1. Explain the classification and the phylogeny of each phyla.

Platyhelminthes, Nematoda, and Rotifera are three phyla of invertebrates whose body plans are more complex than those of sponges and cnidarians. Scientists identify four types of body plans in invertebrates. The relationship among the germ layers differs in each body plan. Germ layers are layers of cells that originate in the developing embryo and become specific structures in the animal. Most animals have three germ layers: the ectoderm, the endoderm and the mesoderm. The mesoderm is the embryonic layer that forms between the ectoderm and the endoderm. In animals muscles, bones, and reproductive organs develop from the mesoderm.

The four body types are defined in part by the presence or absence of a coelom, or body cavity. Acoelomate, for example, means "without a coelom." The four body types, from the least complex to the most complex, are described below:

• The acoelomate body plan with two germ layers is characterized by an ectoderm and an endoderm that are not separated by a cavity. Sponges and cnidarians have this kind of body plan. It is considered the least complex body plan.

• The acoelomate body plan with three germ layers is characterized by an ectoderm, a mesoderm, and an endoderm that are not separated by a cavity. Flatworms are acoelomates with three germ layers.

• The pseudocoelomate body plan is characterized by a pseudocoelom, a cavity that forms between the mesoderm and the endoderm. Roundworms and rotifers are pseudocoelomates.

• The coelomate body plan is characterized by a cavity called a coelom, which develops within the mesoderm. Mollusks, annelids, arthropods, echinoderms, and all chordates, including humans, are coelomates. This is the most complex body plan.

The term worm does not refer to a specific taxon of animals. Though both are called worms, flatworms and roundworms differ so significantly in their fundamental structure that scientists classify them into separate phyla.

Members of the Phylum Platyhelminthes are called flatworms. Flatworms are bilaterally symmetrical organisms that lack respiratory and circulatory systems. In flatworms, as in most animals, the sense receptors and nerves are concentrated at their anterior end, a characteristic called cephalization. The adaptive advantage of cephalization is that an organism that enters an environment with its sense organs leading the way will have an advantage over those that do not.

The more then 13,000 species of flatworms belong to three classes: Turbellaria, Trematoda, and Cestoda. Only turbellarians are free-living. Trematodes and cestodes live as parasites on or inside other animals. The parasite is an organism that lives on or in another organism, called the host, for a long period of time. The parasite derives its nutrition and protection from the host. Yet a parasite can harm the host by depriving it of nutrition and by interfering with its internal organ systems. The majority of parasites do not kill their hosts, an adaptation that helps to ensure that the hosts will not become extinct.

Scientists believe that parasites originated as free-living organisms. During evolution some organs that were advantageous to free living became modified for parasitism; some were lost entirely. Because their food is already digested , some parasites lack mouths or well developed digestive systems. Parasites have hooks or suckers that grip their host. Features that prevent parasites from being digested by their hosts are the tegument, a thick covering of cells, and the cuticle, a nonliving layer secreted by the epidermis.

Colorful feather-duster worms, common earthworms, and blood-sucking leeches are all classified in the Phylum Annelida. The name annelid means "little rings," which refers to the many segments that make up the body of an annelid. The diverse species of annelids share certain basic traits:

• All annelids have a true coelom

• The body is divided into many segments. This phenomenon is called metamerism. Some segments fuse during development, while others remain separate segments, or metameric units.

• All organ systems are well developed in most groups.

• Most annelids have external bristles called setae.

The number of setae and the presence or absence of appendages called parapodia provide the basis for classifying annelids into three classes: Polychaeta, Oligochaeta, and Hirudinea. Marine worms of class polychaeta have many setae; earthworms of class oligochaeta have no parapodia and few setae; and leeches of class hirudinea have no setae or parapodia.

Kingdom - Animalia

Phylum Platyhelminthes: Body flaty and ribbonlike, without true segments; acoelomates; no skeletal, circulatory, or respiratory systems; head provided with two sense organs; nervous system of two longitudinal nerve cords: flatworms

Class Turbellaria: Mostly free-living aquatic or terrestial forms; many with cilia on the epidermis: planarians

Class Trematoda: Parasitic worms with mouth at anterior end; intestine present; no cilia on adults: human blood flukes, sheep liver fluke.

Class Cestoda: Parasitic worms; body a series of proglottids; intestine lacking; hooked scolex: tapeworms.

Phylum Rotifera: Crown of cilia on anterior end; pseudocoelomates; chitinlike jaws and well-developed digestive system; body usually cylindrical: rotifers.

