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33 OVERVIEW Life Without a Backbone At ﬁrst glance, you might mistake the organism shown in Figure 33.1 for a 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 removing small food particles from 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 of a ﬂat bilayer of cells to other species with features such as silk-spinning glands, pivoting spines, and tentacles covered with suction cups. Invertebrate species also show enormous variation in size, from microscopic organisms to organisms that can grow to 18 m long (1.5 times the length of a school bus). In this chapter, we’ll take a tour of the 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. Serving as representatives of invertebrate diversity, many of those phyla are explored in more detail in the rest of this chapter. An Introduction to Invertebrates Figure 33.1 What function do the red whorls of this organism have? EVOLUTION KEY CONCEPTS Porifera 33.1 Sponges are basal animals that lack true tissues 33.2 Cnidarians are an ancient Common ancestor of all animals Cnidaria Lophotrochozoa Bilateria Eumetazoa phylum of eumetazoans 33.3 Lophotrochozoans, a clade identiﬁed by molecular data, have the widest range of animal body forms 33.4 Ecdysozoans are the most species-rich animal group 33.5 Echinoderms and chordates are deuterostomes ANCESTRAL PROTIST Ecdysozoa Deuterostomia Figure 33.2 Review of animal phylogeny. Except for sponges (basal animals in phylum Porifera) and a few other groups, all animals have tissues and are in the clade Eumetazoa. Most animals are in the diverse clade Bilateria. The evolutionary tree shown here provides an overview of material covered in Chapter 32 but omits many groups; for a more complete view of animal relationships, see Figure 32.11. 666 UNIT FIVE The Evolutionary History of Biological Diversity Figure 33.3 Exploring Invertebrate Diversity Kingdom Animalia encompasses 1.3 million known species, and estimates of total species range as high as 10–20 million species. Of the 23 phyla surveyed here, 12 are discussed more fully in this chapter, Chapter 32, or Chapter 34; cross-references are given at the end of their descriptions. Porifera (5,500 species) Placozoa (1 species) Animals in this phylum 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 body (see Concept 33.1). A sponge Cnidaria (10,000 species) Cnidarians include corals, jellies, and hydras. These animals have a diploblastic, 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 adhaerens, doesn’t even look like an animal. It consists of a simple bilayer of a few thousand cells. Placozoans are thought to be basal animals, but it is not yet known how they are related to other early-diverging animal groups such as Porifera and Cnidaria. Trichoplax can reproduce by dividing into two individuals or by budding off many multicellular individuals. A placozoan (LM) Ctenophora (100 species) A jelly Acoela (400 species) 1.5 mm 0.5 mm Acoel ﬂatworms have a simple nervous system and a saclike gut, and thus were once placed in phylum Platyhelminthes. Molecular analyses, however, indicate that Acoela is a separate lineage that diverged before the three main bilaterian clades (see Concept 32.4). A ctenophore, or comb jelly Acoel flatworms (LM) Ctenophores (comb jellies) are diploblastic and radially symmetrical like cnidarians, suggesting that both phyla diverged from other animals very early (see Figure 32.11). Comb jellies make up much of the ocean’s plankton. They have many distinctive traits, including eight “combs” of cilia that propel the animals through the water. When a small animal contacts the tentacles of some comb jellies, specialized cells burst open, covering the prey with sticky threads. Lophotrochozoa 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). Rotifera (1,800 species) Despite their microscopic size, rotifers have specialized organ systems, including an alimentary canal (a digestive tract with both a mouth and an anus). They feed on microorganisms suspended in water (see Concept 33.3). 100 μm Platyhelminthes (20,000 species) A rotifer (LM) A marine flatworm Ectoprocta (4,500 species) Brachiopoda (335 species) Ectoprocts (also known as bryozoans) live as sessile colonies and are covered by a tough exoskeleton (see Concept 33.3). Ectoprocts 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, as well as a crown of cilia called a lophophore (see Concept 33.3). A brachiopod Continued on next page CHAPTER 33 An Introduction to Invertebrates 667 Figure 33.3 (continued) Exploring Invertebrate Diversity Lophotrochozoa (continued) Acanthocephala (1,100 species) Cycliophora (1 species) Acanthocephalans are Curved called spiny-headed worms hooks because of the curved hooks on the proboscis at the anterior end of their body. All species are parasites. Some acanthocephalans manipulate the behavior of their intermediate hosts (generally arthropods) in ways that increase their chances of An acanthocephalan (LM) reaching their ﬁnal hosts (generally vertebrates). For example, acanthocephalans that infect New Zealand mud crabs 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. Some phylogenetic analyses place the acanthocephalans within Rotifera. The only known cycliophoran species, Symbion pandora, was discovered in 1995 on the mouthparts of a lobster. This tiny, vaseshaped creature has a unique body plan and a particularly bizarre life cycle. Males impregnate females that are still developing in their 100 μm mothers’ bodies. The fertilized females then escape, A cycliophoran (colorized SEM) settle elsewhere on the lobster, and release their offspring. The offspring apparently leave that lobster and search for another one to which they attach. Nemertea (900 species) Also called proboscis worms or ribbon worms, nemerteans swim through water or burrow in sand, extending a unique proboscis to capture prey. Like ﬂatworms, they lack a true coelom. However, unlike ﬂatworms, nemerteans have an alimentary canal and a closed circulatory system in which the blood is contained in vessels and hence is distinct from ﬂuid in the body cavity. A ribbon worm Annelida (16,500 species) Mollusca (93,000 species) Annelids, or segmented worms, are distinguished from other worms by their body segmentation. Earthworms are the most familiar annelids, but the phylum consists primarily of marine and freshwater species (see Concept 33.3). Molluscs (including snails, clams, squids, and octopuses) have a soft body that in many species is protected by a hard shell (see Concept 33.3). A marine annelid An octopus Ecdysozoa Loricifera (10 species) Loriciferans (from the Latin lorica, corset, and ferre, to bear) are tiny animals that inhabit the deep-sea bottom. A loriciferan can telescope its head, neck, and thorax in and out of the lorica, a pocket formed by six plates surrounding the abdomen. Though the natural history of loriciferans is mostly a mystery, at least some species likely eat bacteria. Priapula (16 species) 50 μm A loriciferan (LM) 668 UNIT FIVE 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 cm in length, most species burrow through seaﬂoor sediments. Fossil evidence suggests that priapulans were among the major predators during the Cambrian period. Ecdysozoa (continued) Onychophora (110 species) Tardigrada (800 species) 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 Tardigrades (from the Latin tardus, slow, and gradus, step) 100 μm are sometimes called 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. As many as 2 million tardigrades can be found on a square meter of moss. Harsh conditions may cause tardigrades Tardigrades (colorized SEM) to enter a state of dormancy; while dormant, they can survive temperatures as low as ⫺272°C, close to absolute zero! Nematoda (25,000 species) A roundworm (colored SEM) Arthropoda (1,000,000 species) Also called roundworms, nematodes are enormously abundant and diverse in the soil and in aquatic habitats; many species parasitize plants and animals. Their most distinctive feature 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 scorpion (an arachnid) Deuterostomia Hemichordata (85 species) An acorn worm Like echinoderms and chordates, hemichordates are members of the deuterostome clade (see Chapter 32). Hemichordates share some traits with chordates, such as gill slits and a dorsal nerve cord. The largest group of hemichordates is 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) 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. A tunicate Echinodermata (7,000 species) Echinoderms, such as sand dollars, sea stars, and sea urchins, are marine 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). A sea urchin CHAPTER 33 An Introduction to Invertebrates 669 CONCEPT 33.1 Sponges are basal animals that lack true tissues Animals in the phylum Porifera are known informally as sponges. (Recent molecular studies indicate that sponges are monophyletic, and that is the phylogeny we follow here; this remains under debate, however, as some studies suggest that sponges are paraphyletic.) 