C: Chapter 2: Mollusks, Worms, Arthropods, and Echinoderms

36 ◆ C

What do fiddler crabs run-ning along the beach havein common with pill bugs

and millipedes you might discoverunder a rock in your yard? How manymosquitoes have bitten you? Theseanimals and more than a millionother species belong to the largest,most diverse group of animals—thearthropods. In this chapter, you willlearn about arthropods, as well asmollusks, worms, and echinoderms.

What do you think?Science Journal Look at the picturebelow with a classmate. Discuss whatyou think this might be. Here’s a hint:It can be found in your house. Writeyour answer or best guess in your Science Journal.

22 Mollusks, Worms,Arthropods, Echinoderms

C ◆ 37

If you’ve ever walked along a beach, especially after a storm, you’ve probably seen many seashells.

They come in many different colors, shapes, and sizes.If you look closely, you will see that some shells havemany rings or bands. In the following activity, find

out what the bands tell you about the shell and the organism that made it.

Examine a clam’s shell1. Use a hand lens to examine

a clam’s shell.

2. Count the number of rings or bands on the shell. Count as number one the large, top point called the crown.

3. Compare the distances between the bands of the shell.

ObserveDo other students’ shells have the same number of bands? Are all of the bands on your shell the same width? What do you think the bands represent, and why are some wider than others? Record your answers in your Science Journal.

EXPLOREACTIVITY

Making an Organizational Study Fold Make the following Foldable to help you organize your thoughts into clear categoriesabout groups of invertebrates.

1. Place a sheet of paper in front of you so the long side is at the top. Foldthe paper in half from the left side to the right side and then unfold.

2. Fold each side in to the centerfold line to divide the paper into fourths.Fold the paper in half from top to bottom and unfold.

3. Through the top thickness of paper, cut along both of the middle foldlines to form four tabs as shown. Label the tabs Mollusks, Worms,Arthropods, and Echinoderms as shown.

4. Before you read the chapter, write what you know about each group ofinvertebrate under the tab. As you read the chapter, add to and correctwhat you have written.

FOLDABLESReading & StudySkills

FOLDABLESReading & Study Skills

Mollusks Worms

Arthro-pods

Echino-derms

Gill Anus Mantle cavity

Radula Mouth

HeartShell

StomachMantle Foot

38 ◆ C CHAPTER 2 Mollusks, Worms, Arthropods, Echinoderms

MollusksS E C T I O N

Characteristics of Mollusks Mollusks (MAH lusks) are soft-bodied invertebrates with

bilateral symmetry and usually one or two shells. Their organsare in a fluid-filled cavity. The word mollusk comes from theLatin word meaning “soft.” Most mollusks live in water, but somelive on land. Snails, clams, and squid are examples of mollusks.More than 110,000 species of mollusks have been identified.

Body Plan All mollusks, like the one in Figure 1, have a thinlayer of tissue called a mantle. The mantle covers the bodyorgans, which are located in the visceral (VIH suh rul) mass.Between the soft body and the mantle is a space called the man-tle cavity. It contains gills—the organs in which carbon dioxidefrom the mollusk is exchanged for oxygen in the water.

The mantle also secretes the shell or protects the body if themollusk does not have a shell. The shell is made up of severallayers. The inside layer is the smoothest. It is usually the thickestlayer because it’s added to throughout the life of the mollusk.The inside layer also protects the soft body.

The circulatory system of most mollusks is an open system.In an open circulatory system, the heart moves blood throughvessels and out into open spaces around the body organs. Thebody organs are surrounded completely by blood that containsnutrients and oxygen.

Most mollusks have a well-developed head with a mouthand some sensory organs. Somemollusks, such as squid, havetentacles. On the underside of amollusk is the muscular foot,which is used for movement.

■ Identify the characteristicsof mollusks.

■ Describe gastropods, bivalves,and cephalopods.

■ Explain the environmental importance of mollusks.

Vocabularymantlegillopen circulatory systemradulaclosed circulatory system

Mollusks are a food source for manyanimals. They also filter impuritiesfrom the water.