Phylum Nematoda: Body slender and elongated; pseudocoelomates; unsegmented body wall in three layers; body cavity present; bilaterally symmetrical; free-living and parasitic worms: roundworms.

Phylum Annelida: Segmented coelomate worms; all organ systems present; bristles or setae on most; spiral cleavage in early developmental stages: segmented worms.

Class Polychaeta: Fleshy outgrowths, or parapodia, extending from segments; marine; many bristles: sandworm.

Class Oligochaeta: Head not well developed; few bristles; terrestrial and freshwater forms: earthworm.

Class Hirudinea: Body flattened from top to bottom; no bristles on body; suckers at both ends; mostly freshwater forms, but may occur as terrestrial or marine organisms: leeches.

2. Describe the characteristics typical for each phyla.

The majority of the 3000 species of the class Turbellaria live in marine environments. However, the most familiar turbellarian is the freshwater planarian Dugesia.

Dugesia’s posterior end is tapered, and its anterior end is shaped like a spade. As an adaptation to its free-living way of life, much of the planarians body is covered with cilia. A planarian moves by swimming with an undulating motion or by laying a layer of mucus over which it slides, propelled by the beating of its cilia.

The Planarian is a scavenger; it forages for carrion. It also preys on smaller organisms, such as protozoa. Its mouth is located in the middle of its body. A planarian feeds by extending a muscular tube, the pharynx, out of its mouth. Food is sucked in through the pharynx and passes into the branched intestine. Nutrients are absorbed through the intestinal wall, and undigested food is excreted through the pharynx and mouth.

Chemical wastes and excess water are eliminated through a network of ducts that run the length of the body. Each duct contains many flame cells. A flame cell is a cell that encloses a tuft of cilia that beats, resembling a candle flame. The beating of the cilia moves wastes into the duct to the excretory pores, where the wastes leave the body.

The planarian nervous system and sense organs are also adapted for free living. It is more organized and specialized than the nerve net of the hydra. Planarians sense the intensity and direction of light with two anterior eyespots that contain photosensitive cells. Receptors for touch, taste and smell are concentrated at the anterior end. Two clusters of nerve cells, called ganglia, form a simple brain. The ganglia receive information from sensory cells and then send impulses to the rest of the body through a ladderlike system of two longitudinal nerves connected by transverse nerves.

Planarians show a surprising ability to learn. Their memory, such as how to find a way through a maze, is stored chemically. One planarian can learn how to pass through a maze by eating another planarian that already knows the way.

Planarians reproduce sexually and asexually. They are hermaphrodites - single individuals possessing both male and female sex organs. Because they are free-living, they can encounter mates. In sexual reproduction two planarians simultaneously fertilize each other. Eggs, laid in protective capsules that stick to rocks or debris, hatch in two to three weeks. Freshwater planarians reproduce asexually in the summer. They attach their posterior ends to a surface and stretch until they tear in two. Each half then regenerates its missing part.

The class Trematoda consists of about 6000 species of parasitic flukes, most of which are shaped like leaves. Some flukes are endoparasites, meaning that they live inside their host. Others are ectoparasites, meaning that they live on external surfaces of their hosts.

Flukes are adapted to parasitism. they are about 1 cm long, oval shaped, and sheathed in tough, unciliated tegument. Typically a fluke has two sucker mouths - one on its anterior and one on its ventral surface - that it uses to cling to its host. A powerful pharynx in the anterior sucker sucks in blood, cells, and fluids.

While the nervous and excretory systems of the fluke are similar to those of planarians, its life cycle and reproductive structures are more complex. Typically hermaphroditic, the fluke has a long coiled tube, or uterus, that stores eggs until release through a genital pore. A fluke may produce tens of thousands of eggs at a time. A few survive predation and environmental destruction and hatch into larvae, or immature organisms.

The complex life cycle of the economically destructive sheep liver fluke, is very complex. Adult flukes live in the sheep’s liver and gallbladder. Flukes mate in these organs and produce eggs. The eggs enter the intestines and are eliminated with the feces of the sheep. The eggs hatch in water, and the larvae then invade snails. Inside snails, the larvae multiply asexually. The larvae leave the snail and form cysts on blades of grass. A cyst is a dormant larva surrounded by a hard protective covering. When sheep eat the grass, they ingest the cysts, which hatch in the sheep's digestive tract. The flukes then bore through the intestines into the blood and mature and reproduce in the liver, completing the cycle.