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 most species are marine, though a few live in fresh water. Sponges are suspension feeders: 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 spongocoel, 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 Porifera Cnidaria Lophotrochozoa Ecdysozoa Deuterostomia 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 ﬁnger-like projections that form a “collar” around the ﬂagellum). These cells engulf bacteria and other food particles by phagocytosis. 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 of a sponge consists of two layers of cells separated by a gelatinous region called the mesohyl. Because both cell layers are in contact with water, processes such as gas exchange and waste removal can occur by diffusion across the membranes of these cells. Other tasks are performed by cells called amoebocytes, named for their use of pseudopodia. These cells move through the mesohyl and have many functions. For example, they take up food from the surrounding water and from choanocytes, digest it, and carry nutrients to other cells. Amoebocytes also manufacture tough skeletal ﬁbers within the mesohyl. In some 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 Figure 33.4 Anatomy of a sponge. 5 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. Flagellum Collar 4 Spongocoel. Water passing through pores enters a cavity called the spongocoel. Phagocytosis of food particles 3 Pores. Water enters the sponge through pores formed by doughnutshaped cells that span the body wall. Spicules Water flow 1 Mesohyl. The wall of this sponge consists of two layers of cells separated by a gelatinous matrix, the mesohyl (“middle matter”). 670 UNIT FIVE Choanocyte Osculum Azure vase sponge (Callyspongia plicifera) 2 Epidermis. The outer layer consists of tightly packed epidermal cells. Food particles in mucus The Evolutionary History of Biological Diversity Amoebocyte 6 The movement of a choanocyte‘s flagellum also draws water through its collar of finger-like projections. Food particles are trapped in the mucus that coats the projections, engulfed by phagocytosis, and either digested or transferred to amoebocytes. 7 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. being sold as brown bath sponges. Finally, and perhaps most importantly, amoebocytes are capable of becoming other types of sponge cells. This gives the sponge body remarkable ﬂexibility, enabling it to adjust its shape in response to changes in its physical environment (such as the direction of water currents). 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: They function ﬁrst as one sex and then as the other. 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 33.1 1. Describe how sponges feed. 2. WHAT IF? 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. CONCEPT 33.2 Cnidarians are an ancient phylum of eumetazoans Eumetazoa All animals except sponges and a few other groups belong to the clade Eumetazoa, animals with true tissues (see 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 in early members of the group some 560 million years ago. Porifera Cnidaria Lophotrochozoa Ecdysozoa Deuterostomia Polyp Mouth/anus Tentacle Medusa Gastrovascular cavity Gastrodermis Mesoglea Body stalk Epidermis Tentacle Mouth/anus 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 (darker blue; derived from ectoderm) and an inner layer of gastrodermis (yellow; derived from endoderm). Digestion begins in the gastrovascular cavity and is completed inside food 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. The basic body plan of a 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 two 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 (plural, medusae) resembles 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. Enzymes are secreted into the cavity, thus breaking down the prey into a nutrient-rich broth. Cells lining the cavity then absorb these nutrients and complete the digestive process; any undigested remains are expelled through the mouth/anus. The tentacles are armed with batteries of cnidocytes, 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 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 gastrodermis (inner layer) have bundles of microﬁlaments arranged into contractile ﬁbers (see Chapter 6). The gastrovascular CHAPTER 33 An Introduction to Invertebrates 671 Hydrozoans Tentacle Cuticle of prey Thread Nematocyst “Trigger” Thread discharges Cnidocyte Scyphozoans 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 shoots out, puncturing and injecting poison into prey. cavity acts as a hydrostatic skeleton (see Concept 50.6) 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 around the body. Thus, the animal can detect and respond to stimuli from all directions. The phylum Cnidaria is divided into four major clades: Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa (Figure 33.7). (a) Hydrozoa. Some species, such as this one, live as colonial polyps. (b) Scyphozoa. Many jellies are bioluminescent. Food captured by nematocystbearing tentacles is transferred to specialized oral arms (that lack nematocysts) for transport to the mouth. Figure 33.7 Cnidarians. 672 UNIT FIVE Most hydrozoans alternate between the polyp and medusa forms, as seen 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 few cnidarians found in fresh water, are unusual hydrozoans in that they exist only in polyp form. When environmental conditions are favorable, a hydra reproduces asexually by budding, forming outgrowths that pinch off from the parent and live independently (see Figure 13.2). When conditions deteriorate, hydras can reproduce sexually, forming resistant zygotes that remain dormant until conditions improve. The Evolutionary History of Biological Diversity The medusa is the predominant stage in the life cycle of most scyphozoans. The medusae of most species 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. Cubozoans As their name (which means “cube animals”) suggests, cubozoans have a box-shaped medusa stage. Cubozoans can be distinguished from scyphozoans in other ways, such as having complex eyes embedded in the fringe of their medusae. They also are comparatively strong swimmers and as a result are less likely to be stranded on shore. Most cubozoans live in tropical oceans and are equipped with highly toxic cnidocytes. The sea wasp (Chironex ﬂeckeri), a cubozoan that lives off the coast of northern Australia, is one of the deadliest organisms known: 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. (c) Cubozoa. A notorious example is the sea wasp (Chironex fleckeri ). Its poison, which can subdue fish and other large prey, is more potent than cobra venom. (d) Anthozoa. Sea anemones and other anthozoans exist only as polyps. Many anthozoans form symbiotic relationships with photosynthetic algae. 2 Some of the colony’s polyps, equipped with tentacles, are specialized for feeding. 4 Medusae swim off, grow, and reproduce sexually. 