MARINE INVERTEBRATES

Transcription

1 MARINE INVERTEBRATES 8 Marine Worms When you have completed this chapter, you should be able to: IDENTIFY the basic characteristics of the various worm phylums. DESCRIBE the life functions of the different groups of worms. DISCUSS some adaptations of worms to the marine environment. In their general shape, worms tend to resemble one another. You are probably most familiar with earthworms the soft, moist, slowmoving creatures that live in the soil. Most worms move through substrates by wriggling their bodies from side to side. Appendages are missing or greatly reduced in size. But not all worms are alike in how they look and behave. The delicate-looking feather duster worms shown above actually live in stony tubes, which offer them some protection against other animals. The worms colorful featherlike structures are thrust out of their tubes to catch plankton and to take in oxygen. Worms vary in size from microscopic to over several meters in length. In fact, there are enough differences among the groups of worm species to justify their being placed into several phylums. In this chapter, you will learn about some similarities and some differences among the worms that live in marine environments. 8.1 Flatworms and Ribbon Worms 8.2 Roundworms and Segmented Worms 8.3 Giant Tube Worms and Arrow Worms 193

2 8.1 FLATWORMS AND RIBBON WORMS The vertical zone of water that extends from the top of the ocean to its bottom is called the water column. There are few places in the water column where worms cannot be found. They inhabit the seas from the surface down to the seafloor. There are significant differences among the various groups of worms that live at the different depths. Most of the worms you will learn about in this chapter live at the bottom of the water column, near or in the ocean s sediments. Some are found closer to the surface. Yet others are parasitic, living within the bodies of different marine organisms. Flatworms Figure 8-1 The three types of flatworms. Head region Eyespot Look at the group of worms shown in Figure 8-1. As you can see, the bodies of these worms are flat. The flat-body form is the distinguishing trait used to classify these worms in the phylum Platyhelminthes (meaning flatworms ). There are both freshwater and saltwater species of flatworms. Although some flatworms are microscopic, many others can be seen without a microscope. In fact, some species can reach nearly 20 meters in length! There are more than 10,000 species of flatworms. Head region Hooks Sucker Sucker Section Mouth Tail region Planaria Tapeworm Liver fluke 194 Marine Invertebrates

3 The Planarian Look at a macroscopic flatworm, the planarian (Planaria), shown in Figure 8-1. Planarians, which feed on small organisms and organic debris, are found in freshwater and marine habitats. These worms have a mouth on their lower, or ventral, surface through which they take in food as they move along rocks and other substrates. The mouth connects to a branched digestive cavity. Unlike other worms, planarians have a two-way digestive tract. Food enters the mouth and is digested in the intestine; the nutrients diffuse throughout the body (there are no circulatory or respiratory systems). The undigested materials are discharged from the mouth (there is no anus). The planarian glides along the bottom, moving its head from side to side. Locomotion results from the contraction of body muscles and the action of cilia, which are attached to cells on the worm s ventral side. When the cilia beat backward, the planarian moves forward along the surface of a substrate. Planaria have two eyespots in their head region, which are surrounded by the ganglia (nerve cell clusters). The ganglia act like a simple brain, sending nerve impulses along two ventral nerve cords to the rest of the body. This lets the flatworm respond to stimuli (such as light) from its environment. Since the structures on the right side of its body are the same as the structures on the left side of its body, the planarian is said to have bilateral symmetry. In fact, bilateral symmetry, which first appears in the flatworm, is a characteristic feature of all other worms and more structurally complex animals. This type of body plan is associated with the development of a head region, such as that seen in the flatworm. Another feature first seen in the flatworm is the possession of three cell layers: ectoderm, endoderm, and mesoderm (the middle layer). This is important for the development of organ systems, also first seen in the flatworms. Planaria are capable of both asexual and sexual reproduction. During asexual reproduction, a planarian can attach to a substrate, stretch its body, and break in two. Each half can then regenerate the parts needed to form a whole new organism. The planarian, like most flatworms, is a hermaphrodite it contains both male and female reproductive organs. However, Marine Worms 195

