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GUIDE FOR READING After you read the following sections, you will be able to CHAPTER 27-1 Mollusks • Explain how mollusks perform their essential life functions. • Describe and give examples of the three major classes Mollusks 27-1 Mollusks Guide For Reading ¦ What are mollusks, and how do they perform essential life functions? sa What are the three major classes of mollusks? ¦ How do mollusks fit into the world? of mollusks. • Discuss how mollusks affect humans and other living things. and 27-2 Annelids • Describe how annelids perform their essential life functions. • List and give examples of three Annelids classes of annelids. Members of the phylum Mollusca are known as mollusks. Mollusks evolved in the sea more than 600 million years ago and have experienced a long and successful adaptive radiation. Today there are more than 100,000 mollusk species, which are divided into seven classes. Mollusks live everywhere—from deep ocean trenches to mountain brooks to the tops of trees. They range in size from snails as small as a grain of sand to giant squids that may grow more than 20 meters long. And as you can see in Figure 27-1, mollusks come in a wide range of forms and colors. Journal Activity " YOU AND YOUR WORLD The organisms that seem to be exotic flowers swaying in the breeze are actually annelid worms. The worms use their feathery gills for feeding and respiration. The spotted nudibranch, or sea slug, (inset) is a mollusk from the Great Barrier Reef in Australia. Have you ever observed a snail or a slug? In your journal, describe the animal and how it moves. What were you doing when you noticed the ani¬ mal? What characteristic intrigues you most about the animal? H,lave you ever eaten fried dams, broiled scallops, or calamari in tomato sauce? Have you ever gone fishing with live worms as bait? If you have, you are already familiar with some of the more common members of the two phyla that we shall study in this chapter: mollusks and annelids (segmented worms). Both of these phyla are ancient, very large, and remarkably diverse. Both provide many examples of how evolution can mold a single basic animal body plan into many different shapes. And both remind us that animals with ancient and simple body plans can be very well adapted to their environments. What are mollusks and annelids? How are they related to one another? How are they adapted to their environments? What relationships do they have with other living things? You will find the answers to these questions in the pages that follow. Figure 27-1 Chitons are relatively primitive marine mollusks that have a shell made up of a number of plates (inset). Snails are more specialized mollusks that have a one-part shell. The tree snail is creeping over a red Heliconia flower. What Is a Mollusk? Why are animals that look and act so differently grouped in the same phylum? One reason mollusks are classified together is that they share similar developmental patterns. (As you may recall from Chapter 15, many animals are classified on the basis of shared features during early development.) Most mol¬ lusks have a special kind of larva called a trochophore (TROHkoh-for). See Figure 27-2 on page 586. Trochophore larvae swim in open water and feed on tiny floating plants. Trochophore larvae are also seen in segmented worms which belong to the phylum Annelida. Biologists believe that this indicates that mollusks and annelids evolved from a com¬ mon ancestor that existed during the Precambrian Period (more than 580 million years ago) and had a trochophore Figure 27-2 The trochophore larva of a chiton, like other trochophore larvae, has a tuft of cilia making up the "handle" on its top-shaped body and a band of cilia encircling its body. Figure 27-3 The basic body parts of mollusks are the foot, mantle, shell, and visceral mass. Note that the form and function of the foot and shell vary greatly among larva. Because the phyla Mollusca and Annelida are closely re¬ lated to each other, we shall discuss them both in this chapter. Another reason mollusks are placed in a single phylum is that their different forms are the results of variations on the same basic body plan. Mollusks are defined as soft-bodied animals that have an internal or external shell. Their name is derived from the Latin word molluscus, meaning soft. Although a few present-day mollusks lack shells, they are thought to have evolved from shelled ancestors. Form and Function in Mollusks As you can see in Figure 27-3, the body plan of almost all mollusks consists of four basic parts: foot, mantle, shell, and visceral mass. The soft muscular foot usually contains the mouth and other structures associated with feeding. The foot takes many different shapes in mollusks: Flat surfaces adapted to crawling, spade-shaped structures for burrowing, and tenta¬ cles for capturing prey are a few examples. The mantle is a thin, delicate tissue layer that covers most of a mollusk's body, much like a cloak. The shell, which is found in almost all mol¬ lusks, is made by glands in the mantle that secrete calcium car¬ bonate (CaCOs). Just beneath the mantle in most mollusks is the visceral mass, which contains the internal organs. mollusks. These basic body parts have taken on different forms as mollusks evolved adaptations to different habitats. The type of foot and the kind of shell that mollusks have are used to group them into classes. Later in this chapter we shall examine the three major classes of mollusks. FEEDING Mollusks have evolved many types of feeding mechanisms and feed on many kinds of food. In fact it would be simpler to list the few things these animals do not eat than it would be to describe everything they can feed on! Every mode of feeding is seen in the phylum Mollusca. Most mollusks are herbivores, carnivores, or filter feeders, but a few species are detritus feeders and others are parasites. Many mollusks—snails and slugs, for example—feed with a tongue-shaped structure called a radula (RAJ-oo-lah). The radula is a layer of flexible skin that carries hundreds of tiny teeth, which make it look and feel like sandpaper. Inside the radula is a stiff supporting rod of cartilage. When the mollusk feeds, it places the tip of the radula on the food and pulls the sandpapery skin back and forth over the cartilage. Mollusks that are herbivores use their radula to scrape algae off rocks and twigs in the water or to eat the buds, roots, and flowers of land plants. Mollusks that are carnivores use their radula