3 Other polyps, specialized for reproduction, lack tentacles and produce tiny medusae by asexual budding. Reproductive polyp Feeding polyp 1 A colony of interconnected polyps (inset, LM) results from asexual reproduction by budding. Medusa bud MEIOSIS Gonad Medusa Egg SEXUAL REPRODUCTION Sperm ASEXUAL REPRODUCTION (BUDDING) Portion of a colony of polyps FERTILIZATION Zygote Developing polyp Planula (larva) 1 mm Mature polyp Key 6 The planula eventually settles and develops into a new polyp. Figure 33.8 The life cycle of the hydrozoan Obelia. The polyp is asexual, and the medusa is sexual, releasing eggs and sperm. These two stages alternate, one producing the other. Do not confuse this with the alternation of generations that occurs in plants and some 5 The zygote develops into a solid ciliated larva called a planula. 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 have a multicellular haploid generation and a multicellular diploid generation. Anthozoans Sea anemones (see Figure 33.7d) and corals belong to the clade Anthozoa (meaning “ﬂower animals”). These cnidarians occur only as polyps. Corals live as solitary or colonial forms, often forming symbioses with algae (see Chapter 28). Many species secrete a hard external skeleton of calcium carbonate. Each polyp generation builds on the skeletal remains of earlier generations, constructing “rocks” with shapes characteristic of their species. These skeletons are what we usually think of as coral. Coral reefs are to tropical seas what rain forests are to tropical land areas: They provide habitat for many other species. Unfortunately, these reefs are being destroyed at an alarming rate. Pollution and overﬁshing are major threats, and global warming may also be contributing to their demise by raising seawater temperatures above the narrow range in which corals thrive. CONCEPT CHECK Haploid (n) Diploid (2n) WHAT IF? 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? 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. MAKE CONNECTIONS As you read in Concept 25.3 (pp. 518–519), many new animal body plans emerged during and after the Cambrian explosion. In contrast, cnidarians today retain the same diploblastic, radial body plan found in cnidarians 560 million years ago. Are cnidarians therefore less successful or less “highly evolved” than other animal groups? Explain. (See also Concept 25.6, pp. 529–530.) For suggested answers, see Appendix A. CHAPTER 33 An Introduction to Invertebrates 673 CONCEPT 33.3 Lophotrochozoans, a clade identiﬁed by molecular data, have the widest range of animal body forms Bilateria The vast majority of animal species belong to the clade Bilateria, whose members exhibit bilateral symmetry and triploblastic development (see Chapter 32). Most bilaterians also have a digestive tract with two openings (a mouth and an anus) and a coelom. While the sequence of bilaterian evolution is a subject of active investigation, the most recent common ancestor of living bilaterians probably existed in the late Proterozoic eon (about 575 million years ago). Many of the 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 of its members. Some lophotrochozoans develop a structure called a lophophore, a crown of ciliated tentacles that functions in feeding, while others go through a distinctive stage called the trochophore larva (see Figure 32.13). Other members of the 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: Lophotrochozoa includes about 18 phyla, more than twice the number in any other clade of bilaterians. We’ll now introduce six diverse lophotrochozoan phyla: the ﬂatworms, rotifers, ectoprocts, brachiopods, molluscs, and annelids. Porifera Cnidaria Lophotrochozoa Ecdysozoa Deuterostomia 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 environment or in their gut. Because of this proximity to water, gas exchange and the elimination of nitrogenous waste (ammonia) can occur by diffusion across the body surface. Flatworms have no organs specialized for gas exchange, and their relatively simple excretory apparatus functions mainly to maintain osmotic balance with their surroundings. This apparatus consists of protonephridia, networks of tubules with ciliated structures called ﬂame bulbs that pull ﬂuid through branched ducts opening to the outside (see Figure 44.11). Most ﬂatworms 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. Early in their evolutionary history, ﬂatworms separated into two lineages, Catenulida and Rhabditophora. Catenulida is a small clade of about 100 ﬂatworm species, most of which live in freshwater habitats. Catenulids typically reproduce asexually by budding at their posterior end. The offspring often produce their own buds before detaching from the parent, thereby forming a chain of two to four genetically identical individuals—hence their informal name, “chain worms.” The other ancient ﬂatworm lineage, Rhabditophora, is a diverse clade of about 20,000 freshwater and marine species, one example of which is shown in Figure 33.9. We’ll explore this group in more detail, focusing on free-living and parasitic members of this clade. Free-Living Species Free-living rhabditophorans are important as predators and scavengers in a wide range of freshwater and marine habitats. The best-known members of this group are freshwater species in the genus Dugesia, commonly called planarians. Abundant in unpolluted ponds and streams, planarians prey on Flatworms Flatworms (phylum Platyhelminthes) live in marine, freshwater, and damp terrestrial habitats. In addition to freeliving species, ﬂatworms 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); the word platyhelminth means “ﬂat worm.” (Note that worm is not a formal taxonomic name but rather refers to a grade of animals with long, thin bodies.) The smallest ﬂatworms are nearly microscopic free-living species, while some tapeworms are more than 20 m long. 674 UNIT FIVE The Evolutionary History of Biological Diversity 5 mm Figure 33.9 A free-living marine ﬂatworm. 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 rhabditophorans also use their muscles to swim through water with an undulating motion. A planarian’s head is equipped with a pair of lightsensitive 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. 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 cross-fertilize each other. Figure 33.10 Anatomy of a planarian. Pharynx. The mouth is at the tip of a muscular pharynx. Digestive juices are spilled onto prey, and the pharynx 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 extensive surface area. Undigested wastes are egested through the mouth. Gastrovascular cavity Mouth Eyespots Parasitic Species More than half of the known species of rhabditophorans live as parasites in or on other animals. Many have suckers that Ganglia. At the anterior end Ventral nerve cords. From of the worm, near the main sources the ganglia, a pair of attach to the internal organs or outer surfaces of the host anof sensory input, is a pair of ganglia, ventral nerve cords runs imal. In most species, a tough covering helps protect the pardense clusters of nerve cells. the length of the body. asites within their hosts. Reproductive organs occupy nearly the entire interior of these worms. We’ll 1 Mature flukes live in the blood vessels of the human discuss two ecologically and economiintestine. A female fluke fits into a groove running cally important subgroups of parasitic the length of the larger male’s body, as shown in rhabditophorans, the trematodes and the LM at right. the tapeworms. Male Trematodes As a group, trematodes parasitize a wide range of hosts, and most species have complex life cycles with alternating sexual and asexual stages. Many trematodes require an intermediate host in which larvae develop before infecting the ﬁnal host (usually a 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 trematodes called blood ﬂukes (Schistosoma) and suffer from schistosomiasis, a disease whose symptoms include pain, anemia, and diarrhea. Living within different hosts puts demands on trematodes that free-living animals don’t face. A blood 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 camouﬂage for itself. It Female 1 mm 5 These larvae penetrate the skin and blood vessels of humans working in fields irrigated with water contaminated with fluke larvae. 2 Blood flukes reproduce sexually in the human host. The fertilized eggs exit the host in feces. 