4 self-fertilization does not occur. During mating, two flatworms exchange sperm, so that sperm from each of the flatworms fertilizes the eggs of the other flatworm internally. Development of fertilized eggs occurs externally in tiny capsules or cocoons. Offspring hatch from the cocoons in two to three weeks. In the same class as the planaria are other marine flatworms. These worms are also bilaterally symmetrical, although the head region is not obvious as in the planaria. They are characterized by very beautiful colors and patterns, and frilly edges that seem to flutter as they swim. Tapeworms and Trematodes Some flatworm species live as parasites. A parasite is an organism that obtains its food by living in or on the body of another organism. The organism that is fed on is called the host. The tapeworm, shown in Figure 8-1, is sometimes found in the intestines of fish and other animals, including humans. The tapeworm attaches itself to the intestinal lining of its host and absorbs nutrients directly through its thin body wall. As a result, it has no need for a digestive system. This feeding method works well for the tapeworm some can grow to more than 18 meters in length! Another flatworm that is a parasite is the trematode. The trematode, or fluke, lives in the bodies of mollusks, fish, birds, and other animals. The liver fluke, which lives as a parasite in mammals, is shown in Figure 8-1. The blood fluke is a parasite that attaches to a fish s skin and then forms a cyst in which it lives within the fish s tissues. Trematode parasites that are accidentally eaten in raw fish may then reproduce in the digestive tracts of people. Human wastes that are discharged into bodies of water may contain the trematode s eggs. These eggs develop into swimming larvae, which penetrate the soft tissue of their first host a snail. The larvae develop inside the snail and, in time, are discharged into the water, where they seek out their second host a fish or aquatic bird thus completing their life cycle. If you happen to be in the water when the trematode larvae are looking for a host, they may attach to your skin, causing what is called swimmer s itch. 196 Marine Invertebrates

5 Ribbon Worms You have just learned that some flatworms (the tapeworms) can grow nearly 20 meters in length. Interestingly, there are other types of worms that grow very large. The largest free-living worm in the sea is the ribbon worm (Cerebratulus lacteus), shown in Figure 8-2. The nearly 1000 species of ribbon worms are classified in the phylum Nemertea. Although some are relatively small, the average length of a ribbon worm, or nemertean, is about 1 meter. However, ribbon worms 10 to 12 meters long have been observed. All ribbon worms are unsegmented and ciliated, have a milky color, and are thin and flat like ribbons, hence their common name. Although they resemble flatworms in their general shape, ribbon worms have more highly developed circulatory and digestive systems. Ribbon worms live in the intertidal zone. Most are free-living, although some live inside the shells of clams and oysters. Ribbon worms burrow in the sand and also swim with a gently undulating motion among animals that live encrusted on rocks. Prey such as small fish and polychaete worms (see Section 8.2) are speared by a sharp, sticky extension called a proboscis, which the ribbon worm extends from a recessed area in its head region. Food is digested in a one-way digestive tract (that has a stomach and intestine) and wastes exit through the anus. Ribbon worms have a simple, closed circulatory system for the distribution of nutrients and oxygen. Ribbon worms can reproduce asexually by breaking into pieces; each piece is capable of regenerating into a whole new worm. They can also reproduce sexually, having external fertilization and development. Like many other worms, the ribbon worm has a nervous system that consists of ganglia (more concentrated than those of the flatworm), which send impulses to its muscles by way of two nerve cords. Proboscis Figure 8-2 The ribbon worm. 8.1 SECTION REVIEW 1. What two features are first seen in the planarians? 2. Describe asexual and sexual reproduction in the planarian. 3. How do the planarian s and the ribbon worm s digestive and circulatory systems differ? Marine Worms 197