One of the world’s most severe public health problems is caused by three species of blood fluke in the genus Schistosoma. The blood fluke infests 200 to 300 million people, mostly in Asia, Africa, and South America. The resulting disease, called schistosomiasis effects 20 to 30 times more people then cancer does. The life cycle of a schistosome parallels that of other flukes, with the exception that the adult lives in the human bloodstream. The spiny eggs of the schistosomes lodge in the human veins, lungs, intestines, bladder, and liver. They block blood vessels and cause internal bleeding and tissue decay that can be fetal.

The class cestoda consists of about 1500 species. Most are parasitic tapeworms. All vertebrates may host tapeworms. Humans, for example, may harbor any of seven different species.

Like flukes, tapeworms are adapted for the parasitic life. They have tough outer teguments and hooks and suckers to attach themselves to their hosts. Their long ribbonlike bodies may grow up to 12 m. The nervous system extends the length of the body.

Tapeworms lack sense organs, mouths, and digestive tracts. They absorb nutrients directly through their heavily folded teguments. The folds increase the surface area for absorption. The tapeworm grips its host with hooks and suckers on its knobshaped head, or scolex. It grows by producing body sections, called proglottids, immediately behind the scolex. Thus the oldest proglottids are at the posterior end. The excretory system drains the proglottid of wastes.

Each proglottid contains both male and female reproductive organs. Cross-fertilization between two adjacent worms is typical, but self-fertilization between proglottids can also occur. After fertilization the egg-packed proglottid breaks off from the adult and is eliminated with the host’s feces.

The life cycle of the beef tapeworm, begins when cattle eat grass contaminated with proglottids and eggs. Larvae hatch and bore through the cow’s intestine into the bloodstream. The larvae then burrow into muscle tissue and form cysts. Humans become infected when they eat beef that has not been cooked sufficiently to kill the bladder worms inside the cysts. Once the cysts are inside the human intestine, the cyst wall dissolves and releases the bladder worm, which then develops into an adult beef tapeworm.

We will now look closely at the earthworm’s structure, feeding habits, and body systems.

Segmentation is the most distinctive feature of annelids. The more than 100 segments, or metameric units, of the earthworm’s body are separated by partions that divide the coelom. These segments are identical except where fusion has resulted in specialized sections near the anterior and posterior ends. The anterior segments of worm reflect the cephalization that is an adaptation for burrowing. The head contains the specialized sense organs.

Circular and longitudinal muscles line the interior body wall. To move, the worm anchors some segments by their setae and contracts the circular muscles in front of those segments, producing fluid pressure in the anterior coelom cavities. This pressure elongates the animal and pushes the head forward. With the anterior setae gripping the ground, the longitudinal muscles contract, pulling the posterior along. Divisions in the coelom make simultaneous cantraction and expansion of segments possible.

Earthworms burrow and feed on soil and organic matter at the same time. They digest the organic matter and eliminate waste and undigested matter as dirt and feces called castings. The passing of dirt through the worms body loosens and aerates the soil.

Soil is sucked into the gut by the muscular pharynx. The soil then passes through a tubelike esophagus to a temporary storage area called a crop, and from there to the gizzard. The thick, muscular gizzard walls contract and grind the soil, releasing and breaking up organic matter. The gizzard is adapted for burrowing, as it grinds up soil. Food is digested and absorbed by the blood after it passes through the walls of the tubular intestines.

Earthworms transport oxygen, nutrients, and wastes through the body via a closed circulatory system. The blood travels from the anterior to the posterior through a ventral blood vessel and then forward through a dorsal vessel. Five pairs of tubes, the aortic arches, link the major vessels near the anterior. Smaller vessels branch into each segment of the body. Contractions of the ventral vessel and aortic arches force blood through the body.

Earthworms have no gills or other respiratory organs. Oxygen and carbon dioxide diffuse directly across the skin. This exchange can take place only if the skin is moist. Thus earthworms avoid dry ground and extreme heat. Secretions of mucus and the presence of a thin cuticle also help keep the earthworm’s skin moist.

Earthworms eliminate nitrogenous wastes through long tubules called nephridia. Coelomic fluid enters each nephridium through a ciliated funnel opening. As fluid passes through the nephridia, some water is reabsorbed by blood vessels. The remaining fluid empties through pores on the ventral surface.