3 If the human feces reach a pond or other source of water, the eggs develop into ciliated larvae. These larvae infect snails, the intermediate host. 4 Asexual reproduction within a snail results in another type of motile larva, which escapes from the snail host. Snail host Figure 33.11 The life cycle of a blood ﬂuke (Schistosoma mansoni ), a trematode. WHAT IF? 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. CHAPTER 33 An Introduction to Invertebrates 675 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. Doctors have patients take the drug niclosamide by mouth to kill the adult worms. Tapeworms The tapeworms are a second large and diverse group of parasitic rhabditophorans (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 gastrovascular cavity; they simply 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, feces carrying the eggs 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 containing the 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. Rotifers (phylum Rotifera) are tiny animals that inhabit freshwater, marine, and damp soil habitats. Ranging in size from about 50 μm 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 two openings, a mouth and an anus. Internal organs lie within the pseudocoelom, a body cavity that is not completely lined by mesoderm (see Figure 32.8b). Fluid in the pseudocoelom serves as a hydrostatic skeleton. Movement of a rotifer’s body distributes the ﬂuid throughout the body, circulating nutrients. The word rotifer is derived from the Latin meaning “wheelbearer,” 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. Digestion is then completed farther along the alimentary canal. Most other bilaterians also have an alimentary canal, which enables the stepwise digestion of a wide range of food particles. Rotifers exhibit some unusual forms of reproduction. Some species consist only of females that produce more females from unfertilized eggs, a type of asexual reproduction called parthenogenesis. Some other invertebrates (for example, aphids and some bees) and even some vertebrates (for example, some lizards and some ﬁshes) can also reproduce in this way. In addition to being able to produce females by parthenogeneis, some rotifers can also reproduce sexually under certain conditions, such as high levels of crowding. When this occurs, a female produces two types of eggs. Eggs Proglottids with reproductive structures Rotifers Jaws 100 μm Crown of cilia Hooks Scolex Sucker 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 Anus Stomach 0.1 mm Figure 33.13 A rotifer. These pseudocoelomates, smaller than many protists, are generally more anatomically complex than ﬂatworms (LM). of one type develop into females, and eggs of the other type develop into males. In some cases, the males do not feed and survive only long enough to fertilize eggs. The fertilized eggs develop into resistant embryos capable of remaining dormant for years. Once the embryos break dormancy, they develop into a new female generation that reproduces by parthenogenesis until conditions once again favor sexual reproduction. It is puzzling that 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 (see Concept 46.1). 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, researchers have been studying a clade of asexual rotifers named Bdelloidea. Some 360 species of bdelloid rotifers are known, and all of them reproduce by parthenogenesis without any males. Paleontologists have discovered bdelloid rotifers preserved in 35-million-year-old amber, and the morphology of these fossils resembles only the female form, with no evidence of males. By comparing the DNA of bdelloids with that of their closest sexually reproducing rotifer relatives, scientists have concluded that bdelloids have likely been asexual for 100 million years. How these animals manage to ﬂout the general rule against long-lasting 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 lophophore, a crown of ciliated tentacles around their mouth (see Figure 32.13a). As the cilia draw water toward the mouth, the tentacles trap suspended food particles. Other similarities, such as a U-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 ecto, 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 Pectinatella magniﬁca grow on submerged sticks or rocks and can grow into a gelatinous, ball-shaped mass more than 10 cm across. Lophophore Lophophore (a) Ectoprocts, such as this creeping bryozoan (Plumatella repens), are colonial lophophorates. (b) Brachiopods, such as this lampshell (Terebratulina retusa), have a hinged shell. The two parts of the shell are dorsal and ventral. Figure 33.14 Lophophorates. Brachiopods, or lamp shells, superﬁcially resemble clams and other hinge-shelled molluscs, but the two halves of the brachiopod 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, appear 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). There are 93,000 known species, making them the second most diverse phylum of animals (after the arthropods, discussed later). Although the majority of molluscs are marine, roughly 8,000 species inhabit fresh water, and 28,000 species of snails and slugs live on land. All molluscs are soft-bodied (from the Latin molluscus, soft), and most secrete a hard protective shell made of calcium 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 (if one 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 organ called a radula to scrape up food. CHAPTER 33 An Introduction to 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. Visceral mass Coelom The long digestive tract is coiled in the visceral mass. Intestine Gonads Mantle Stomach Shell Mantle cavity Mouth Radula Anus The nervous system consists of a nerve ring around the esophagus, from which nerve cords extend. Gill Foot Nerve cords Esophagus Mouth 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. 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 a ciliated larval stage, the trochophore (see Figure 32.13b), which is also characteristic of marine annelids (segmented worms) and some other lophotrochozoans. The basic body plan of molluscs has evolved in various ways in the phylum’s seven or eight clades (experts disagree on the number). We’ll examine four of those clades here: Polyplacophora (chitons), Gastropoda (snails and slugs), Bivalvia (clams, oysters, and other bivalves), and Cephalopoda (squids, octopuses, cuttleﬁshes, and chambered nautiluses). We will then focus on threats facing some groups of molluscs. Figure 33.16 A chiton. Note the eight-plate shell characteristic of molluscs in the clade Polyplacophora. Chitons Chitons have an oval-shaped body and a shell composed of 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. (a) A land snail Gastropods About three-quarters of all living species of molluscs are gastropods (Figure 33.17). Most gastropods are marine, but there are also freshwater species. Still other gastropods have adapted to life on land, where snails and slugs thrive in habitats ranging from deserts to rain forests. 678 UNIT FIVE The Evolutionary History of Biological Diversity (b) A sea slug. Nudibranchs, or sea slugs, lost their shell during their evolution. Figure 33.17 Gastropods. Mantle cavity Stomach Intestine Anus Mouth Figure 33.18 The results of torsion in a gastropod. Because of torsion (twisting of the visceral mass) during embryonic development, the digestive tract is coiled and the anus is near the anterior end of the animal. Figure 33.19 A bivalve. This scallop has many eyes (dark blue spots) lining each half of its hinged shell. Gastropods undergo a distinctive 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 a separate 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 of other molluscs or for tearing apart prey. In the cone snails, the teeth of the radula act as poison darts that are used to subdue prey (see the Unit 7 interview with Baldomero Olivera on pp. 850–851 to learn more about cone snails and their venom). 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. 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. Most bivalves lead sedentary lives, a characteristic suited to suspension feeding. Mussels secrete strong threads that tether them to rocks, docks, boats, and the shells of other animals. 