to drill through the shells of other animals. Once they have made a hole through the shell, these carnivores extend their mouth and radula into the shell and tear up and swallow the prey's soft tissue. In the carnivorous snails called cone shells, the tiny rasping teeth of the radula have evolved into long hollow darts that are attached to poison glands. A cone shell uses these darts to stab and poison prey such as small fish. Although they may have a radula, carnivorous mollusks such as octopi and certain sea slugs typically use sharp jaws to eat their prey. Like cone shells, some octopi produce poison to subdue their prey. Although cone shells and octopi generally feed on fish and other small animals, the poisons produced by some species are strong enough to hurt or even kill humans. Mollusks such as clams, oysters, and scallops are filter feeders. They use their feathery gills to sift food from the water. As these animals pass water over their gills, phytoplankton (tiny photosynthetic organisms) in the water become trapped in a layer of sticky mucus. Cilia on the gills move the mixture of mucus and food into the mouth. Figure 27-4 Many mollusks scrape bits of food into their mouth by pulling the tooth-covered skin of the radula back and forth over a supporting rod of cartilage. The scanning electron micrograph shows the teeth on the radula of a land snail. RESPIRATION Gills serve as organs of respiration as well Shell as filters for food. In fact, in most species gills are used only for breathing. Aquatic mollusks such as snails, clams, and octopi breathe by using gills located inside their mantle cavities. But Mantle cavity land snails and slugs breathe by using a specially adapted Visceral mass mantle cavity that is lined with many blood vessels. The mantle is wrinkled or folded to fit a larger surface within the limited Radula teeth Figure 27-5 The nudibmnch (right) breathes through its skin and tuft of gills. Janthina (left), which uses a raft of air bubbles to float at the ocean surface, breathes with gills inside its shell. Many land snails (bottom) use their mantle cavity as a lung. space of the cavity. This surface is constantly kept moist so that oxygen can enter the cells. Because the mantle loses water in dry air, most land snails and slugs must live in moist places. They prefer to move around at night, during rainstorms, and at other times when the air is humid. INTERNAL TRANSPORT Oxygen that is taken in by the respiratory system and nutrients that are the products of di¬ gestion are carried by the blood to all parts of a mollusk's body. The blood is pumped by a simple heart through what is called an open circulatory system. "Open" does not mean that blood can spill to the outside of the animal! It means that blood does not always travel inside blood vessels. Instead, blood works its way through body tissues in open spaces called sin¬ uses. These sinuses lead to vessels that pass first through the gills, where oxygen and carbon dioxide are exchanged, and then back to the heart. Open circulatory systems work well for slow-moving or sessile (attached to one spot) mollusks like snails and clams. But the flow of blood through sinuses is not efficient enough for fast-moving octopi and squids. Those ani¬ mals have closed circulatory systems, in which blood always moves inside blood vessels. EXCRETION Like other animals, mollusks must eliminate waste products. Undigested food becomes solid waste that leaves through the anus in the form of feces. Cellular metabo¬ lism produces nitrogen-containing waste in the form of ammo¬ nia. Because ammonia is poisonous, it must be removed from body fluids. Mollusks get rid of ammonia by using simple tubeshaped organs called nephridia (neh-FRiHD-ee-ah; singular: nephridium). Nephridia remove ammonia from the blood and release it to the outside. RESPONSE Mollusks vary greatly in the complexities of their nervous systems and their abilities to respond to environ¬ mental conditions. Clams and other two-shelled mollusks, many of which lead basically inactive lives burrowing in mud or sand, have simple nervous systems. They have several small ganglia near the mouth, a few nerve cords, and simple sense organs such as chemical and touch receptors, statocysts (sim¬ ple organs for balance), and ocelli (eyespots). Octopi and other tentacied mollusks, on the other hand, are active and in¬ telligent predators that have the most highly developed ner¬ vous systems of all members of their phylum. Because of their well-developed brain, these animals can remember things for long periods of time, and they may even be more intelligent than some vertebrates. The numerous complex sense organs these mollusks possess help them distinguish shapes by sight and texture by touch. Octopi can be trained to perform differ¬ ent tasks in order to obtain a reward or avoid punishment. Be¬ cause of these abilities, octopi are often studied by psychologists interested in the way animals learn. REPRODUCTION As with almost all other essential func¬ tions, mollusks accomplish the function of reproduction in dif¬ ferent ways. In most mollusks, the sexes are separate and fertilization is external. These mollusks—which include many snails, almost all two-shelled mollusks, and most of the species in the four minor classes of mollusks—release eggs and sperm into open water in enormous numbers. Eggs and sperm find each other by chance, and free-swimming larvae develop from the resulting fertilized eggs. In tentacied mollusks and certain snails, fertilization takes place inside the body of the female. Fertilization is also internal in some hermaphrodites (organ¬ isms that have both male and female reproductive organs). For example, many hermaphroditic snails get together in pairs and fertilize each other's eggs at the same time. Some other her¬ maphroditic mollusks, such as certain oysters, switch from one sex to the other. Sometimes they are male (and thus produce sperm) and sometimes female (and thus produce eggs)! Figure 27-6 Like most mollusks, coquina clams (inset) have an open circulatory system. Cephalopod mollusks, such as cuttlefish, have a closed circulatory system. Clams move slowly, whereas cuttlefish can jet backwards through the water at high speeds. Figure 27-7 A scallop gathers information about its environment with tiny round eyespots and sensory tentacles. Using those eyespots and other sense organs, scallops stay alert for enemies, such as starfish. Unlike most bivalves, scallops can sometimes escape from enemies by clapping their shells