their shell to strain food from the wat:el: Bamades were not rerogniz.ed as crustaceans until the I~ when naturoJ· ists discovered that bamacIe larvae resemble the larvae of other crustaceans. The remarkable mix of unique traits and crustarean homologies fourKI. in barnacles was a major inspiration to Oiarles Darwin as he developed his theory ofevolution. 692 UNIT fiVE The Evolutionary History of Biological Diversity (a) Ghost crabs live on sandy ocean beaches worldwide. Pnmarily nocturnal. they take shelter In burrows dunng the day_ (b) Planktonic crustaceans known as I:::rin are consumed In vast quantities by some whales. (el The JOinted appendages prOjectlng from the shells of these barnacles capture organISms and organic partlCles suspended in the water. • Figure 33.38 Crustaceans. CONCEPT CHECK 33.4 I. How do nematode and annelid body plans differ? 2. In contrast to our jaws. which move up and down, the mouthparts of arthropods move side to side. Explain this feature ofarthropods in terms of the origin of their mouthparts. 3. Describe two adaptations that have enabled insects to thrive on land. 4. i1iM Traditionally. annelids and arthropods were thought to be closely related because both groups show body segmentation. Yet molecular data indicate that annelids belong to one clade (Lophotrochozoa) and arthropods to another (Ecdysozoa). Could traditional and molecular hypotheses be tested by studying the expression of Hox genes that control body segmentation? Explain. -lim For suggested answers. see Appendix A. -..J Deuterostomia is defined primarily by DNA similarities-not developmental similarities. r;:~~::;e~~;~nd chordates are Echinoderms deuterostomes C~IQlfe~ ~Md Sili(e~ !~P.;;;;;;;;! Cmdana Lopholrochozoa Ecdysozoa Deuterostomia Sea stars and most other echinoderms (from the Greek echin, spiny, and derma, skin) are slow-moving or sessile marine animals. A thin epidermis covers an endoskeleton of hard calcareous plates. Most echinoderms are prickly from skeletal bumps and spines. Unique to echinoderms is the water vascular system, a network of hydraulic canals branching into extensions called tube feet that function in locomotion, feeding, and gas exchange (Figure 33.39). Sexual reproduction of echinoderms usually involves separate male and female individuals that release their gametes into the water. The internal and external parts of most adult echinoderms radiate from the center, often as five spokes. However, echinoderm larvae have bilateral symmetry. Furthermore, the symmetry of adult echinoderms is not truly radial. For example, the opening (madreporite) ofa sea star's water vascular system is not central but shifted to one side. Sea stars, sea urchins, and other e<hinoderms (phylum Echino~ dermata) may seem to have lit- tle in common with phylum Chordata, which includes the vertebrates-animals that have a backbone. In fact, however, echinoderms and chordates share features characteristic of a deuterostome mode of development, such as radial cleavage and formation of the mouth at the end of the embryo opposite the blastopore (see Figure 32.9). Molecular systematics has reinforced Deuterostomia as a clade of bilaterian animals. But molecular evidence also indicates that some animal phyla with members that have deuterostome developmental features, including ectoprocts and brachiopods, are not in the deuterostome dade (see Chapter 32). Hence, despite its name, the clade The surface of a sea star is covered by spines that help defend against predators, as well as by small gills that provide gas exchange. Ashort digestive tract runs from the mouth on the bottom of the central disk to the anus on top of the disk. ~ Central disk. The central disk has a nerve ring and nerve cords radiating from the ring into the arms. r Madreporite. Water can flow in or out of the water vascular system into the surrounding water through the madreporite. Digestive glands secrete digestive juices and aid in the absorption and storage of nutrients. / Radial canal. The water vascular system consists of a ring canal in the central disk and five radial canals, each running in a groove down the entire length of an arm. Branching from each radial canal are