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. Hinge area Mantle Coelom Gut Heart Adductor muscle (one of two) Digestive gland Anus Mouth Excurrent siphon Bivalves The molluscs of the clade Bivalvia are all aquatic and include many species of clams, oysters, mussels, and scallops. Bivalves have a shell divided into two halves (Figure 33.19). The halves are hinged, 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 of their 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 small food Shell Palp Foot 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 and the palps to the mouth. CHAPTER 33 An Introduction to Invertebrates 679 Cephalopods Cephalopods are active marine predators (Figure 33.21). They use their tentacles to grasp prey, which they then bite with beak-like jaws and immobilize with a poison present 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. The mantle covers the visceral mass of cephalopods, but the shell is generally reduced and internal (in most species) or missing altogether (in some cuttleﬁshes and some octopuses). One small group of cephalopods with external shells, the chambered nautiluses, survives today. Cephalopods are the only molluscs with a closed circulatory system, in which the blood remains separate from ﬂuid in the body cavity. They also have well-developed sense organs and 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 Table 25.1). Most species of squid are less than 75 cm long, but some are considerably larger. The giant squid Architeuthis 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 Mesonychoteuthis hamiltoni 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 twice as large! Unlike A. dux, which has large suckers and small teeth on its tentacles, M. hamiltoni has two rows of sharp hooks at the ends of its tentacles that can inﬂict deadly lacerations. It is likely that A. dux and M. hamiltoni 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. hamiltoni has yet to be observed in nature. Overall, these marine giants remain among the great mysteries of invertebrate life. Protecting Freshwater and Terrestrial Molluscs Squids are speedy carnivores with beak-like jaws and well-developed eyes. Octopuses are considered among the most intelligent invertebrates. Chambered nautiluses are the only living cephalopods with an external shell. Figure 33.21 Cephalopods. 680 UNIT FIVE The Evolutionary History of Biological Diversity Species extinction rates have increased dramatically in the last 400 years, raising concern that a sixth, human-caused mass extinction may be under way (see Chapter 25). Among the many taxa under threat, molluscs have the dubious distinction of being the animal group with the largest number of documented extinctions (Figure 33.22). Threats to molluscs are especially severe in two groups, freshwater bivalves and terrestrial gastropods. The pearl mussels, a group of freshwater bivalves that can make natural pearls (gems that form when a mussel or oyster secretes layers of a lustrous coating around a grain of sand or other small irritant), are among the world’s most endangered animals. Roughly 10% of the 300 pearl mussel species that once lived in North America have become extinct in the last 100 years, and over two-thirds of those that remain are threatened by extinction. Terrestrial gastropods, such as the snail in Figure 33.22, fare no better. Hundreds of Paciﬁc island land snails have disappeared since 1800. Overall, more than 50% of the Paciﬁc island land snails are extinct or under imminent threat of extinction. Threats faced by freshwater and terrestrial molluscs include habitat loss, pollution, and competition or predation by non-native species introduced by people. Is it too late to protect these molluscs? In some locations, reducing water pollution and changing how water is released from dams have led to dramatic rebounds in pearl mussel populations. Such results provide hope that with corrective measures, other endangered mollusc species can be revived. Figure 33.22 Figure 33.23 A polychaete. Hesiolyra bergi lives on the seaﬂoor around deep-sea hydrothermal vents. I M PA C T Molluscs: The Silent Extinction M olluscs account for a largely unheralded but sobering 40% of all documented extinctions of An endangered Pacific island animal species. These extinctions land snail, Partula suturalis have resulted from habitat loss, pollution, introduced species, overharvesting, and other human actions. Many pearl mussel populations, for example, were driven to extinction by overharvesting for their shells, which were used to make buttons and other goods. Today, remaining populations of these and other freshwater bivalves face threats from pollution and introduced species. Terrestrial gastropods such as the species pictured above are highly vulnerable to the same threats and are among the world’s most imperiled animal groups. Molluscs Other invertebrates Amphibians Insects Birds Fishes Mammals Recorded extinctions of animal species. Reptiles (excluding birds) (Data: International Union for Conservation of Nature, 2008) Parapodia Annelids Annelida means “little rings,” referring to the annelid body’s resemblance to a series of fused rings. Annelids are segmented 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 of a giant Australian earthworm. The phylum Annelida can be divided into two main groups: Polychaeta (the polychaetes) and Oligochaeta (the earthworms and their relatives, and the leeches). Some recent phylogenetic analyses have suggested that the oligochaetes are actually a subgroup of the polychaetes. However, since this idea continues to be debated, we discuss polychaetes and oligochaetes separately. Polychaetes Workers on a mound of pearl mussels killed to make buttons (ca. 1919) WHY IT MATTERS The extinctions of molluscs represent an irreversible loss of biological diversity and greatly threaten other organisms, too. Land snails, for example, play a key role in nutrient cycling, while the ﬁltering activities of freshwater bivalves purify the waters of streams, rivers, and lakes. FURTHER READING C. Lydeard et al., The global decline of nonmarine mollusks, BioScience 54:321–330 (2004). MAKE CONNECTIONS Freshwater bivalves feed on and can reduce the abundance of photosynthetic protists and bacteria. As such, would the extinction of freshwater bivalves likely have weak or strong effects on aquatic communities (see Concept 28.7, p. 597)? Explain. Each segment of a polychaete (from the Greek poly, many, and chaitē, long hair) has a pair of paddle-like or ridge-like structures called parapodia (“beside feet”) that function in locomotion (Figure 33.23). Each parapodium has numerous chaetae, bristles made of chitin. In many polychaetes, the parapodia are richly supplied with blood vessels and also function as gills. Polychaetes make up a large and diverse group, 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-dwellers, such as the fan worms, build their tubes by mixing mucus with bits of sand and broken shells. Others, such as Christmas tree worms (see Figure 33.1), construct tubes using only their own secretions. Oligochaetes Oligochaetes (oligos, few, and chaitē, long hair) are named for their relatively sparse chaetae (far fewer per segment than in polychaetes). Molecular data indicate that these segmented worms form a diverse clade that includes the earthworms and their aquatic relatives, along with the leeches. Earthworms Earthworms eat their way through the soil, extracting nutrients as the soil passes through the alimentary canal. CHAPTER 33 An Introduction to Invertebrates 681 Coelom. The coelom of the earthworm is partitioned by septa. 