together rapidly, using jet propulsion to help them jump off the ocean bottom and scuttle away. See Figure 27-10. Figure 27-8 The ringed top snail (bottom, left) is found in the oceans of the Pacific Northwest. Despite their lack of a protective shell, the marine nudibranch (bottom, right) and terrestrial banana slug (top) are not likely to be eaten by predators— their bright colors and patterns indicate that these gastropods are poisonous. Snails, Slugs, and Their Relatives colors of the nudibranch and avoid it in the future! (While this Members of the class Gastropoda are called gastropods (GAS-troh-pahdz). The name gastropod literally means stomach foot. This name is quite appropriate because most gastropods move by means of a broad, muscular foot located on their ven¬ does not help the first nudibranch, it does protect others of its kind.) Thus shell-less gastropods do have means of protection. tral (stomach) side. Gastropods include the familiar pond snails and land slugs as well as more exotic mollusks such as abalones, sea butterflies, sea hares, and nudibranchs. Many gastropods have a one-piece shell that protects their soft bodies. This shell may be simple and shieldlike, as in lim¬ pets, or coiled, as in snails. When threatened, many snails can pull up completely into their coiled shells. Some snails are ad¬ ditionally protected by a hard disk on their foot that forms a solid "door" at the mouth of their shell when they withdraw. Some gastropods have small shells or, as is the case with slugs, lack shells completely. This would seem to make them easy prey for hungry predators. However, these gastropods are not entirely helpless. Most land slugs are protected by their be¬ havior—they spend the daylight hours hiding under rocks and logs, hidden from birds and other animals that might eat them. Some sea hares have a special ink-producing gland that they use when threatened to squirt ink into the surrounding water. This confuses predators and allows the sea hare to escape under its "smoke screen." Some gastropods, such as sea but¬ terflies, escape predators by swimming rapidly. Many nudi¬ branchs, or sea slugs, have chemicals in their bodies that taste bad or are poisonous. When a predator nibbles on one of these bad-tasting morsels, it gets sick. In addition, some nudibranchs use nematocysts from the cnidarians they eat to sting preda¬ tors. The bad-tasting, poisonous, stinging, or otherwise booby- trapped nudibranchs are usually brightly colored. The bright colors warn predators to stay away. If a predator ignores the warning colors and eats a nudibranch, the consequences usually guarantee that the predator will remember the bright Two-Shelled Mollusks Members of the class Bivalvia (bi- means two; valve means shell) have two shells that are hinged together at the back and held together by one or two powerful muscles. Common bivalves include clams, oysters, and scallops. Bivalves may be tiny or as large as the giant clam Tridacno, which has been known to grow as large as 1.9 meters in length. Although bivalve larvae are free-swimming, they soon set¬ tle down to a relatively quiet life on the bottom of a body of water. Some bivalves, such as clams, burrow in mud or sand. Others, such as mussels, secrete sticky threads to attach them¬ selves to rocks. Although most adult bivalves are sessile, some, such as scallops, can move around rapidly by flapping their shells when threatened. The mantles of bivalves, like those of most other mollusks, contain glands that manufacture the shells. These mantle glands also keep the shell's inside surfaces smooth and com¬ fortable by secreting layers of mother-of-pearl. If a foreign ob¬ ject—a sand grain or small pebble, for example—gets caught between mantle and shell, the mantle glands cover it with this secretion. After many years these objects become completely coated and are called pearls. Figure 27-10 The internal structures of a clam, a typical bivalve, are shown in the diagram on the left. Another bivalve, the scallop, can swim by rapidly opening and closing its shell (top). Pearls—objects coated by smooth, shiny secretions of a bivalve's mantle—may be beautiful and valuable gems or they may be fascinating curios, like the pearl fish (bottom). Nephridium Posterior adductor muscle Anterior muscle Excurrent Mouth siphon ncurrent siphon Intestine Gonad (reproductive organ) Figure 27-9 When threatened, a sea hare releases purple ink into the water. This confuses predators and allows the sea hare to make its escape. Tentacled Mollusks How Mollusks Fit into the World Cephalopods (SEHF-uh-loh-pahdz)—members of the class Cephalopoda—are among the most active and interesting mol¬ lusks. This class includes octopi, squids, cuttlefish, and nauti¬ luses. Cephalopoda means head-foot (cephalo- means head-pod means foot). This name refers to the fact that a cephalopod's head is attached to its foot, which is divided into tenta¬ cles, or arms. Cephalopods range in size from tiny cuttlefish less than 2 centimeters long to giant squids, which are thought to grow to more than 20 meters long. Mollusks play many different roles in living systems. For example, they feed on plants, prey on animals, and "clean up" their surroundings by eating detritus. Some of them are hosts to symbiotic algae or to parasites; others are themselves para¬ sites. In addition, mollusks are an important source of food for many organisms, including humans. Modern-day scientists have found some new uses for mol¬ lusks. Because filter-feeding bivalves concentrate dangerous pollutants and microorganisms in their tissues, careful checks Most cephalopods have eight flexible tentacles equipped of bivalves can warn biologists and public health officials of with a number of round sucking disks that are used to grab and hold fish and other prey. In addition to these tentacles, cuttle¬ fish and squids also have two long, slender arms with suckers on the end. Nautiluses have many more tentacles (38 to 90) than other cephalopods. Their tentacles lack suckers but are made sticky by a mucuslike covering. Although fossil evidence indicates that their ancestors had large external cone-shaped or coiled shells, most modern ce¬ phalopods have small internal shells or no shells at all. The only present-day cephalopods with shells are the few species of nautiluses. These cephalopods look much like fossil cepha¬ lopods from the beginning of the Cambrian Period, more than 500 million years ago. Cuttlefish have small shells that are found inside their bodies. The shells of some