6 8.2 ROUNDWORMS AND SEGMENTED WORMS Roundworms Figure 8-3 A marine roundworm. Mouth Esophagus Female genital pore Larvae in uterus End of esophagus Midgut Eggs in uterus Ovary Anus The most numerous of all worms in the sea in numbers of individuals and of species are the roundworms. There are more than 10,000 species of roundworms. Very tiny roundworms live in the sand and mud at the bottom of the water column. When the tide goes out, roundworms can be found in the moist substrate of the intertidal zone. Gravel samples taken from an aquarium tank and examined under a microscope will also reveal a number of roundworms. (See Figure 8-3.) Typical whipping movements of the roundworm s body propel it through the spaces between sand grains. Some roundworms can also swim through the water. In the roundworm s head region are the ganglia, which connect to nerves that run through the body. These nerves control the muscles that enable the worm to move and actively respond. Roundworms are classified in the phylum Nematoda; they are often referred to as nematodes. As their name implies, roundworms are characterized by a cylindrical body shape, which is tapered at both ends. Most roundworms are small, but some reach up to a meter in length. The sexes are usually separate (a few nematode species are hermaphrodites), and reproduction is sexual. Fertilization is internal; development of eggs is external. Males and females tend to differ in their size and shape. Most marine nematodes live freely in bottom sediments and feed on organic debris. Like the ribbon worm, roundworms have a one-way digestive tract. Food enters the mouth, is digested in the stomach and intestine, and undigested wastes pass out through the anus, located at the opposite end. Like the flatworm, the roundworm has no circulatory or respiratory systems. Nutrients diffuse into its cells; respiratory gases and cellular wastes pass through its skin. Segmented Worms Of all the worms, the earthworm is probably the most familiar to you. Earthworms are typically found in soil and other moist 198 Marine Invertebrates

7 DISCOVERY The Cells of C. Elegans Dozens of scientists around the world are focusing their efforts, and their microscopes, on Caenorhabditis elegans, a transparent roundworm just 1 millimeter in length. This common worm found scavenging on debris in moist sediments everywhere offers scientists an uncommon chance to investigate biological processes such as cell division, embryology, and disease. Many scientists choose to study C. elegans because the adult worm contains a total of only 959 cells compared with millions of cells in other multicellular animals. The worm s transparent skin enables scientists to observe its muscle cells and nerve fibers at work. And it allows them to trace the pathway that the worm s cells take during cell division, from the fertilized egg cell to the 959 cells of the adult just three days later. That means that the researchers have observed differentiation, the process by which embryonic cells develop into adult muscle, nerve, and skin cells. Scientists also found that some cells in the embryo are predestined to die rather than differentiate. It is not known why the cells die off during development. However, researchers think that the study of these cells may help them find out which genes control cell death. This, in turn, may lead to a better understanding of what happens during diseases such as Alzheimer s, in which a person s nerve cells inexplicably die, resulting in severe memory loss. One of the most spectacular achievements in the last few years was the deciphering of the complete genome of C. elegans the first multicellular organism for which the entire DNA was mapped! Scientists discovered that C. elegans has more than 19,000 genes, which code for over 19,000 different proteins. Deciphering an organism s genome is the first step in learning which genes are responsible for a cell s fate. It is hoped that, in the years to come, further research on C. elegans will continue to throw more light on our understanding of cell processes and diseases in humans. QUESTIONS 1. State three advantages of using C. elegans over other animals for cell research. 2. How might studying C. elegans help us understand Alzheimer s disease in humans? 3. Why was deciphering of the roundworm s genome a significant achievement? Marine Worms 199

8 Figure 8-4 An earthworm (partly cutaway view). Anus Clitellum Brain Hearts Artery Mouth Nerve cord Vein Food tube Segments sediments. Some of the internal and external features of an earthworm are shown in Figure 8-4. Notice that the worm s body is divided into compartments, or segments. Referred to as the segmented worms, or annelids, earthworms and all other annelids are classified in the phylum Annelida (meaning little rings ). There are three classes of annelids, comprising more than 10,000 species. Earthworms, and their aquatic relatives that feed on organic matter in sediments, belong to the class Oligochaeta (meaning few bristles ). The Sandworm A common marine annelid is the clamworm, or sandworm (Nereis). (See Figure 8-5.) Sandworms live in muddy sands in the intertidal and subtidal zones of inland bays and marshes. When the tide comes in, sandworms crawl around on the bottom and prey on tiny invertebrates. When the tide moves out, sandworms burrow in the sand and scavenge on organic debris. Some live within tubes. How do sandworms carry out their life functions? Nereis captures its prey with two sharp hooks located in its mouth. The sandworm also has a proboscis, which it can extend to catch prey or bits of seaweed. Food is digested in a one-way digestive tube; waste products of digestion are eliminated through the anus. The digestive tract is separated from the skin by a fluid-filled space called the coelom. (All annelids have a coelom.) Sandworms belong to the class Polychaeta (meaning many bristles ). They wriggle through the wet sand by using their paddlelike appendages called parapodia (singular, parapodium). The parapodia are located on each segment. Each parapodium also has 200 Marine Invertebrates