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 50.35). These muscles work against the noncompressible coelomic fluid, which acts as a hydrostatic skeleton. Epidermis Many of the internal structures are repeated within each segment of the earthworm. Cuticle Septum (partition between segments) Circular muscle Longitudinal muscle Chaetae. Each segment has four pairs of chaetae, bristles that provide traction for burrowing. Metanephridium. Each segment of the worm contains a pair of excretory tubes, called metanephridia, with ciliated funnel-shaped openings called nephrostomes. The metanephridia remove wastes from the blood and coelomic fluid through exterior pores. Anus Dorsal vessel Tiny blood vessels are abundant in the earthworm’s skin, which functions as its respiratory organ. The blood contains oxygencarrying hemoglobin. Intestine photo to come from 7e archive Fused nerve cords Nephrostome Ventral vessel Clitellum Pharynx Esophagus Metanephridium Crop 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. Gizzard Mouth Subpharyngeal ganglion 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 animal, as do the digestive tract and longitudinal blood vessels. Figure 33.24 Anatomy of an earthworm, an oligochaete. 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 50,000 earthworms, producing 18 tons of castings per year.) A guided tour of the anatomy of an earthworm, which is representative of annelids, is shown in Figure 33.24. Earthworms are hermaphrodites, but they do cross-fertilize. Two 682 UNIT FIVE The Evolutionary History of Biological Diversity earthworms mate by aligning themselves in opposite directions in such a way that they exchange sperm, and then they separate. The received sperm are stored temporarily while an organ called the clitellum 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. Figure 33.25 A leech. A nurse applied this medicinal leech (Hirudo medicinalis) to a patient’s sore thumb to drain blood from a hematoma (an abnormal accumulation of blood around an internal injury). CONCEPT 33.4 Ecdysozoans are the most species-rich animal group Although deﬁned primarily by molecular evidence, the clade Ecdysozoa includes animals that shed a tough external coat (cuticle) as they grow; in fact, the group derives its name from this process, which is called ecdysis, or molting. Ecdysozoa consists of about eight animal phyla and contains more known species than all other animal, protist, fungus, and plant groups combined. Here we’ll focus on the two largest ecdysozoan phyla, the nematodes and arthropods, which are among the most successful and abundant of all animal groups. Porifera Cnidaria Lophotrochozoa Ecdysozoa Deuterostomia Leeches Most leeches inhabit fresh water, but there are also marine species and terrestrial leeches, which live in moist vegetation. Leeches range in length from 1 to 30 cm. Many are predators that feed on other invertebrates, but some are parasites that suck blood by attaching temporarily to other animals, including humans (Figure 33.25). Some parasitic species use bladelike jaws to slit the skin of their 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 of the 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 the 20th 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 puriﬁed hirudin to dissolve unwanted blood clots that form during surgery or as a result of heart disease. Several forms of hirudin have been developed using recombinant DNA techniques; two of these were recently approved for clinical use. As a group, Lophotrochozoa encompasses a remarkable range of body plans, as illustrated by members of such phyla as Rotifera, Ectoprocta, Mollusca, and Annelida. Next we’ll explore the diversity of Ecdysozoa, a dominant presence on Earth in terms of sheer number of species. CONCEPT CHECK Nematodes Among the most ubiquitous of 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 1 m long, often tapering to a ﬁne tip at the posterior end and to a blunter tip at the anterior end (Figure 33.26). A nematode’s body is covered by a tough cuticle (a type of exoskeleton); 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. 33.3 1. Explain how tapeworms can survive without a coelom, a mouth, a digestive system, or an excretory system. 2. Annelid anatomy can be described as “a tube within a tube.” Explain. 3. MAKE CONNECTIONS Explain how the molluscan foot in gastropods and the excurrent siphon in cephalopods represent an example of descent with modiﬁcation (see Concept 22.2, pp. 457–460). For suggested answers, see Appendix A. 25 μm Figure 33.26 A free-living nematode (colorized SEM). CHAPTER 33 An Introduction to Invertebrates 683 Multitudes of nematodes 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 of Earth or its organisms remained but nematodes, they would still preserve the outline of the planet and many of its 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, Caenorhabditis elegans, however, is very well studied and has become a model research organism in biology (see Chapter 47). Ongoing studies of C. elegans are revealing some of the mechanisms involved in aging in humans, among other ﬁndings. Phylum Nematoda includes many species that parasitize plants, and some are major agricultural pests that attack the roots of crops. Other nematodes parasitize animals. Some of these species beneﬁt humans by attacking insects such as cutworms that feed on the roots of crop plants. On the other hand, 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 33.27). 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. Within 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. Some species inject their plant hosts with molecules Encysted juveniles Muscle tissue 50 μm that induce the development of root cells, which then supply nutrients to the parasites. Trichinella, which parasitizes animals, 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 promote the growth of new blood vessels, which then supply the nematode with nutrients. Arthropods Zoologists estimate that there are about a billion billion (1018) arthropods living on Earth. More than 1 million arthropod species have been described, most of which are insects. In fact, two out of every three known species are arthropods, and members of the 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 regarded as the most successful of all animal phyla. Arthropod Origins Biologists hypothesize that the diversity and success of arthropods are related to their body plan—their segmented body, 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 lobopods, 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 33.28). 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 of labor among different body regions. Figure 33.28 A trilobite fossil. Trilobites were common denizens of the shallow seas throughout the Paleozoic era but disappeared with the great Permian extinctions about 250 million years ago. Paleontologists have described about 4,000 trilobite species. Figure 33.27 Juveniles of the parasitic nematode Trichinella spiralis encysted in human muscle tissue (LM). 684 UNIT FIVE The Evolutionary History of Biological Diversity INQUIRY Figure 33.29 Did the arthropod body plan result from new Hox genes? 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 (Ubx) 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: Origin of Ubx and abd-A Hox genes? Other ecdysozoans Arthropods Common ancestor of onychophorans and arthropods Onychophorans 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, researchers studied Hox genes in onychophorans (see Figure 33.3), close relatives of arthropods (Figure 33.29). 