Figure 1The general mollusk body planis shown by this snail. Mostmollusks have a head, foot,and visceral mass.

Classification of Mollusks The first thing scientists look at when they classify mollusks

is whether or not the animal has a shell. Mollusks that haveshells are then classified by the kind of shell and kind of footthat they have. The three most common groups of mollusks aregastropods, bivalves, and cephalopods.

Gastropods The largest group of mollusks, the gastropods,includes snails, conchs like the one in Figure 2A, abalones,whelks, sea slugs, and garden slugs shown in Figure 2B. Conchsare sometimes called univalves. Except for slugs, which have noshell, gastropods have a single shell. Many have a pair of tenta-cles with eyes at the tips. Gastropods use a radula (RA juhluh)—a tonguelike organ with rows of teeth—to obtain food.The radula works like a file to scrape and tear food materials.That’s why snails are helpful to have in an aquarium—theyscrape the algae off the walls and keep the tank clean.

How do gastropods get food?

Slugs and many snails are adapted to life on land. They moveby rhythmic contractions of the muscular foot. Glands in thefoot secrete a layer of mucus on which they slide. Slugs andsnails are most active at night or on cloudy days when they canavoid the hot Sun. Slugs do not have shells but are protected bya layer of mucus instead, so they must live in moist places. Slugsand land snails damage plants as they eat leaves and stems.

SECTION 1 Mollusks C ◆ 39

Figure 2Conchs, sometimes called

marine snails, have a single shellcovering their internal organs.

Garden slugs are molluskswithout a shell.

Bivalves Mollusks that have a hinged,two-part shell joined by strong muscles arecalled bivalves. Clams, oysters, and scallops,as shown in Figure 3, are bivalve mollusksand are a familiar source of seafood. Theseanimals pull their shells closed by contract-ing powerful muscles near the hinge. Toopen their shells, they relax these muscles.

Bivalves are well adapted for living inwater. For protection, clams burrow deepinto the sand by contracting and relaxingtheir muscular foot. Mussels and oystersattach themselves with a strong thread or

cement to a solid surface. This keeps waves and currents fromwashing them away. Scallops escape predators by rapidly open-ing and closing their shells. As water is forced out, the scallopmoves rapidly in the opposite direction.

Cephalopods The most specialized and complex mollusksare the cephalopods (SE fuh luh pawdz). Squid, octopuses, cut-tlefish, and chambered nautiluses belong to this group. Theword cephalopod means “head-footed” and describes the bodystructure of these invertebrates. Cephalopods, like the cuttlefishin Figure 4, have a large, well-developed head. Their foot isdivided into many tentacles with strong suction cups or hooksfor capturing prey. All cephalopods are predators. They feed onfish, crustaceans, worms, and other mollusks.

Squid and octopuses have a well-developed nervous systemand large eyes similar to human eyes. Unlike other mollusks,cephalopods have closed circulatory systems. In a closed circu-latory system, blood containing food and oxygen movesthrough the body in a series of closed vessels, just as your bloodmoves through your blood vessels.

Figure 3Scallops force water betweentheir valves to move away fromsea stars and other predators.They can move up to 1 m witheach muscular contraction.

Figure 4Most cephalopods, like this cuttlefish, have an internal shell.

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Cephalopod Propulsion Allcephalopods live in oceans and areadapted for swimming. Squid and

other cephalopods have a water-filled cavity between an outermuscular covering and its internal organs. When the cephalo-pod tightens its muscular covering, water is forced out throughan opening near the head, as shown in Figure 5. The jet of waterpropels the cephalopod backwards, and it moves away quickly.According to Newton’s third law of motion, when one objectexerts a force on a second object, the second object exerts a forceon the first that is equal and opposite in direction. The move-ment of cephalopods is an example of this law. Muscles exertforce on water under the mantle. Water being forced out exerts aforce that results in movement backwards.

A squid can propel itself at speeds of more than 6 m/s usingthis jet propulsion and can briefly outdistance all but whales,dolphins, and the fastest fish. A squid even can jump out of thewater and reach heights of almost 5 m above the ocean’s surface.It then can travel through the air as far as 15 m. However, squidcan maintain their top speed for just a few pulses. Octopusesalso can swim by jet propulsion, but they usually use their tenta-cles to creep more slowly over the ocean floor.