hundreds of hollow, muscular tube feet filled with fluid. Each tube foot consists of a bulb-like ampulla and a podium (foot portion). When the ampulla squeezes. water is forced into the podium, which expands and contacts the substrate. Adhesive chemicals are then secreted from the base of the podium, attaching it to the substrate. To detach the tube foot, de-adhesive chemicals are secreted and muscles in the podium contract, forcing water back into the ampulla and shortening the podium. As it moves, a sea star leaves an observable Nfootprint of adhesive material on the substrate. H ... Figure 33.39 Anatomy of a sea star, an echinoderm. CHAPTER THIRTY·THREE Invertebrates 693 ''''33.6 Classes of Phylum Echinodermata Class and Examples Main Characteristics Asteroidea (sea stars; see Figure 33.39 and Figure 33.4Oa) Star.shaped body with multiple arms; mouth directed to substrate Ophiuroidea (brittle stars; see Figure 33.4Ob) Distinct central disk; long, flexible arms; incomplete digestive system Echinoidea (sea urchins, sand dollars; see Figure 33.4Oc) Roughly spherical or disk-shaped; no arms; five rows of tube feet, enabling slow movement; mouth ringed by complex, jaw-like structure Crinoidea (sea lilies, feather stars; see Figure 33.4Odl Feathered arms surrounding upward-pointing mouth; suspension feeders Holothuroidea (sea cucumbers; see Figure 33.4Oe) Cucumber-shaped body; five rows of tube feet; tube feet around the mouth are modified as feeding tentacles; reduced skeleton; no spines Concentricycloidea (sea daisies; see Figure 33.4Of) Armless, disk-shaped body ringed with small spines: incomplete digestive system; live on submerged wood Living echinoderms are divided into six classes (Table 33.6; Figure 33.40): Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (sea lilies and feather stars), Holothuroidea (sea cucumbers), and Concentricycloidea (sea daisies). Sea Stars Sea stars have multiple arms radiating from a central disk, the undersurfaces of which bear tube feet. By a combination of muscular and chemical actions, the tube feet can attach to or detach from a substrate. The sea star adheres firmly to rocks or creeps along slowly as its tube feet extend, grip, release, extend, and grip again. Although the base of a sea star tube foot has a flattened disk that resembles a suction cup, the gripping action results from adhesive chemicals, not suction (see Figure 33.39). Sea stars also use their tube feet to grasp prey, such as clams and oysters. The arms of the sea star embrace the closed bivalve, clinging tightly with the tube feet. The sea star then turns part of its stomach inside out, everting it through its mouth and into the narrow opening between the halves of the bivalve's shell. The digestive system of the sea star secretes juices that begin digesting the soft body of the mollusc within its own shell. Sea stars and some other echinoderms have considerable powers of regeneration. Sea stars can regrow lost arms, and 694 UNIT fiVE The Evolutionary History of Biological Diversity members of one genus can even regrow an entire body from a single arm if part of the central disk remains attached. Brittle Stars Brittle stars have a distinct central disk and long, flexible arms. They move primarily by lashing their arms in serpentine movements. The base of a brittle star tube foot lacks the flattened disk found in sea stars but does secrete adhesive chemicals. Hence, like sea stars and other echinoderms, brittle stars can use their tube feet to grip substrates. Some species are suspension feeders; others are predators or scavengers. Sea Urchins and Sand Dollars Sea urchins and sand dollars have no arms, but they do have five rows of tube feet that function in slow movement. Sea urchins also have muscles that pivot their long spines, which aid in locomotion as well as protection. The mouth of a sea urchin is ringed by highly complex, jaw-like structures that are well adapted to eating seaweed. Sea urchins are roughly spherical, whereas sand donars are flat disks. Sea Lilies and Feather Stars Sea lilies live attached to the substrate by a stalk: feather stars crawl about by using their long, flexible arms. Both use their arms in