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.) General Characteristics of Arthropods Over the course of evolution, the appendages of some arthropods have become modiﬁed, specializing in functions such as walking, feeding, sensory reception, reproduction, and defense. Like the appendages from which they were derived, these modiﬁed structures are jointed and come in pairs. Figure 33.30 illustrates the diverse appendages and other arthropod characteristics of a lobster. According to this hypothesis, Ubx and abd-A would not have been present in the common ancestor of arthropods and onychophorans; hence, onychophorans should not have these genes. To ﬁnd out whether this was the case, Carroll and colleagues examined the Hox genes of the onychophoran Acanthokara kaputensis. RESULTS The onychophoran A. kaputensis has all arthropod Hox genes, including Ubx and abd-A. Cephalothorax Antennae (sensory reception) Abdomen Thorax Head Red indicates the body regions of this onychophoran embryo in which Ubx or abd-A genes were expressed. (The inset shows this area enlarged.) Swimming appendages (one pair per abdominal segment) Ant = antenna J = jaws L1–L15 = body segments Walking legs CONCLUSION Since A. kaputensis, an onychophoran, has the arthro- pod Hox genes, the evolution of increased body segment diversity in arthropods must not have been related to the origin of new Hox genes. SOURCE J. K. Grenier, S. Carroll, et al., Evolution of the entire arthropod Hox gene set predated the origin and radiation of the onychophoran/ arthropod clade, Current Biology 7:547–553 (1997). WHAT IF? If Carroll and colleagues had found that A. kaputensis did not have the Ubx and abd-A Hox genes, how would their conclusion have been affected? Explain. Pincer (defense) Mouthparts (feeding) Figure 33.30 External anatomy of an arthropod. Many of the distinctive features of arthropods are apparent in this dorsal view of a lobster, along with some uniquely crustacean characteristics. The body is segmented, but this characteristic 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. CHAPTER 33 An Introduction to Invertebrates 685 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 of attachment for the muscles that move the appendages. But it also means that an arthropod cannot grow without 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. When the arthropod exoskeleton ﬁrst evolved in the sea, its main functions were probably protection and anchorage for muscles, but it later 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 of water. 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, although there are interesting exceptions. Female butterﬂies, for example, “taste” plants using sensory organs on their feet. 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 closed 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, phylogenetic analyses suggest that the open circulatory systems of molluscs and arthropods arose independently. A variety of specialized gas exchange organs have evolved in arthropods. These organs allow the diffusion of respiratory 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. 686 UNIT FIVE The Evolutionary History of Biological Diversity Figure 33.31 Horseshoe crabs (Limulus 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 chelicerates that once ﬁlled the seas. Morphological and molecular evidence suggests that living arthropods consist of four major lineages that diverged early in the evolution of the phylum: chelicerates (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). Chelicerates Chelicerates (subphylum Chelicerata; from the Greek cheilos, lips, and cheir, arm) are named for clawlike feeding appendages called chelicerae, which serve as pincers or fangs. Chelicerates have an anterior cephalothorax and a posterior abdomen. They lack antennae, and most have simple eyes (eyes with a single lens). The earliest chelicerates were eurypterids, or water scorpions. 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 of the marine chelicerates, including all of the eurypterids, are extinct. Among the marine chelicerates that survive today are the sea spiders (pycnogonids) and horseshoe crabs (Figure 33.31). The bulk of modern chelicerates are arachnids, a group that includes scorpions, spiders, ticks, and mites (Figure 33.32). Ticks and many mites are among a large group of parasitic arthropods. Nearly all ticks are bloodsucking parasites that live on the body surfaces of reptiles or mammals. Parasitic mites live on or in a wide variety of vertebrates, invertebrates, and plants. Arachnids have a cephalothorax that has six pairs of appendages: the chelicerae; a pair of appendages called pedipalps that function in sensing, feeding, or reproduction; and four pairs of walking legs (Figure 33.33). Spiders use their fang-like chelicerae, which are equipped with poison glands, to attack 50 μm (a) Millipede Scorpions have pedipalps that are pincers specialized for defense and the capture of food. The tip of the tail bears a poisonous stinger. (b) Centipede Figure 33.34 Myriapods. prey. As the chelicerae pierce the prey, the spider secretes digestive juices onto the prey’s torn tissues. 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 33.33). The extensive surface area of these respiratory organs is a structural adaptation that enhances the exchange of O2 and CO2 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, indicating that this complex behavior is inherited. Various spiders also use silk in other ways: as droplines for rapid escape, as a cover for eggs, and even as “gift wrap” for food that males offer females during courtship. Many small spiders also extrude silk into the air and let themselves be transported by wind, a behavior known as “ballooning.” Dust mites are ubiquitous scavengers in human dwellings but are harmless except to those people who are allergic to them (colorized SEM). Web-building spiders are generally most active during the daytime. Figure 33.32 Arachnids. Stomach Intestine Heart Myriapods Brain Digestive gland Eyes Ovary Poison gland Anus Book lung Spinnerets Silk gland Gonopore (exit for eggs) Sperm receptacle Figure 33.33 Anatomy of a spider. Chelicera Pedipalp Millipedes and centipedes belong to the subphylum Myriapoda (Figure 33.34). 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 have a large number of legs, though fewer than the thousand their name implies. Each trunk segment is formed from two fused segments and bears two pairs of legs (see Figure 33.34a). Millipedes eat decaying leaves and other plant matter. They may have been among the earliest animals on land, living on mosses and early vascular plants. CHAPTER 33 An Introduction to Invertebrates 687 Unlike millipedes, centipedes are carnivores. Each segment of a centipede’s trunk region has one pair of legs (see Figure 33.34b). Centipedes have poison claws on their foremost trunk segment that paralyze prey and aid in defense. 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 insect ﬂight evolved during the Carboniferous and Permian periods, it spurred an explosion in insect diversity. A fossil record of diverse 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 increases in the diversity of particular insect groups often were associated with radiations of the ﬂowering plants on which they fed. 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 (Figure 33.36). Because the wings are extensions of the 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. Insect wings may have ﬁrst evolved as extensions of the cuticle that helped the insect 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. The insect body has three regions: head, thorax, and abdomen. The segmentation of the thorax and abdomen is obvious, but the segments that form the head are fused. Abdomen Thorax Head Heart. The insect heart drives hemolymph through an open circulatory system. Compound eye Antennae Dorsal artery 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. Crop Anus Malpighian tubules. Metabolic wastes are removed from the hemolymph by excretory organs called Malpighian tubules, which are outpocketings of the digestive tract. Vagina Ovary 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. Figure 33.35 Anatomy of a grasshopper, an insect. 