Origin of Mollusks Some species of mollusks, such as thechambered nautilus, have changed little from their ancestors.Mollusk fossils date back more than 500 million years. Manyspecies of mollusks became extinct about 66 million years ago.Today’s mollusks are descendants of ancient mollusks.

SECTION 1 Mollusks C ◆ 41

Figure 5Squid and other cephalopodsuse jet propulsion to movequickly away from predators.

By about 245 million yearsago, many mollusks hadbecome extinct. What werethe major physical eventsof the time that could havecontributed to changingthe environment? Writeyour answers in your Sci-ence Journal.

Water in

Water out

Directionof squid

Value of MollusksMollusks have many uses. They are food for

fish, sea stars, birds, and humans. Many peoplemake their living raising or collecting mollusks tosell for food. Other invertebrates, such as hermitcrabs, use empty mollusk shells as shelter. Manymollusk shells are used for jewelry and decoration.Pearls are produced by several species of mollusks,but most are made by mollusks called pearl oysters,shown in Figure 6. Mollusk shells also provideinformation about the conditions in an ecosystem,including the source and distribution of waterpollutants. The internal shell of a cuttlefish is the

cuttlebone, which is used in birdcages to provide birds with cal-cium. Squid and octopuses are able to learn tasks, so scientistsare studying their nervous systems to understand how learningtakes place and how memory works.

Even though mollusks are beneficial in many ways, they alsocan cause problems for humans. Land slugs and snails damageplants. Certain species of snails are hosts of parasites that infecthumans. Shipworms, a type of bivalve, make holes in submergedwood of docks and boats, causing millions of dollars in damageeach year. Because clams, oysters, and other mollusks are filterfeeders, bacteria, viruses, and toxic protists from the water canbecome trapped in the animals. Eating these infected molluskscan result in sickness or even death.

Section Assessment

1. What are the characteristics used to classifymollusks?

2. Name the three groups of mollusks. Iden-tify a mollusk from each group, and explainwhy it is in that group.

3. Explain how a squid and other cephalopodscan move so rapidly.

4. Describe some positive and negative waysthat mollusks affect humans.

5. Think Critically Why is it unlikely thatyou would find garden slugs and landsnails in a desert?

6. Interpreting Scientific Illustrations Observethe images of gastropods and bivalves in thissection. Infer how bivalves are not adapted tolife on land and gastropods are. For more help,refer to the Science Skill Handbook.

7. Using a Word Processor Make a data tablethat compares and contrasts the following forgastropods, bivalves, and cephalopods: methodsof obtaining food, movement, circulation, andhabitat. For more help, refer to the TechnologySkill Handbook.


Figure 6A pearl starts as an irritant—agrain of sand or a parasite—toan oyster. The oyster coats theirritant with a material thatforms smooth, hard layers.It can take years for a pearl to form. Culturing pearls is acommercial industry in somecountries.

SECTION 2 Segmented Worms C ◆ 43

Segmented Worm CharacteristicsThe worms you see crawling across sidewalks after a rain and

those used for fishing are called annelids (A nul udz). The wordannelid means “little rings” and describes the bodies of theseworms. They have tube-shaped bodies that are divided intomany segments.

Have you ever watched a robin try to pull an earthworm outof the ground or tried it yourself? Why don’t they slip out of thesoil easily? On the outside of each body segment are bristlelikestructures called setae (SEE tee). Segmented worms use theirsetae to hold on to the soil and to move. Segmented worms alsohave bilateral symmetry, a body cavity that holds the organs,and two body openings—a mouth and an anus. Annelids can befound in freshwater, salt water, and moist soil. Earthworms, likethe one in Figure 7, marine worms, and leeches are examples ofannelids.

What is the function of setae?