9 Figure 8-5 The sandworm Nereis. Mouth Hooks Parapodium Segment Bristles Anus hairlike bristles called setae sticking out from it. Movement results when longitudinal and circular muscles located under the worm s skin are contracted. The sandworm has a nervous system to coordinate the movements of its body. Eyelike receptors that receive light stimuli, and touch receptors in the skin that pick up other external stimuli, send impulses to a ventral nerve cord. The impulses travel to the brainlike ganglia. Movement occurs when the brain sends the impulses via nerve cords to muscles, causing them to contract. Like earthworms, sandworms breathe through their skin in this case, through their parapodia. Because the worm s skin is thin and moist, oxygen diffuses from the surrounding water into the body, and carbon dioxide diffuses from inside the body to the outer environment. Nutrients and respiratory gases are transported around the body in a closed circulatory system. This consists of a dorsal blood vessel, arteries, veins, and capillaries. The dorsal blood vessel contracts and pumps the blood. Arteries and veins carry the blood back and forth; capillaries connect arteries with veins. Sandworms have a well-developed excretory system. Waste products of metabolism are excreted from a pair of coiled tubes located in each body segment. These organs, which carry out excretion for the annelids as our kidneys do for us, are called the Marine Worms 201

10 Mouth Bristles Anus Figure 8-6 The bloodworm. nephridia. The sandworm reproduces sexually; the sexes are separate (unlike the earthworm, which is a hermaphrodite). The Bloodworm and Other Polychaetes Feathery gills Stony tube Figure 8-7 The fan worm. Another segmented worm found in marine sediments is the bloodworm (Glycera). (See Figure 8-6.) Like the sandworm, the bloodworm is a polychaete. However, unlike the sandworm, the bloodworm has an open circulatory system in which the blood circulates through the tissue spaces rather than through blood vessels. You can see a bloodworm s blood through its skin, hence its common name. Polychaete worms also include a variety of other bristle worms, such as the plumeworm and the paddle worm. Sandworms and bloodworms usually burrow in sand and hide in seaweed to avoid detection by predators. Some polychaete worms have developed a different strategy to avoid being eaten they live inside a tube, which they make themselves. Most of these tubedwelling worms are small and threadlike in appearance. The fan worm (Sabella) constructs its hard tube by mixing sand grains with mucus, which it secretes from special sacs. (See Figure 8-7.) The parchment worm (Chaetopterus variopedatus) secretes a tube formed from a tough fibrous material, in which it lies buried in sediments below low tide. Both of these worms thrust their feathery gills out of the tube into the water to obtain oxygen, give off carbon dioxide, and capture plankton. The tube of the trumpet worm (Pectinaria gouldii) looks like an ice-cream cone fashioned from sand grains and bits of shell cemented together. This worm extends its ciliated tentacles into the sediment to obtain food. Another polychaete tube worm, called the Atlantic tube worm (Hydroides), secretes a hard tube composed of calcium carbonate, which is cemented to the surfaces of mollusks, rocks, and corals. 202 Marine Invertebrates