suspension feeding. The arms encircle the mouth, which is directed upward, away from the substrate. Crinoidea is an ancient class whose evolution has been very conservative; fossilized sea lilies some 500 million years old are extremely similar to present-day members of the class. Sea Cucumbers On casual inspection, sea cucumbers do not look much like other echinoderms. They lack spines, and their endoskeleton is much reduced. They are also elongated in their oral-aboral axis, giving them the shape for which they are named and further disguising their relationship to sea stars and sea urchins. Closer examination, however, reveals that sea cucumbers have five rows of hlbe feet. Some of the tube feet around the mouth are developed as feeding tentacles. Sea Daisies Sea daisies were discovered in 1986, and only three species are known (one near New Zealand, a second in the Bahamas, and a third in the North Pacific). All live on submerged wood. A sea daisy's body is armless and is typically disk-shaped; it has a five-sided organization and measures less than a centimeter in diameter. The edge of the body is ringed with small spines. Sea daisies absorb nutrients through the membrane surrounding their body. The relationship of sea daisies to other echinoderms remains unclear; some systematists consider sea daisies the sister group of sea stars. (a) Asea star (class Asteroidea) (b) Abrittle star (class Ophiuroidea) «) Asea urchin (class E(hinoidea) (d) Afeather star (class Crinoidea) (e) Asea cucumber (class Holothuroidea) (f) A sea daisy (class Con(entricycloidea) .. Figure 33.40 Echinoderms. Chordales Phylum Chordata consists of m'o subphyla of invertebrates as well as the hagfishes and the vertebrates. Chordates are bilaterally symmetrical coelomates with segmented bodies. The dose relationship between echinoderms and chordates does not mean that one phylum evolved from the other. In fact, echinoderms and chordates have evolved independently of one another for at least 500 million years. We will trace the phylogeny ofchordates in Chapter 34, focusing on the history of vertebrates. CONCEPT CHECK 33.5 1. Describe how sea star tube feet attach to substrates. 2. Are characteristics shared by a sea star and a sea daisy homologous or analogous? Explain. 3. •i,il:O'I,. The insect Drosophila me/anogasler and the nematode Caenurhabditis e/egans are prominent model organisms. Are these species the most appropriate invertebrates for making inferences about humans and other vertebrates in Deuterostomia? Explain. For suggested answers. see AppendiK A. (HAPTER THIRTY·THREE Invertebrates 695 -N·if.• Go to the Study Area at www.masteringbio.com for BioFli~ SUMMARY OF KEY CONCEPTS 3·D Animations, MP3 Tutors, Videos, Practice Tests, an eBook, and more. The table below summarizes the groups of animals surveyed in this chapter. Selected Animal Phyla Key Concept Phylum Description Concept 33.1 Calcarea, Silicea (sponges) Lack true tissues; have choanocytes (collar cells-flagellated cens that ingest bacteria and tiny food particles) Sponges are basal animals that lack true tissues (pp.670-671) Concept 33.2 Cnidarians are an ancient phylum of eumetaloans Cnidaria (hydras. jellies, sea anemones, corals) (pp.671-673) Concept 33.3 LQphotrochOloans, a clade identified by molecular data, have the widest range of animal body forms Unique stinging structures (nematocysts) housed in specialized cells (cnidocytes); diploblastic; radially symmetrical; gastrovascular cavity (digestive compartment with a single opening) Platyhelminthes (flatworms) Dorsoventrally flattened, unsegmented acoelomates; gastrovascular cavity or no digestive tract Rotifera (rotifers) Pseudocoelomates with alimentary canal (digestive tube with mouth and anus); jaws (trophi) in pharynx; head with ciliated crown Lophophorates: Ectoprocta, Brachiopoda Coelomates with lophophores (feeding structures bearing ciliated tentacles) Mollusca (clams, snails, squids) Coelomates with three main body parts (muscular foot, visceral mass, mantle); coelom reduced; most have hard shell made of calcium carbonate Annelida (segmented worms) Coelomates with segmented body wall and internal organs (except digestive tract, which