Earthworm Body SystemsThe most well-known annelids are earthworms. They have a

definite anterior, or front end, and a posterior, or back end.Earthworms have more than 100 body segments. The segmentscan be seen on the outside and the inside ofthe body cavity. Each body segment, exceptfor the first and last segments, has fourpairs of setae. Earthworms move by usingtheir setae and two sets of muscles in thebody wall. One set of muscles runs thelength of the body, and the other set circlesthe body. When an earthworm contracts itslong muscles, it causes some of the seg-ments to bunch up and the setae to stickout. This anchors the worm to the soil.When the circular muscles contract, thesetae are pulled in and the worm can moveforward.

■ Identify the characteristics of segmented worms.

■ Describe the structures of anearthworm and how it takes inand digests food.

■ Explain the importance of segmented worms.

Vocabularysetaecropgizzard

Earthworms condition and aerate thesoil, which helps increase crop yields.

Segmented WormsS E C T I O N

Figure 7One species of earthwormthat lives in Australia cangrow to be 3.3 m long.

Digestion and Excretion As an earthworm bur-rows through the soil, it takes soil into its mouth. Earth-worms get energy from the bits of leaves and otherorganic matter found in the soil. The soil ingested by anearthworm moves to the crop, which is a sac used forstorage. Behind the crop is a muscular structure calledthe gizzard, which grinds the soil and the bits oforganic matter. This ground material passes to the intes-tine, where the organic matter is broken down and thenutrients are absorbed by the blood. Wastes leave theworm through the anus. When earthworms take in soil,they provide spaces for air and water to flow through itand mix the soil. Their wastes pile up at the openings totheir burrows. These piles are called castings. Castings,like those in Figure 8, help fertilize the soil.

Circulation and Respiration Earthworms have a closedcirculatory system, as shown in Figure 9. Two blood vesselsalong the top of the body and one along the bottom of the bodymeet in the front end of the earthworm. There, they connect toheartlike structures called aortic arches, which pump bloodthrough the body. Smaller vessels go into each body segment.

Earthworms don’t have gills or lungs. Oxygen and carbondioxide are exchanged through their skin, which is covered witha thin film of watery mucus. It’s important never to touch earth-worms with dry hands or remove their thin mucous layer,because they could suffocate. But as you can tell after a rain-storm, earthworms don’t survive in puddles of water either.


Nerve

Setae

Aortic arches

Mouth

Brain

EsophagusCrop

Gizzard

Intestine

Figure 9An earthworm has fiveaortic arches that pumpblood throughout its body.

Figure 8Earthworm castings—also calledvermicompost—are used as anorganic fertilizer in gardens.


Nerve Response and Reproduction Earthworms have asmall brain in their front segment. Nerves in each segment jointo form a main nerve cord that connects to the brain. Earth-worms respond to light, temperature, and moisture.

Earthworms are hermaphrodites (hur MAF ruh dites)—meaning they produce sperm and eggs in the same body. Eventhough each worm has male and female reproductive structures,an individual worm can’t fertilize its own eggs. Instead, it has toreceive sperm from another earthworm in order to reproduce.

Marine WormsMore than 8,000 species of marine worms, or polychaetes,

(PAH lee keets) exist, which is more than any other kind ofannelid. Marine worms float, burrow, build structures, or walkalong the ocean floor. Some polychaetes even produce their ownlight. Others, like the ice worms in Figure 10, are able to live 540 m deep. Polychaetes, like earthworms, have segments withsetae. However, the setae occur in bundles on these worms. Theword polychaete means “many bristles.”

Sessile, bottom-dwelling polychaetes, such as the marineworms shown in Figure 11A, have specialized tentacles that areused for exchanging oxygen and carbon dioxide and gatheringfood. Some marine worms build tubes around their bodies.When these worms are startled, they retreat into their tubes.Free-swimming polychaetes, such as the bristleworm shown inFigure 11B, have a head with eyes; a tail; and parapodia (pur uhPOH dee uh), which are paired, fleshy outgrowths on their seg-ments. The parapodia help in feeding and locomotion.

Figure 10Ice worms, a type of marine polychaete, were discovered firstin 1997 living 540 m deep in theGulf of Mexico.