cuttlefish are thin and coiled, whereas others (which serve as the cuttlebone on health problems long before scientists can detect these dangers in the open water. Besides acting as environmental monitors, mollusks also serve as subjects in biological re¬ search. Some current investigations are based on the observa¬ tion that snails and other mollusks never seem to develop any form of cancer. If scientists can determine what protects the cells of these animals from cancer, they will gain valuable in¬ sights into how to fight cancer in humans. Although mollusks are beneficial in many ways, they do have some negative relationships with humans. For example, land slugs and snails are plant eaters that can do much damage to gardens and crops. The bivalves called shipworms, which which pet birds condition their beaks) are flat, platelike, and do not resemble shells at all. In both nautiluses and cuttlefish, gases in the shell help the cephalopod remain upright and allow it to float in the water. A squid's internal shell has evolved into a thin, flexible supporting rod known as a pen. Oc¬ topi have lost their shells completely. Although most cephalopods lack a protective shell, they do have other means of protection. Most cephalopods can move quickly, either by swimming or crawling. They can also move by using a form of jet propulsion. The cephalopods draw water into their mantle cavities and then force that water out through use their shells to drill their way slowly through pieces of wood in the water, are sometimes described as the termites of the sea, They settle on wood in large numbers and can reduce a good-sized log to a pile of wet sawdust over the course of a few years. Shipworms cause millions of dollars worth of damage to wooden boats and docks every year. Another problem with mollusks is associated with their use as food. Clams and oys¬ ters, which are among the few marine animals that are farmed in the sea, are filter feeders and thus gather and concentrate particles floating in the water—including bacteria, viruses, and the toxic protists that cause red tides. Eating bivalves that con¬ tain high concentrations of pathogens (things that cause dis¬ ease), toxins, or pollutants can result in sickness or even death. the tubelike siphon. By pointing the siphon in different direc¬ tions, they can shoot out a jet of water that propels them back¬ ward, away from danger. In addition, many cephalopods can release large amounts of dark-colored, foul-tasting ink when they are frightened. After squirting out a large cloud of ink, they make a hasty retreat. Perhaps most fascinating of all, oc¬ topi can quickly change color to match the colors of their sur¬ roundings. The match is often close enough that the octopi are nearly invisible. Figure 27-11 The luminescent squid (top), chambered nautilus (center), and extremely venomous blue-ringed octopus (bottom) are examples of cephalopods. f >: | SECTION Cr'-'i REVIEW 1. What are mollusks? List the three major classes of mollusks and give an example of each. 2. Describe some of the ways mollusks affect humans. 3. What are some different ways mollusks use a radula? is dwindling rapidly. What effect would this have on the Figure 27-12 One trend in cephalopod evolution has been a reduction in the size of the shell. Most modem species have a small shell or no shell at all. The "shell" of the paper nautilus is actually an egg surrounding area? On the human community? cose. 4. How do mollusks protect themselves? 5. Connection—Ecology The number of oyster beds in Chesapeake Bay, which is an arm of the Atlantic Ocean, 593 Guide For Reading 27-2 Annelids : What are annelids, and how do they perform essential life functions? ¦ What are the three classes of annelids? ¦ How do annelids fit into the world? Have you ever dug in a garden? If so, you have probably made the acquaintance of the long, thin, pink earthworm. The soft-bodied earthworm is the most common terrestrial, or landdwelling, segmented worm. But this species is only one of ap¬ Esophagus proximately 9000 species of segmented worms that live in moist soil, in fresh water, and in the sea. Segmented worms, or annelids, live just about everywhere in the world. But because most segmented worms live in the sea, and many others spend their lives underground, only a few species are familiar to us. What Is an Annelid? Members of the phylum Annelida are known as annelids, or segmented worms. An annelid is a round, wormiike animal that has a long, segmented body. The name Annelida is de¬ rived from the Latin word annellus, which means little ring, and refers to the ringlike appearance of the body segments. Annelids range in size from tiny aquatic worms less than half a millimeter long to giant earthworms more than 3 meters long. Although they also vary greatly in color, patterning, number of bristles, and other superficial features, most anne¬ lids are quite wormlike in appearance. Form and Function in Annelids The many segments of an annelid's body are separated by internal walls called septa (singular: septum). Most of the body Figure 27-13 Many polychaete annelids, such as the sandworm Nereis, use hooklike jaws to capture prey or nibble on algae. segments are virtually identical to one another. However, some segments are modified to perform special functions. For exam¬ ple, the first few segments may carry one or more pairs of eyes, several pairs of antennae, and other sense organs. Intestine Anus it. When the pharynx returns to its normal position, it carries these food particles back into the gut. In other detritus feeders, such as earthworms, the pharynx acts like a pump. It sucks a mixture of soil and detritus through the mouth and forces it down into the gut. In parasites, such as leeches, the pharynx is used to suck blood and tissue fluids from the host. Annelids have a number of other structures that are used in feeding. For example, some annelids filter-feed by fanning water through their tubelike burrows and catching passing food particles in a mucus bag. In other filter-feeding annelids, such as the plume worm shown in Figure 27-15, the first seg¬ ment forms featherlike structures that sift detritus and plank¬ ton from the surrounding water. These feeding structures are also used as gills for respiration. RESPIRATION Aquatic annelids often breathe through gills. In some of these annelids, such as feather-duster worms, the large brightly colored feathery gills protrude from the opening of the worm's burrow or tube. In other annelids, small FEEDING The digestive