688 UNIT FIVE The Evolutionary History of Biological Diversity 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 33.36 Ladybird beetle in 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 protect 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 insect 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 33.37). Reproduction in insects is usually sexual, with separate male and female individuals. Adults come together and recognize each other as members of the same species by advertising with bright colors (as in butterﬂies), sounds (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 of copulation, though in some species the male deposits a sperm packet outside the female, and the female picks it up. An internal structure in the female called the spermatheca 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 more than 30 orders, 8 of which are introduced in Figure 33.38. (a) Larva (caterpillar) (b) Pupa (c) Later-stage pupa (d) Emerging adult Figure 33.37 Complete metamorphosis of a butterﬂy. (a) The larva (caterpillar) spends its time eating and growing, molting as it grows. (b) After several molts, the larva develops into a pupa. (c) Within the pupa, the larval tissues are broken down, and the adult is built by the division 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 veins as struts supporting the wings. The insect will ﬂy off and reproduce, deriving much of its nourishment from the food reserves stored by the feeding larva. CHAPTER 33 (e) Adult An Introduction to Invertebrates 689 Figure 33.38 Exploring Insect Diversity Although there are more than 30 orders of insects, we’ll focus on just 8 here. Two earlydiverging groups of wingless insects are the bristletails (Archaeognatha) and silverﬁsh (Thysanura). Evolutionary relationships among the other groups discussed here are under debate and so are not depicted on the tree. Archaeognatha (bristletails; 350 species) These wingless insects are found under bark and in other moist, dark habitats such as leaf litter, compost piles, and rock crevices. They feed on algae, plant debris, and lichens. Thysanura (silverﬁsh; 450 species) These small, wingless insects have a ﬂattened body and reduced eyes. They live in leaf litter or under bark. They can also infest buildings, where they can become pests. Winged insects (many orders; six are shown below) Complete metamorphosis Incomplete metamorphosis Coleoptera (beetles; 350,000 species) Hemiptera (85,000 species) Beetles, such as this male snout weevil (Rhiastus lasternus), constitute the most species-rich order of insects. They have two pairs of wings, one of which is thick and stiff, the other membranous. They have an armored exoskeleton and mouthparts adapted for biting and chewing. Hemipterans include so-called “true bugs,” such as stink bugs, bed bugs, and assassin bugs. (Insects in other orders are sometimes erroneously called bugs.) Hemipterans have two pairs of wings, one pair partly leathery, the other pair membranous. They have piercing or sucking mouthparts and undergo incomplete metamorphosis, as shown in this image of an adult stink bug guarding its offspring (nymphs). Diptera (151,000 species) Dipterans have one pair of wings; the second pair has become modiﬁed into balancing organs called halteres. Their mouthparts are adapted for sucking, piercing, or lapping. Flies and mosquitoes are among the best-known dipterans, which live as scavengers, predators, and parasites. Like many other insects, ﬂies such as this red tachinid (Adejeania vexatrix) have well-developed compound eyes that provide a wideangle view and excel at detecting fast movements. Orthoptera (13,000 species) Hymenoptera (125,000 species) Most hymenopterans, which include ants, bees, and wasps, are highly social insects. They have two pairs of membranous wings, a mobile head, and chewing or sucking mouthparts. The females of many species have a posterior stinging organ. Many species, such as this European paper wasp (Polistes dominulus), build elaborate nests. Lepidoptera (120,000 species) Proboscis 690 UNIT FIVE Butterﬂies and moths have two pairs of wings covered with tiny scales. To feed, they uncoil a long proboscis, visible in this photograph of a hummingbird hawkmoth (Macroglossum stellatarum). This moth’s name refers to its ability to hover in the air while feeding from a ﬂower. Most lepidopterans feed on nectar, but some species feed on other substances, including animal blood or tears. The Evolutionary History of Biological Diversity Grasshoppers, crickets, and their relatives are mostly herbivorous. They have large hind legs adapted for jumping, two pairs of wings (one leathery, one membranous), and biting or chewing mouthparts. This aptly named spear-bearer katydid (Cophiphora sp.) has a face and legs well adapted to making a threatening display. Male orthopterans commonly make courtship sounds by rubbing together body parts, such as ridges on their hind legs. Animals as numerous, diverse, and widespread as insects are bound to affect the lives of most other terrestrial organisms, including humans. Insects consume enormous quantities of plant matter; play key roles as predators, parasites, and decomposers; and are an essential source of food for larger animals such as lizards, rodents, and birds. Humans depend on bees, ﬂies, and many other insects to pollinate crops and orchards. In addition, people in many parts of the world eat insects as an important source of protein. 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). Insects also compete with humans for food. In parts of Africa, for instance, insects claim about 75% of the crops. In the United States, billions of dollars are spent each year on pesticides, spraying crops with massive doses of some of the 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.” (a) Ghost crabs live on sandy ocean beaches worldwide. Primarily nocturnal, they take shelter in burrows during the day. Crustaceans While arachnids and insects thrive on land, crustaceans, for the most part, have remained in marine and freshwater environments. Crustaceans (subphylum Crustacea) typically have highly specialized appendages. Lobsters and crayﬁshes, for instance, have a toolkit of 19 pairs of appendages (see Figure 33.30). The anterior-most 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 the next molt. Small crustaceans exchange gases across thin areas of the cuticle; larger species have gills. Nitrogenous wastes 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 crayﬁshes, 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 of the largest groups of crustaceans (numbering over 11,000 species) is the isopods, which include terrestrial, freshwater, and marine species. Some isopod species are abundant in habitats at the bottom of the deep ocean. Among the terrestrial isopods are the pill bugs, or wood lice, common on the undersides of moist logs and leaves. Lobsters, crayﬁshes, crabs, and shrimps are all relatively large crustaceans called decapods (Figure 33.39a). The cuticle of (b) Planktonic crustaceans known as krill are consumed in vast quantities by some whales. (c) The jointed appendages projecting from the shells of these barnacles capture organisms and organic particles suspended in the water. Figure 33.39 Crustaceans. decapods is hardened by calcium carbonate; the portion that covers the dorsal side of the 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 of copepods, which are among the most numerous of all animals. Some copepods are grazers that feed upon algae, while others are predators that eat small animals (including smaller copepods!). Copepods are rivaled in abundance by the shrimplike krill, which grow to about 5 cm long (Figure 33.39b). A major food source for baleen whales (including blue 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. With the exception of a few parasitic species, barnacles are a group of sessile crustaceans whose cuticle is hardened into a CHAPTER 33 An Introduction to Invertebrates 691 shell containing calcium carbonate (Figure 33.39c). Most barnacles anchor themselves to rocks, boat hulls, pilings, and other submerged surfaces. Their natural adhesive is as strong as synthetic glues. These barnacles feed by extending appendages from their shell to strain food from the water. Barnacles were not recognized as crustaceans until the 1800s, when naturalists discovered that barnacle larvae resemble the larvae of other crustaceans. The remarkable mix of unique traits and crustacean homologies found in barnacles was a major inspiration to Charles Darwin as he developed his theory of evolution. CONCEPT CHECK 33.4 1. How do nematode and annelid body plans differ? 2. Describe two adaptations that have enabled insects to thrive on land. 3. In contrast to mammalian jaws, which move up and down, the mouthparts of arthropods move side to side. Explain this feature of arthropods in terms of the origin of their mouthparts. 4. MAKE CONNECTIONS Traditionally, annelids and arthropods were viewed as closely related because both have body segmentation. Yet DNA sequence 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 H

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