Figure 11These Christmas tree worms

filter microorganisms from the water to eat. Some free-swimming polychaetesswim backwards and forwards,so some have eyes at both ends of their body.


Research Visit the Glencoe Science Web site atscience.glencoe.com forinformation about the uses of chemicals from leechsaliva.Communicate to yourclass what you learn.

Figure 12Medical leeches are usedsometimes to prevent bloodfrom clotting or accumulatingin damaged skin.

LeechesA favorite topic for scary movies is leeches. If you’ve ever had

to remove a leech from your body after swimming in a fresh-water pond, lake, or river, you know it isn’t fun. Leeches are seg-mented worms, but their bodies are not as round or as long asearthworms are, and they don’t have setae. They feed on theblood of other animals. A sucker at each end of a leech’s body isused to attach itself to an animal. If a leech attaches to you, youprobably won’t feel it. Leeches produce many chemicals, includ-ing an anesthetic (a nus THEH tik) that numbs the wound soyou don’t feel its bite. Why is producing an anesthetic an advan-tage to a leech? After the leech has attached itself, it cuts into theanimal and sucks out two to ten times its own weight in blood.Even though leeches prefer to eat blood, they can survive by eat-ing aquatic insects and other organisms instead.

Leeches and MedicineSometimes, leeches are used after surgery to keep blood

flowing to the repaired area, as shown in Figure 12. For exam-ple, the tiny blood vessels in the ear quickly can become blockedwith blood clots after surgery. To keep blood flowing in suchplaces, physicians might attach leeches to the surgical site. As theleeches feed on the blood, chemicals in their saliva prevent theblood from coagulating. Besides the anti-clotting chemical,leech saliva also contains a chemical that dilates blood vessels,which improves the blood flow and allows the wound to healmore quickly. These chemicals are being studied to treat patientswith heart or circulatory diseases, strokes, arthritis, or glaucoma.

Why are leeches sometimes used after surgery?

http://science.glencoe.com

Value of Segmented WormsDifferent kinds of segmented worms are helpful to

other animals in a variety of ways. Earthworms help aer-ate the soil by constantly burrowing through it. By grind-ing and partially digesting the large amount of plantmaterial in soil, earthworms speed up the return of nitro-gen and other nutrients to the soil for use by plants.

Researchers are developing drugs based on the chemi-cals that come from leeches because leech saliva preventsblood clots. Small marine worms and their larvae arepart of ocean plankton, which is food for many fish,invertebrates, and mammals. Bottom-dwelling marineworms are a food source for many fish.

Origin of Segmented WormsSome scientists hypothesize that segmented worms

evolved in the sea. The fossil record for segmented wormsis limited because of their soft bodies. The tubes of marineworms are the most common fossils of the segmentedworms. Some of these fossils date back about 620 million years.

Similarities between mollusks and segmented worms suggestthat they could have a common ancestor. These groups were thefirst animals to have a body cavity with space for body organs todevelop and function. Mollusks and segmented worms have aone-way digestive system with a separate mouth and anus. Theirlarvae, shown in Figure 13, are similar and are the best evidencethat they have a common ancestor.


Stomach

Mouth

Anus

Ring of cilia

Section Assessment

1. What is the most distinguishing character-istic of annelids?

2. How does an earthworm exchange oxygenand carbon dioxide with its environment?

3. Describe how an earthworm takes in anddigests its food.

4. Why would farmers promote the use ofearthworms in their fields?

5. Think Critically What advantages domarine worms with tubes have over free-moving polychaetes?

6. Comparing and Contrasting Compare howearthworms and marine worms obtain oxygen.For more help, refer to the Science Skill Handbook.

7. Using Proportions Suppose you find sixearthworms in a 10-cm3 volume of soil. Basedon this sample, calculate the number of earth-worms you would find in a 10-m3 volume of soil. For more help, refer to the Math SkillHandbook.

Figure 13Some mollusk larvae havemany structures that are similarto those of some annelid larvae.