tract, or gut, is a long tube within Figure 27-14 The digestive system of an earthworm is shown here. The pharynx pumps a mixture of food and soil into a tube called the esophagus. The food then moves through the crop, where it con be stored, and through the gizzard, where it is ground into smaller pieces. The food is digested in the intestine. Undigested materials pass through the intestine and are eliminated through the anus. Figure 27-15 The spaghetti worm (left) uses long tentacles to pluck bits of detritus from the ocean floor. In plume worms (right), a brushshaped structure on the head is used in filter feeding and in respiration. the body cavity of the worm that extends from the mouth to the anus (in the tip of the "tail"). Food enters through the mouth and travels through the gut, where it is digested. Like mollusks, annelids have evolved structures and behaviors that allow them to use a wide variety of foods. One feeding organ that has evolved many different forms in different groups of annelids is the pharynx, or the muscular front end of the digestive tube. Many annelids can extend the pharynx through the mouth. In carnivorous annelids, this type of pharynx usually has two or more sharp jaws attached to it. When a suitable animal approaches, the worm lunges forward, rapidly extends the pharynx, and grabs the prey with its jaws. Jaws are also present in herbivores, which use them to tear off bits of algae. In some detritus feeders, the pharynx is covered with sticky mucus. When these worms extend the pharynx and press it against the sea-floor sediments, food particles stick to 595 "Hearts" Ring (pumping ring vessel Dorsal blood vessel vessels) Clitellum il : RESPONSE Many annelids are active animals with welldeveloped nervous systems. The brain sits on top of the gut at the front end of the body. Two large nerves pass around the gut and connect the brain with a pair of ganglia below. From these ganglia, a ventral nerve cord runs the entire length of the worm. Nerves from each segment of the worm enter and leave the nerve cord at a pair of small ganglia. These nerves help carry messages from sense organs and coordinate the move¬ ments of muscles. Ventral' nerve cord Male reproductive organs [Circulatory System Female reproductive organs Excretory System Ventral blood vessel Nephridia Nervous System Reproductive System Figure 27-16 The circulatory, excretory, nervous, and ri reproductive systems of an earthworm are shown here. How many "hearts" does an earthworm have? delicate gills are located on the sides of the body. The tube- dwelling annelids with this type of gill breathe by fanning water through their tubes. Many annelids take in oxygen and give off carbon dioxide through their skin. Because the skin must stay moist to make gas exchange possible, the worms die if the skin dries out. To help guard against this, terrestrial an¬ nelids, such as earthworms, secrete a thin protective coating called a cuticle to hold moisture around them. i'l'i I, i '! INTERNAL TRANSPORT Annelids typically have closed circulatory systems organized around two blood vessels that run the length of their bodies. Blood moves toward the head of the worm in the dorsal (top side) vessel and toward the back of the worm in the ventral (bottom side) vessel. In each body segment is a pair of smaller vessels called ring vessels that connect the dorsal and ventral vessels and supply blood to the internal organs. In annelids such as earthworms, several of the ring vessels near the anterior (front) end of the worm are larger than the other ring vessels and have muscle tissue in their walls. These vessels are often called hearts because they contract rhythmically and help pump blood through the sys¬ tem. In other annelids, blood is moved through the body by muscle contractions when the worm moves. EXCRETION Like other animals, annelids produce two kinds of wastes. Solid wastes pass out through the anus at the end of the gut. Wastes resulting from cellular metabolism are eliminated by nephridia (simple tube-shaped excretory organs). A pair of nephridia in each body segment removes waste products from the body fluids and carries them to the outside. 596 Sense organs are best developed in the free-living marine species of annelids. Many of these annelids have sensory ten¬ tacles, statocysts, chemical receptors, and two or more pairs of eyes. Although the eyes are usually simple light detectors, in a few species the eyes can actually perceive objects. Most tubedwelling species have light-sensitive cells either on their gills or near their mouths. These cells allow the animals to detect the shadows of predators passing overhead. When a shadow is detected, the worm pulls back into the shelter of its tube with amazing speed. In addition to specialized sense organs, these free-living marine worms also have various types of isolated sensory cells scattered along their epidermis. These cells re¬ spond to light, chemicals, and vibration. Many other annelids have much simpler sensory systems. For example, earthworms have no specialized sense organs. They rely on simple sensory cells in the skin that are similar to those found in the skin of marine annelids. Most free-living annelids do not have body structures that protect them from predators. Many depend on rapidly burrow¬ ing or swimming away from danger. Some, like earthworms, grab onto the walls of their burrows to make it harder to pull Figure 27-17 Sense organs are best developed in free-swimming annelids such as the paddleworm, which has a pair of beady eyes and a number of sensory tentacles on its them out, Others, such as the marine fanworms, secrete pro¬ head. tective tubes of calcium carbonate into which they withdraw if frightened. But some annelids do fight back. Several carnivo¬ rous annelids use their sharp jaws to attack animals that try to eat them. And the marine fireworms have tufts of poisonous bristles that easily break off and penetrate skin, causing painful sores and a burning sensation. MOVEMENT Annelids have two major groups of muscles in their body walls. One group, called longitudinal muscles, runs from the front of the worm to the rear. When these mus¬ cles contract, they make the worm shorter. Another group of muscles runs in circles around the body of the worm. When these muscles contract, they make the worm skinnier. Marine annelids can swim by using these muscles to wriggle through the water. Burrowing annelids use their muscles to force their way through heavy sediment—not an easy thing for a soft- bodied