Earthworms are sensitive to light, touch, moisture, chemicals, temperature, and vibrations. Light receptors concentrated in the head and tail sense direction. Most other sense organs and the nerves that control individual muscle contractions are present in each segment. The nerves in each segment form a pair of ganglia that coordinate movements with neighboring segments. A cerebral ganglion or brain in the head controls total body actions. A ventral nerve cord connects the brain with all the ganglia.

Earthworms are hermaphrodites, but an individual worm cannot fertilize its own eggs. Mating occurs when two earthworms join head to tail. Together they form a mucus coat around part of their intertwined bodies. Each injects sperm into the mucus. Sperm from one worm moves to the pouchlike seminal receptacle of the other. Simultaneously eggs in the body cavity move through oviducts to the female genital pore. After several days a mucus and chitin sheath is secreted by the clitellum, a swelling around the sex organs. As the worm wriggles to slip the sheath off its body, eggs and sperm are joined and fertilization occurs.

Polychaetes are annelids that live in virtually all marine habitats. Some are free-swimming predators that use their strong jaws to feed on small animals. Some burrowing species excavate tunnels by eating the sediment, while others dig and pump water through their bodies or scour the bottom with tentacles.

Polychaetes have numerous setae that help them move. The setae project from the parapodia. Some parapodia function in gas exchange. Polychaetes differ from other annelids by having antennae and specialized mouthparts.

Hirudinea is the smallest class of annelids, consisting of about 300 species of leeches. A leech has no setae. At each end of the body the leech has a sucker, which it uses to effect a crude walking motion. Most leeches live in calm bodies of fresh water.

3. Describe the life cycles typical of organisms in this phylum.

Flukes are adapted to parasitism. they are about 1 cm long, oval shaped, and sheathed in tough, unciliated tegument. Typically a fluke has two sucker mouths - one on its anterior and one on its ventral surface - that it uses to cling to its host. A powerful pharynx in the anterior sucker sucks in blood, cells, and fluids.

While the nervous and excretory systems of the fluke are similar to those of planarians, its life cycle and reproductive structures are more complex. Typically hermaphroditic, the fluke has a long coiled tube, or uterus, that stores eggs until release through a genital pore. A fluke may produce tens of thousands of eggs at a time. A few survive predation and environmental destruction and hatch into larvae, or immature organisms.

The complex life cycle of the economically destructive sheep liver fluke, is very complex. Adult flukes live in the sheep’s liver and gallbladder. Flukes mate in these organs and produce eggs. The eggs enter the intestines and are eliminated with the feces of the sheep. The eggs hatch in water, and the larvae then invade snails. Inside snails, the larvae multiply asexually. The larvae leave the snail and form cysts on blades of grass. A cyst is a dormant larva surrounded by a hard protective covering. When sheep eat the grass, they ingest the cysts, which hatch in the sheep's digestive tract. The flukes then bore through the intestines into the blood and mature and reproduce in the liver, completing the cycle.

Each proglottid contains both male and female reproductive organs. Cross-fertilization between two adjacent worms is typical, but self-fertilization between proglottids can also occur. After fertilization the egg-packed proglottid breaks off from the adult and is eliminated with the host’s feces.

The life cycle of the beef tapeworm, begins when cattle eat grass contaminated with proglottids and eggs. Larvae hatch and bore through the cow’s intestine into the bloodstream. The larvae then burrow into muscle tissue and form cysts. Humans become infected when they eat beef that has not been cooked sufficiently to kill the bladder worms inside the cysts. Once the cysts are inside the human intestine, the cyst wall dissolves and releases the bladder worm, which then develops into an adult beef tapeworm.

Earthworms burrow and feed on soil and organic matter at the same time. They digest the organic matter and eliminate waste and undigested matter as dirt and feces called castings. The passing of dirt through the worms body loosens and aerates the soil.

Soil is sucked into the gut by the muscular pharynx. The soil then passes through a tubelike esophagus to a temporary storage area called a crop, and from there to the gizzard. The thick, muscular gizzard walls contract and grind the soil, releasing and breaking up organic matter. The gizzard is adapted for burrowing, as it grinds up soil. Food is digested and absorbed by the blood after it passes through the walls of the tubular intestines.

Earthworms transport oxygen, nutrients, and wastes through the body via a closed circulatory system. The blood travels from the anterior to the posterior through a ventral blood vessel and then forward through a dorsal vessel. Five pairs of tubes, the aortic arches, link the major vessels near the anterior. Smaller vessels branch into each segment of the body. Contractions of the ventral vessel and aortic arches force blood through the body.