11 The Leech An example of a segmented worm without bristles is the leech, which is placed in class Hirudinea. (See Figure 8-8.) Some leeches are free-living, while others are parasites that can be found attached to the gills and mouths of fish. The leech attaches to its host by means of two suckers, located at the anterior (front) and posterior (back) ends of its body. Sharp teeth in the sucker at the anterior end pierce the host s skin. The leech then draws blood from its host. At the same time, the leech secretes a chemical anticoagulant, called hirudin, into the wound. This prevents the host s blood from clotting, thus enabling the blood to flow freely into the leech. While feeding, leeches can increase their body weight threefold. After feeding, the leech drops off the fish and fasts while it digests its blood meal. Posterior sucker Digestive tract Anterior sucker Figure 8-8 The leech. 8.2 SECTION REVIEW 1. In what part of the marine environment are roundworms usually found? What do they feed on there? 2. Describe two adaptations polychaetes have to avoid predators. 3. How do polychaete tube worms feed and protect themselves? 8.3 GIANT TUBE WORMS AND ARROW WORMS Giant Tube Worms You have just read about the segmented worms that live in tubes. There are other types of worms that live in protective tubes, and some of these grow quite large. An interesting group of deep-sea gutless worms belongs to the phylum Pogonophora. In 1977, a new species of worm belonging to this phylum was discovered living near hot-water vents on the deep seafloor, thousands of meters below the ocean s surface. First photographed by the submersible Alvin, these giant tube worms (Riftia pachyptila) measure up to 1 meter long and live clustered in water that is rich in hydrogen Marine Worms 203

12 Feathery plume Segments Ventral blood vessel Feeding body Bacteria live inside Tube Deep seafloor Figure 8-9 The giant tube worm. sulfide. The tubes in which they live (which are made of proteins and minerals) may measure up to 2 meters in length. (See Figure 8-9.) Marine biologists have discovered unique bacteria that live inside the giant tube worms. These bacteria are able to use the hydrogen from the hydrogen sulfide in the water and combine it with carbon dioxide from seawater to produce sugars. The energyrich sugars are also used by the worms. This way of obtaining energy makes the giant tube worms different from most other animals that live on Earth. Remember, most animal life depends on the energy of the sun, energy that is captured by plants and algae during the process of photosynthesis. Yet sunlight cannot reach the great depths at which the giant tube worms live. So the worms live by utilizing carbohydrates made by the bacteria, which get their energy from the oxidation of hydrogen sulfide. (The worms excrete the sulfur from the hydrogen sulfide compound.) The process by which the bacteria produce energy-rich compounds from inorganic chemicals is called chemosynthesis. Arrow Worms At the other end of the water column, near the surface of the ocean, lives the tiny, transparent arrow worm (Sagitta), shown in Figure The arrow worm is classified in the phylum Chaetognatha (meaning bristlejaw ). Arrow worms are just a few centimeters long. They have tiny fins that enable swimming, but they mostly drift as part of the plankton community. An active hunter, the arrow worm uses its mouth bristles, which are modified as hooks, to prey on other animal plankton such as copepods, fish eggs, and fish larvae. Arrow worms have a one-way digestive tract. Food is digested in the worm s narrow intestine, and undigested wastes are eliminated through the anus. Like the flatworms and roundworms, the arrow worm has no circulatory or respiratory systems. Digested nutrients diffuse into its cells. Respiratory gases and cellular wastes are exchanged between the arrow worm and the outer environment through the worm s thin skin. Arrow worms have a simple nervous system that lets them respond to stimuli. Two eyes in the head region are sensitive to light. Sensory projections (called papillae) 204 Marine Invertebrates

13 Hooks (mouth bristles) Eyes Sensory papillae Ventral nerve ganglion Intestine Anterior lateral fin Eggs in ovary Anus Posterior lateral fin Testis Tail fin Anterior teeth Posterior teeth Head Trunk located along the surface of the worm s body are sensitive to touch. Ganglia are located in the head and trunk regions. Arrow worms are hermaphrodites, and reproduction is sexual. Each worm produces both sperm and eggs, but self-fertilization does not occur. The sperm and eggs are shed directly into the water, where the sperm from one arrow worm fertilizes the eggs of another. Tail Figure 8-10 The arrow worm. 8.3 SECTION REVIEW 1. How are giant tube worms able to feed at such great depths? 2. In what part of the water column are arrow worms found? What do they eat? 3. How are the arrow worms means of locomotion and feeding adaptive in their particular habitat? Marine Worms 205