is unsegmented) Nematoda (roundworms) Cylindrical. unsegmented pseudocoelomates with tapered ends; no circulatory system; undergo ecdysis Arthropoda (crustaceans, insects, spiders) Coelomates with segmented body, jointed appendages, and exoskeleton made of protein and chitin Echinodermata (sea stars, sea urchins) Coelomates with bilaterally symmetrical larvae and five-part body organization as adults; unique water vascular system; endoskeleton Chordata (lancelets, tunicates, vertebrates) Coelomates with notochord; dorsal, hollow nerve cord; pharyngeal slits; post-anal tail (see Chapter 34) (pp.674-682) Concept 33.4 -" Ec:dysoloans are the most species.rich animal group (pp.683-692) In\'estigation How Ar~ Insect Specic-< Identified? Concept 33.5 Ec:hinoderms and chordates are deutcrostomes (pp.693-695) ,\etl\ity Oar.cter;stics of Invertcbrat~s 6% UNIT fiVE The Evolutionary History of Biological Diversity TESTING YOUR KNOWLEDGE SELF-QUIZ I. \Vhich two main clades branch from the most recent common ancestor of the eumetazoans? a. Calcarea and $ilicea b. Lophotrochozoa and Ecdysozoa c. Cnidaria and Bilateria d. Rotifera and Deuterostomia e. Deuterostomia and Bilateria 2. A land snail, a clam. and an octopus all share a. a mantle. d. embryonic torsion, b. a radula. e. distinct cephalization. c. gills. 3. \Vhich phylum is characterized by animals that have a segmented body? d. Arthropoda a. Cnidaria b. Platyhelminthes e. Mollusca c. Silicea 4. Which of the following characteristics is probably most responsible for the great diversification of insects on land? a. segmentation d. bilateral symmetry b. antennae e. exoskeleton c. eyes 5. The water vascular system of echinoderms a. functions as a circulatory system that distributes nutrients to body cells. b. functions in locomotion, feeding, and gas exchange. c. is bilateral in organization, even though the adult animal is not bilaterally symmetrical. d. moves water through the animal's body during suspension feeding. e. is analogous to the gastrovascular cavity of flatworms. EVOLUTION CONNECTION 7. ••I;f.W"1 Draw a phylogenetic tree ofBilateria that includes the ten phyla ofbilaterians discussed in detail in this chapter. Label each br,lllch that leads to a phylum with a C, P, or A, depending on whether members of the phylum are coelomates (C), pseudocoelomates (P), or acoelomates (A). Use your labeled tree to answer the following questions: (a) For each of the three major clades of bilaterians, what (if anything) can be inferred about whether the common ancestor of the clade had a true coelom? (b) To what extent has the presence of a true coelom in animals changed over the course of evolution? SCIENTIFIC INQUIRY 8. A marine biologist has dredged up an unknown animal from the seafloor. Describe some of the characteristics she should look at to determine the ph}~um to which the animal should be assigned. SCIENCE, TECHNOLOGY, AND SOCIETY 9. Construction of a dam and irrigation canals in an African country has enabled fJrmers to increase the amount of food they can grow. In the past, crops were planted only after spring floods; the fields were too dry the rest of the year. Now fields can be watered year-round. But improved crop yields have had an unexpected cost-a tremendous increase in the incidence of schistosomiasis. Imagine that your Peace Corps assignment is to help local health officials control the disease. Reviewing the blood fluke life cycle in Figure 33.11, why do you think the irrigation project increased the incidence of schistosomiasis? It is difficult and expensive to control this disease with drugs. Suggest three other methods that could be tried to prevent people from becoming infected. 6. Which of the following combinations of phylum and description is incorrect? a. Echinodermata-bilateral symmetry as a larva, coelom present b. Nematoda-roundworms, pseudocoelomate c. Cnidaria-radial symmetry, polyp and medusa body forms d. Platyhelminthes-flatworms, gastrovascular cavity, acoelomate e. Calcarea-gastrovascular cavity, coelom present For Self-Quiz tlnSWer.l, sec Appendix A. -$14",,- Visit the Study Area at www.masteringbio.com for a Practice Test. CHAPTER THIRTY·THREE Invertebrates 697