Earthworms have no gills or other respiratory organs. Oxygen and carbon dioxide diffuse directly across the skin. This exchange can take place only if the skin is moist. Thus earthworms avoid dry ground and extreme heat. Secretions of mucus and the presence of a thin cuticle also help keep the earthworm’s skin moist.

Earthworms eliminate nitrogenous wastes through long tubules called nephridia. Coelomic fluid enters each nephridium through a ciliated funnel opening. As fluid passes through the nephridia, some water is reabsorbed by blood vessels. The remaining fluid empties through pores on the ventral surface.

Earthworms are sensitive to light, touch, moisture, chemicals, temperature, and vibrations. Light receptors concentrated in the head and tail sense direction. Most other sense organs and the nerves that control individual muscle contractions are present in each segment. The nerves in each segment form a pair of ganglia that coordinate movements with neighboring segments. A cerebral ganglion or brain in the head controls total body actions. A ventral nerve cord connects the brain with all the ganglia.

Earthworms are hermaphrodites, but an individual worm cannot fertilize its own eggs. Mating occurs when two earthworms join head to tail. Together they form a mucus coat around part of their intertwined bodies. Each injects sperm into the mucus. Sperm from one worm moves to the pouchlike seminal receptacle of the other. Simultaneously eggs in the body cavity move through oviducts to the female genital pore. After several days a mucus and chitin sheath is secreted by the clitellum, a swelling around the sex organs. As the worm wriggles to slip the sheath off its body, eggs and sperm are joined and fertilization occurs.

Liverfluke - the liver fluke matures in a humans liver ; its eggs leave the human with feces; the feces enters water where the larvae hatch and enter snails. There larvae multiple asexually then they enter fish where they form cysts. When human eats the fish the fluke travels with it to the intestines where it burrows through the intestinal wall and enters the blood stream from which it enters the liver where it matures and reproduces.

Bloodfluke - as a human walks the ceracaria burrows into his foot; there it enters his blood and after reproducing its eggs come out with the feces; eggs come in contact with water and hatch; then they enter a snail and develop into a ceracaria

Ascaris - enters humans through contaminated food or water in form of eggs; After hatching in intestines larvae bore into bloodstream and are carried into lungs; they crawl up throat and are swallowed down the intestines where the mature and mate and eggs are carried out with the feces

Swimmers itch - Cerkeria a parasite from other species such as birds enters skin, however since it is a parasite for different organisms it is not adapted for human immune system which kills it causing itch.

Hookworm - enters humans through burrowing through foot; After hatching in intestines larvae bore into bloodstream and are carried into lungs; they crawl up throat and are swallowed down the intestines where the mature and mate and eggs are carried out with the feces and hatch out on warm soil

Trichina worm - Human eats undercooked pork that is contaminated with cysts; cysts release larvae which burrow into the wall of small intestine, where they mature into adults; the adults produce larvae that pass into blood and form cysts in the muscles.

Pin worm - through persons anus the worm enters the large intestine usually transmitted from a dog or a cat; there they feed off organic material and at night they crawl out the large intestine and lay eggs in the anus; eggs are then transmitted into someone else.

Filarial worms - mosquito picks up eggs; eggs hatch into mosquito and larvae develop; mosquito bites the human and larvae enter the human; the larvae block the body’s lymphatic systems(elephantiasis), they reproduce and eggs leave the body through feces.

Guinea worms - human drinks water with christatian infected with worm and becomes infected, the worms burrow into the skin exposing meat in your hand where they develop a cyst which is then secreted into water, there the cysts enter the chrystatian.

4. List all the animals which are used as examples in class, films, text, etc...

Phylum Platyhelminthes. Flatworms

Class Turbellaria. Free-living flatworms. Planaria, Dugesia, Leptoplana.