animal to do! REPRODUCTION Although a few annelids are able to reproduce asexually by budding, most annelids reproduce sexually. Some species have separate sexes and external 597 ,1 dlelike appendages on their body segments. These appendages are tipped with the bristles that give this class its name. In the sea mouse, shown in Figure 27-19, the bristles are so long that they extend over the back of the worm and look like hair or fur. Polychaetes live in cracks and crevices in coral reefs, in sand, mud, and piles of rocks, and even out in the open water. Some burrow through or crawl over sediments. Others live al¬ most entirely in tubes they build for themselves. Some poly¬ chaetes are dull in color and rather uninteresting; some are brightly colored, iridescent, or even luminescent. Earthworms and Their Relatives Figure 27-18 Although they look very different from each other, both the fanworm (left) and the fire worm (right) are polychaetes. The fanworm is a filter feeder that retreats into its tube when threatened. The fireworm defends itself with poisonous bristles that break off and penetrate skin at the slightest touch. The pain caused by these bristles gives the fireworm its name. fertilization. This means that females and males release eggs and sperm, respectively, into the open water where fertilization takes place. Of course, the chances of fertilization taking place are enhanced if many worms in an area release their eggs and sperm at the same time. This is exactly what happens in some species. In the South Pacific, islanders eagerly await the au¬ tumn spawning season of the annelids called palolo worms. At a particular phase of the moon, hundreds of thousands of male and female palolo worms swarm at the surface of the water to release their eggs and sperm. Just before sunrise, the sea is lit¬ erally covered with these worms. The islanders, who consider these worms a great delicacy, join sea birds and fishes that gather to feast on the spawning worms. Some annelids, such as earthworms and leeches, are her¬ maphrodites that undergo internal fertilization. Although an in¬ dividual worm produces both sperm and eggs, it rarely fertilizes its own eggs. Instead, worms pair up, attach them¬ selves to each other, and exchange sperm. Each worm stores the sperm it has received in special sacs. When eggs are ready for fertilization, a band of thickened, specialized segments called the clitellum (cligh-TEHL-um) secretes a mucus ring into which eggs and sperm are released. The ring then slips off the worm's body and forms a cocoon that shelters the eggs. Sandworms, Bloodworms, and Their Relatives The class Polychaeta (poly- means many; chaeta refers to bristles) contains many common and important marine worms. Polychaetes (PAHL-ee-keets) are characterized by paired pad¬ The class Oligochaeta contains earthworms and related species. Two oligochaetes (AHL-ih-goh-keets) that you might be familiar with are earthworms and tubifex worms. Earth¬ worms are long pink worms that often show up on the surfaces of lawns and sidewalks after it rains, are dug up in gardens, or are sold as fishing bait. Tubifex worms are red threadlike aquatic worms that are sold as tropical-fish food in pet stores. Most oligochaetes live in soil or fresh water, although some species live in the ocean. As the name of the class indicates (oligo- means few), oligochates have fewer bristles than poly¬ chaetes. These bristles, which can be felt as a roughness on the ventral (bottom) side of an earthworm, help anchor it in its Figure 27-19 The long bristles of the sea mouse look like iridescent fur. burrow. Although earthworms spend most of their lives hidden under ground, an observant person may find evidence of their presence above ground in the form of squiggles of mud known as castings. Recall that an earthworm (which swallows just about anything it can get into its mouth) uses its pharynx to suck a mixture of detritus and soil particles into its mouth. As the mixture of food and soil passes through the intestine, part of it is digested. Sand grains, clay particles, and indigestible or¬ ganic matter pass out through the anus in large quantities, pro¬ ducing castings. Some tropical earthworms produce enormous castings—as large as 18 centimeters long and 2 centimeters in diameter! Figure 27-20 Earthworms are hermaphrodites that undergo internal fertilization. The sperm from one worm fertilizes the eggs of its partner, and vice versa. Leeches The class Hirudinea contains the leeches, most of which live in moist tropical countries. Leeches are typically no more than 6 centimeters long, but there are some tropical species that are as long as 30 centimeters. Most leeches are freshwater organisms that exist as external parasites, drinking the blood and body fluids of their host. However, there are some marine and terrestrial leeches. And roughly one fourth of all leeches are carnivores rather than parasites. Carnivorous leeches, which feed on soft-bodied invertebrates such as snails, worms, and insect larva, either swallow their prey whole or suck all the soft parts from its body. 599 SCIENCE, All leeches have powerful suckers at both ends of their bodies. These suckers—especially the anterior one, which usually surrounds the mouth—are used to attach a leech to its host. The posterior sucker is also used to anchor a leech to rocks, leaves, and other objects as it waits for a host to come by. Leeches penetrate the skin of their host in one of two ways Some leeches use a muscular proboscis (proh-BAHS-ihs), or tu¬ bular organ, that they force into the tissue of their host. Others slice into the skin of their hosts with a razor-sharp pair of jaws Once the wound has been made, the leech uses its muscular pharynx to suck blood from the area. Both types of leeches re¬ lease a special secretion from their salivary glands to prevent the blood from clotting as they drink it. Some leeches also pro¬ duce a substance that anesthetizes the wound—thus keeping the host from knowing it has been bitten! During feeding, a leech can swallow as much as 10 times its weight in blood. Such a huge meal can take the leech up to 200 Figure 27-21 A leech attaches to its host with suckers on its anterior and posterior ends (top). As it feeds, the body of the leech swells to accommodate as much as ten times its mass in blood (bottom). days to digest, with the help of symbiotic bacteria that live in its gut. A leech can live for a year before it must feed again. AND SOCIETY ciiiiiiiic i null jm Leeches: Modern Applications of Ancient