14 Laboratory Investigation 8 Adaptations of the Sandworm PROBLEM: How is the sandworm adapted for carrying out life functions? SKILL: Observing features of the sandworm by means of dissection. MATERIALS: Preserved sandworm, dissecting pan, water, hand lens, forceps, scissors, pins. PROCEDURE 1. Place a sandworm in a dissecting pan and cover it with water so that you can see the tissues more clearly. Notice that the body is divided into segments. Count the number of segments and record the number. 2. Which end of the sandworm is the front, or anterior, end? Use your hand lens to locate the mouth, which is located in the first two segments at the anterior end. With the forceps, gently squeeze the area. Two sharp hooks (jaws) should stick out from the mouth, showing that this is the worm s front. 3. Which side of the sandworm is the upper surface and which is the lower? Notice that the lower surface is lighter in color than the upper surface. The lower surface is the ventral side, and the upper surface is the dorsal side. 4. Notice the many tiny fiberlike appendages projecting from each segment. Examine one of the appendages with your hand lens. The appendage, called a parapodium, has a fleshy paddle shape. The parapodia contain bristles called setae. These appendages are used for swimming and for burrowing. 5. Sandworms breathe through the thin, moist skin of the parapodia. Use your hand lens to examine the parapodia. 6. Now place the sandworm in a dry dissecting pan with its dorsal surface up. Pin the sandworm to the tray at the worm s anterior and posterior ends. Beginning at the posterior end, cut through the skin with the scissors. Make the cut just slightly to the right of the midline (center of worm, lengthwise). Carefully cut the skin without cutting the underlying tissues. As you cut, pin the skin on both sides to the tray. Cut all the way up to the anterior end. 206 Marine Invertebrates

15 Pharynx Crop Intestine Dorsal blood vessel Mouth Esophagus Gizzard Figure 8-11 The internal anatomy of a sandworm. Hearts Ventral blood vessel 7. To see the internal structures more clearly, pour just enough water over the sandworm to cover it. (See Figure 8-11.) Look at the inside body wall. The wall is composed of longitudinal and circular muscles for movement. 8. Use your hand lens to locate the brain, or cerebral ganglia, at the anterior end near the dorsal surface. The brain is a white structure with two lobes. Locate the connection of the brain to a double ventral nerve cord. Trace the ventral cord and notice that it extends the entire length of the sandworm. Locate the tiny lateral nerve branches going to the muscles in each segment. The ventral nerve cord relays messages to and from the brain. 9. Food enters the worm s mouth; then it is digested in a food tube, the digestive tract. The mouth is connected to a wider part of the digestive tract called the pharynx. 10. The pharynx connects with a narrow esophagus, located in segments 6 through 14. Food moves from the esophagus into the crop. The crop, in segments 15 and 16, temporarily stores the food. Then the food passes into the gizzard, where it is ground up with sand that is ingested with it. 11. Next, the food passes into the intestine, where it is further digested and then absorbed into the blood. Finally, solid wastes are eliminated through the anus, located in the last two segments. Note that the digestive tract is separated from the skin by the fluid-filled space called the coelom. 12. Nutrients and oxygen are transported in the blood. Use your hand lens to locate the aortic arches ( hearts ) and the dorsal blood vessel (on top of the food tube), which connects with the arteries, veins, and capillaries. 13. Locate the ventral blood vessel under the digestive tract. The smaller blood vessels connect the dorsal and ventral blood vessels. The blood vessels are the circulatory system of the sandworm. Blood flows only inside this network of blood vessels; thus the sandworm has a closed circulatory system. Marine Worms 207

16 14. Wastes are removed by paired, coiled tubes called nephridia, located in most segments. Use your hand lens to look for tiny white tubes attached to the inside body wall. (You may have to push the digestive tract aside.) 15. Two sandworms are required for sexual reproduction to occur; the sexes are separate. With your hand lens, locate a pair of testes in segments 10 and 11, or the ovaries in segments 12 and 13. Both fertilization and development are external; the larvae are planktonic. OBSERVATIONS AND ANALYSES 1. What adaptations does the sandworm have for breathing? 2. Describe the structures of the sandworm that enable locomotion. 3. How is the sandworm adapted for carrying out ingestion and digestion? 208 Marine Invertebrates