Class Trematoda. Flukes. Fasciola, Schistosoma, Prosthogonimus

Class Cestoda. Tapeworms. Taenia, Dipylidium, Mesocestoides

Phylum Nematoda. Roundworms or nematodes. Ascaris, Trichinella, Necator, Enterobius, Ancylostoma, heterodera

Phylum Annelida. Segmented worms

Class Polychaeta. Sandworms, tubeworms, etc.. Nereis, Chaetopterus, Aphrodite, Diopatra, Arenicola, Hydroides, Sabella

Class Oligochaeta. Earthworms and many freshwater annelids. Tubifex, Enchytraeus, Lumbricus, Dendrobaena

Class Hirudinoidea . Leeches. Trachelobdella, Hirudo, Macrobdella, Haemadipsa

5. Make a vocabulary list and briefly define them. Use all words printed in bold print.

1. Coelom - body cavity

2. Acoelomate - "without a coelom", without body cavity

3. Pseudocoelom - a cavity that forms between the mesoderm and the endoderm

4. Parasite - an organism that lives on or in another organism

5. Host - the organism that hosts the parasite

6. Tegument - a thick covering of cells preventing parasites from being digested

7. Cuticle - a nonliving layer secreted by the epidermis preventing parasites from being digested

8. Pharynx - a muscular tube extended out of planarians mouth

9. Flame cell - cell that encloses a tuft of cilia that beats, resembling a candle flame.

10. Uterus - long coiled tube that stores eggs until ready for release through the genital pore in a fluke

11. schistosomiasis - a disease caused by the blood fluke schstosoma

12. Scolex - a tapeworms knobshaped head

13. Proglottids - tapewroms body sections

14. Anus - the opening through which wastes are eliminated

15. Metamerism - the phenomenon of body divided into segments. (Annelida)

16. Setae - external bristles in annelids

17. Parapodia - the number of setae and the presence or absence of fleshy appendages

18. Esophagus - a tube in worm that leads to the crop

19. Crop - temporary storage area

20. Gizzard - an organ whose walls contract and grind soil, releasing and breaking up organic matter.

21. Aortic arches - five pairs of tubes that link the major vessels near the anterior.

22. Nephridia - tubules in earthworms which eliminate nitrogenous wastes

23. Seminal receptacle - the puchlike receptacle which accepts other earthworms sperm during mating.

24. Clitellum - an organ that secretes mucus and chitin sheath around the eggs and sperm after several days after earthworms have mated.

6. List the special adaptations which make this phylum successful on earth.

Members of the phyla Nematoda and Rotifera are characterized by the presence of a pseudocoelom. This body cavity is lined on the inside by endoderm and on the outside by mesoderm. The fluid-filled space contains organs, supports the body, provides the hydrostatic pressure against which muscles can contract, and serves as a storage area for wastes or for eggs and sperm.

Roundworms are classified in the phylum Nematoda. The number of nematode species is estimated at 10,000 to 80,000. Roundworms have long slender bodies that taper at both ends. Most are covered with a flexible protective cuticle. They range in length from less then 1 mm to as long as the female guinea worm, which can be 120 cm. Roundworms have a digestive tract with two openings, an anterior mouth and a posterior anus, the opening through which wastes are eliminated. The sexes are distinct in most species.

The vast majority of roundworm species are free-living on land, in salt water, and in fresh water. About 50 species are plant and animal parasites that cause enormous economic damage and physical suffering throughout the world. Humans are host to about 50 different roundworm species. All of these roundworms have adaptations for parasitism.

Most of the 1,750 species of the phylum Rotifera are transparent, free-swimming, microscopic animals that live in marine or fresh water. These microscopic rotifers are between 100 and 500 µm long, though the male is much smaller then the female. The crown of cilia that surrounds the rotifers mouth is an adaptation for free-living. the cilia sweeps food into the mouth. Under a microscope this beating of the cilia looks like rotating wheels. Rotifers feed on unicellular algae, bacteria and protozoa. This food moves through the pharynx to the mastax, a muscular organ that chops up the food. The rotifer has no skeleton. It maintains its shape by hydrostatic pressure within the pseudocoelom. The nervous system is composed of anterior ganglia and two long nerves that run the length of the body. Cephalization is indicated by the two anterior eyespots.

Colorful feather-duster worms, common earthworms, and blood-sucking leeches are all classified in the Phylum Annelida. The name annelid means "little rings," which refers to the many segments that make up the body of an annelid. The diverse species of annelids share certain basic traits:

• All annelids have a true coelom

• The body is divided into many segments. This phenomenon is called metamerism. Some segments fuse during development, while others remain separate segments, or metameric units.

• All organ systems are well developed in most groups.

• Most annelids have external bristles called setae.

The number of setae and the presence or absence of appendages called parapodia provide the basis for classifying annelids into three classes: Polychaeta, Oligochaeta, and Hirudinea. Marine worms of class polychaeta have many setae; earthworms of class oligochaeta have no parapodia and few setae; and leeches of class hirudinea have no setae or parapodia.