Medicine There are few medical techniques as ancient as leeching, or applying leeches to a pa¬ tient. The earliest known reference to leeching was written by a Greek physician more than 2200 years ago. And experts believe that #• leeching is much older than that! Many people once believed that diseases could be cured by using leeches to remove blood from the patient. However, when people began to better understand the nature of dis¬ ease, leeches ceased to be popular medical tools. After all, it seemed senseless to remove blood from a patient when it was clear that mi¬ • % How Annelids Fit into the World Annelids are important in many habitats. Small polychaetes and their larvae are members of the ocean plankton, where they are food for many fishes, crabs, and lobsters. Bot¬ tom-dwelling polychaetes are important items in the diets of food fishes such as flounder. Oligochaetes, particularly earthworms, perform an essen¬ tial task in conditioning soil, as Charles Darwin noted in a lengthy and detailed study. By constantly burrowing through the ground, earthworms help aerate the soil. And by grinding and partially digesting the incredible amount of soil and de¬ tritus that passes through their guts, earthworms speed the re¬ turn of nitrogen and other important nutrients from dead organisms to forms that can be used by plants. Without the continual efforts of these annelids, the structure and fertility of farm soils would degenerate quickly, lowering crop yields. 1. What is an annelid? List and give examples of three classes of annelids. 2. Discuss three adaptations for feeding in annelids. 3. Describe the structure of the digestive tract in an earthworm. 4. Critical Thinking—Making Inferences Explain why it is advantageous for an earthworm to have more lightsensitive cells in its anterior and posterior segments than in other parts of its body. ease. But interestingly enough, leeches are once again in the medical spotlight. One modern medical problem faced by surgeons is that blood tends to collect in body parts reattached by microsurgery. Here-is where leeches come in handy. They are used to remove the excess blood until the blood vessels in the reattached part have healed. The chemicals in leech saliva make it pos¬ sible to use leeches for a variety of other medi¬ cal purposes. These chemicals prevent blood from clotting, dissolve existing blood clots, ex¬ pand blood vessels (to keep blood flowing), loosen the connections between cells (to help disperse the other chemicals), and anesthetize the area of the bite. Researchers are currently developing medicines based on chemicals ex¬ tracted from leech saliva. These new medi¬ cines may soon be used to clear blocked blood vessels and to treat a variety of circulatorysystem diseases. Leeches also produce chemicals that harm bacteria—chemicals that they may inject into the host as they feed. Symbiotic bacteria in¬ side the leech's gut produce an antibiotic that keeps stored blood fresh by killing bacteria. And the chemical in leech saliva that loosens or dissolves connections between host cells may also dissolve the protective coating on bacteria, thus making them vulnerable to an attack by the immune system. 4 /, I. V croorganisms—not "bad blood"—caused dis¬ SECTION REVIEW 600 TECHNOLOGY, -> r, V • 1 ( ¦ • The leech shown here is being "milked" for its saliva. Researchers are currently developing medicines based on the chemicals found in leech saliva. As medical researchers discover new uses for leeches, they are reminded that the ancient practice of leeching may not have been quite as senseless as it seemed. They are also re¬ minded of the role evolution plays in shaping and refining the relationships between organ¬ isms (such as leeches and vertebrates). Of course, the leeches themselves didn't decide to perform a useful function for humans. But recall that parasites must evolve along with their hosts. Under pressure from natural se¬ lection, leeches have evolved adaptations that enable them to feed effectively on vertebrate hosts, including humans. Although leeches ap¬ peared on Earth long before humans, the chemicals they produce still affect us. Why? Due to common descent, our body chemistry is similar to that of other vertebrates—includ¬ ing the leech's original vertebrate host. 601 SUMMARIZING THE CONCEPTS PROBLEM How do live earthworms respond to moisture and light? The key concepts in each section of this chapter are listed below to help you review the chapter content. Make sure you understand each concept and its relationship to other concepts and to the theme of this chapter. MATERIALS (per group) 2 live earthworms piece of cardboard in a storage container desk lamp tray medicine dropper paper towels PROCEDURE i II 1. Open the storage container and examine the earthworms. Record your observations of their physical characteristics. Fill the medi¬ cine dropper with water and use it to give your earthworms a "bath." Note: Make sure you keep your earthworms moist by giving them frequent baths. If an earthworm's skin dries out, it dies. 2. Fold a dry paper towel and place it on one side of your tray, as shown in the accompany¬ ing figure. Fold a dampened paper towel and place it on the other side of the tray. 3. Place the earthworms in the center of the tray, between the dry paper towel and the moist paper towel. Cover the tray with the piece of cardboard. 4. After 5 minutes, remove the cardboard and observe the location of the earthworms. Record your observations. 5. Return the earthworms to their storage con¬ tainer. Using the dropper, moisten the earth¬ tray. 8. Cover one half of the tray with the piece of cardboard. Position the lamp above the open side of the tray. 9. After 5 minutes, observe the location of the earthworms. Record your observations. 