17 Chapter 8 Review Answer the following questions on a separate sheet of paper. Vocabulary The following list contains all the boldface terms in this chapter. annelids, arrow worm, bilateral symmetry, bloodworm, coelom, flatworms, ganglia, giant tube worms, hirudin, host, leech, nematodes, nephridia, parapodia, parasite, planarian, proboscis, ribbon worm, roundworms, sandworm, segmented worms, setae, tapeworm, trematode, water column Fill In Use one of the vocabulary terms listed above to complete each sentence. 1. A worm s nerve cell clusters that act like a brain are the. 2. Sandworms wriggle through the sand by means of. 3. The segmented worms are also referred to as the. 4. A sharp organ used by ribbon worms to spear prey is the. 5. The chemical secreted by a leech to make its victim s blood flow is. Think and Write Use the information in this chapter to respond to these items. 6. Diagram and discuss the life cycle of the parasitic trematode. 7. Describe the structures that enable sandworms to move. 8. Why is chemosynthesis important for giant tube worms? Marine Worms 209

18 Inquiry Base your answers to questions 9 through 12 on the diagram below, which shows the anatomy of an arrow worm, and on your knowledge of marine biology. 9. Does the arrow worm have a oneway or a two-way digestive system? Explain your answer. 10. Which is an accurate statement regarding reproduction in the arrow worm? a. Arrow worms are separate males and females that reproduce sexually. b. Arrow worms are separate males and females that reproduce asexually. c. Arrow worms are hermaphrodites that reproduce sexually. d. Arrow worms are hermaphrodites that reproduce asexually. 11. Based on the diagram, you could conclude that arrow worms a. have no adaptations for foodgetting b. have a closed circulatory system c. have a simple nervous system d. are normally tube-dwelling worms. Anterior teeth Hooks (mouth bristles) Eyes Sensory papillae 12. What kind of locomotion is the arrow worm capable of? a. ameboid b. free-swimming c. deep-diving d. crawling Ventral nerve ganglion Intestine Anterior lateral fin Eggs in ovary Posterior lateral fin Anus Testis Tail fin Multiple Choice Choose the response that best completes the sentence or answers the question. 13. Two examples of parasitic flatworms are the a. tapeworm and leech b. tapeworm and fluke c. sandworm and tapeworm d. sandworm and leech. 210 Marine Invertebrates

19 14. The worms are not all classified into a single phylum because a. there are too many worms b. they live in such varied habitats c. they are too difficult to classify d. they are too varied in structure to be in one group. 15. Members of phylum Annelida are all characterized by a. having a thick skin b. the absence of appendages c. living in the same habitat d. having a segmented body. 16. What part of a worm s anatomy is shown in the diagram below? a. the anterior end b. the posterior end c. the dorsal side d. the ventral side Pharynx Crop Intestine Mouth Esophagus Gizzard 17. Segmented worms such as the sandworm have a fluid-filled space called the a. water column b. proboscis c. coelom d. nephridia. 18. Which of the following worms always lives as a parasite? a. trematode b. bloodworm c. sandworm d. ribbon worm 19. A worm that does not live inside a substrate is the a. arrow worm b. sandworm c. tube worm d. roundworm. 20. The largest free-living marine worm is the a. arrow worm b. giant tube worm c. ribbon worm d. sandworm. 21. The most numerous worm species in the sea are the a. tube worms b. roundworms c. bloodworms d. sandworms. 22. The pairs of coiled tubes that excrete wastes in annelids are the a. setae b. parapodia c. ganglia d. nephridia. 23. Another term used to refer to roundworms is a. platyhelminthes b. nematodes c. annelids d. tube worms. Marine Worms 211

20 24. What worms would you most likely see under the microscope in a sample of aquarium gravel? a. ribbon worms b. roundworms c. arrow worms d. sandworms 25. Which of the following marine worms depend on chemosynthesis by bacteria to obtain their food? a. roundworms b. flatworms c. sandworms d. giant tube worms Research/Activity Roundworms are commonly found in wet beach sand from the intertidal zone. If available, obtain a sample and prepare a wet mount slide. Examine the slide under a microscope s low power. Look for the typical whipping action by which the roundworm propels itself. Design an investigation to determine if changes in temperature, ph, or salinity affect its motion. 212 Marine Invertebrates