10. Return the earthworms to their storage con- 602 27-2 Annelids • Mollusks are soft-bodied animals such as snails, clams, and squids that usually have an internal or external shell. The body plan of mollusks consists of four basic parts: foot, mantle, shell, and visceral mass. • Annelids, which are also known as seg¬ mented worms, are round, wormlike animals with long, segmented bodies. An annelid's segments are very similar to one another and are separated by internal partitions. • Most gastropods move by means of a broad, muscular ventral foot. Many gastropods have • Polychaetes have a pair of paddlelike appen¬ dages on each segment. Most polychaetes are free-living marine worms. a one-piece shell. • Bivalves have a hinged two-part shell. Al¬ though larvae are motile, most adult bivalves are sessile. • Cephalopods have a well-developed nervous system, relatively advanced sense organs, and a closed circulatory system. A cephalopod's foot is divided into eight or more tainer. Using the dropper, moisten the earth¬ worms with water. Cover the container. tentacles. • Mollusks play many roles in the natural world. Many products that are important or • Oligochaetes have few bristles and lack ap¬ pendages. Many are burrowing worms. Most live in fresh water or in soil. • Leeches are typically blood-sucking external parasites that live in fresh water, • Annelids interact in many different ways with other parts of the living world. Burrow¬ ing annelids such as earthworms are impor¬ tant in aerating soil. OBSERVATIONS 1. Which kind of surface did the earthworms prefer—moist or dry? 2. Do earthworms prefer light or darkness? REVIEWING KEY TERMS 3. Describe the earthworms' color, texture, ex¬ ternal features, and other physical character¬ istics. worms with water. 6. Cover the entire bottom of the tray with a damp paper towel. 7. Place the earthworms in the center of the 27-1 Mollusks ANALYSIS AND CONCLUSIONS 1. How does an earthworm's response to mois¬ ture help it survive? 2. Does an earthworm's response to light have any protective value? Explain. 3. How is an earthworm's body adapted for movement into and through soil? 4. Would you expect to find earthworms in hard soil? Explain. Vocabulary terms are important to your understanding of biology. The key terms listed below are those you should be especially familiar with. Review these terms and their meanings. Then use each term in a complete sentence. If you are not sure of a term's meaning, return to the appropriate section and review its definition. 27-1 WSoIlosks gill 27-2 Annelids m0l]usk open circulatory system annelid foot closed circulatory system polychaete nephridium oligochaete she,! gastropod visceral mass bivalve leech radula cephalopod 603 CONTENT REVIEW B. Replace the underlined definition with the correct vocabulary word. Multiple Choice Choose the letter of the answer that best completes each statement 1. Which characteristic do many mollusks and annelids have in common? a. segmented body b. one- or two-part shell c. open circulatory system d. trochophore larvae 2. One major class of mollusks is a. Cephalopoda. c. Oligochaeta. 5. A bristly marine worm that has paired ap¬ pendages on each segment belongs to Class a. Annelida. c. Oligochaeta b. Polychaeta. d. Cephalopoda. 6. An oligochaete probably a. is a parasite. b. has paired appendages. c. has septa. b. Hirudinea. d. Polychaeta. 3. A mollusk that swims by flapping its broad muscular foot is probably a d. has a mantle, foot, and visceral mass. 7. A scraping organ used for feeding is a a. nephridium. c. mantle. b. pharynx. d. radula. a. bivalve. c. cephalopod. b. gastropod. d. polychaete. 8. 4. Which organ is used for both respiration and filter feeding in some animals? a. nephridium c. gill b. radula d. ganglion In earthworms, the clitellum a. is involved in asexual reproduction. b. secretes a cocoon for the eggs. c. often has a pair of jaws. d. grinds food particles into smaller pieces. True or False Determine whether each statement is true or false. If it is true, write "true." If it is false, change the underlined word or words to make the statement true. !• A pharynx is an organ used in excretion 2. Softbodied animals that typically have a shell are known as oligochaetes. 3. Segmented worms belong to phylum Hirudinea. 4. Pearls and the shells of mollusks are formed by secretions from the radula. 5. 6. 7. 8. Many leeches are blood-sucking parasites. Cephalopods are characterized by a onepart shell and a broad, muscular foot. Gastropods are usually sessile as adults. Hermaphrodites usually undergo external fertilization. " 5. Octopi have a circulatory system in which the blood is always contained in blood vessels. 6. Earthworms are members of the segmented worms phylum. 7. The part of a mollusk that contains the mouth and is often used in locomotion in cephalopods is divided into tentacles. CON CERT M AST E RY Use your understanding of the concepts developed in the chapter to answer each of the following in a brief paragraph. nf oarasranh. 1. How are mollusks adapted to different modes of feeding? 2. Compare the ways in which polychaetes and oligochaetes perform their essential functions. CRITICAL AND CREATIVE THINKING Discuss each of the following in a brief paragraph. 1. Assessing concepts Although a number of animals are hermaphrodites, they rarely fertilize their own eggs. Explain why crossfertilization is usually better than selffertilization. Under what circumstances might self-fertilization be better than cross-fertilization? 2. Making inferences Some oligochaetes can survive in areas that have little oxygen and can even tolerate a complete lack of oxygen for short periods of time. Some of these oligochaetes die when exposed to normal oxygen levels for a long period of time. What is probably the natural habitat of these oligochaetes? Explain. 3. Developing a hypothesis Female octopi die after brooding their eggs (tending and Word Relationships A. An analogy is a relationship between two pairs of words or phrases eenerallv — the wnner: abed The symbol: is read "is to."and the symbol ,s read as. For example. cot:animal,rose:planl is read "cat is to animal as rose is to plant." e u tut is 10 In the analogies that follow, a word or phrase is missing. Complete each analogy by providing the missing word or phrase. 1. one-part shell:gastropod::two-part shell2. cocoon:clitellum:;shell: 3. shelI;snaiI::nematocyst and chemicals: 4. light detection:ocelli::balance: 3. How are clams adapted for burrowing in mud and sand? 4. How do mollusks fit into the world? 5. Explain why a person might purchase earthworms to put in a garden. Although many nudibranchs are simultaneously male and female, they do not fertilize their own eggs. protecting eggs until they hatch). However, if certain glands near the brooding octopus's eyes are surgically removed, the octopus stops brooding, resumes feeding, and has a life span longer than the normal three to four years. Develop a hypothesis to explain this phenomenon. How might you go about testing your hypothesis? 4. Using the writing process Suppose that the topic of a debate is, "Resolved: It is better to be a free-swimming polychaete than a sessile one." Take either the affirmative or the negative stance and prepare a persuasive argument for your position. J 605