Stomach

Mouth

Anus

Ring of cilia

ArthropodsS E C T I O N

Characteristics of ArthropodsThere are more than a million different species of arthro-

pods, (AR thruh pahdz) making them the largest group of ani-mals. The word arthropoda means “jointed foot.” The jointedappendages of arthropods can include legs, antennae, claws,and pincers. Arthropod appendages are adapted for movingabout, capturing prey, feeding, mating, and sensing their envi-ronment. Arthropods also have bilateral symmetry, segmentedbodies, an exoskeleton, a body cavity, a digestive system withtwo openings, and a nervous system. Most arthropod specieshave separate sexes and reproduce sexually. Arthropods areadapted to living in almost every environment. They vary in sizefrom microscopic dust mites to the large, Japanese spider crab,shown in Figure 14.

Segmented Bodies The bodies of arthropods are dividedinto segments similar to those of segmented worms. Somearthropods have many segments, but others have segments thatare fused together to form body regions, such as those of insects,spiders, and crabs.

Exoskeletons All arthropods have a hard, outer coveringcalled an exoskeleton. It covers, supports, and protects theinternal body and provides places for muscles to attach. In many

land-dwelling arthropods, for exampleinsects, the exoskeleton has a waxy layerthat reduces water loss from the animal.

An exoskeleton cannot grow as theanimal grows. From time to time, it isshed and replaced by a new one in aprocess called molting. While the ani-mals are molting, they are not well pro-tected from predators because the newexoskeleton is soft. Before the newexoskeleton hardens, the animal swal-lows air or water to increase its exoskele-ton’s size. This way the new exoskeletonallows room for growth.

■ Determine the characteristics thatare used to classify arthropods.

■ Explain how the structure of theexoskeleton relates to its function.

■ Distinguish between completeand incomplete metamorphosis.

Vocabularyappendageexoskeletonmoltingspiraclemetamorphosis

Arthropods, such as those that carrydiseases and eat crops, affect yourlife every day.

Figure 14The Japanese spider crab has legsthat can span more than 3 m.

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InsectsMore species of insects exist

than all other animal groups com-bined. More than 700,000 speciesof insects have been classified, andscientists identify more each year.Insects have three body regions—ahead, a thorax, and an abdomen, asshown in Figure 15. However, it isalmost impossible on some insectsto see where one region stops andthe next one begins.

Head An insect’s head has a pair of antennae, eyes, and amouth. The antennae are used for touch and smell. The eyes aresimple or compound. Simple eyes detect light and darkness.Compound eyes, like those in Figure 16, contain many lensesand can detect colors and movement. The mouthparts of insectsvary, depending on what the insect eats.

Thorax Three pairs of legs and one or two pairs of wings, ifpresent, are attached to the thorax. Some insects, such as silver-fish and fleas, don’t have wings, and other insects have wingsonly for part of their lives. Insects are the only invertebrate ani-mals that can fly. Flying allows insects to find places to live, foodsources, and mates. Flight also helps them escape from theirpredators.

How does flight benefit insects?

Abdomen The abdomen has neither wings nor legs but it iswhere the reproductive structures are found. Females lay thou-sands of eggs, but only a fraction of theeggs develop into adults. Think abouthow overproduction of eggs might ensurethat each insect species will continue.

Insects have an open circulatory sys-tem that carries digested food to cells andremoves wastes. However, insect blooddoes not carry oxygen because it does nothave hemoglobin. Instead, insects haveopenings called spiracles (SPIHR ih kulz)on the abdomen and thorax throughwhich air enters and waste gases leave theinsect’s body.

SECTION 3 Arthropods C ◆ 49

Figure 16Each compound eye is made up of small lenses that fit together.Each lens sees a part of the picture to make up the wholescene. Insects can’t focus theireyes. Their eyes are always openand can detect movements.

Figure 15One of the largest types of antsis the carpenter ant. Like allinsects, it has a head, thorax,and abdomen.

Head

Thorax

Abdomen

Magnification: 5�

From Egg to Adult Many insects go through changes in body form as they grow. This series of changes is called metamorphosis (meh tuh MOR fuh sus). Grasshoppers, silver-fish, lice, and crickets undergo incomplete metamorphosis,shown in Figure 17A. The stages of incomplete metamorphosisare egg, nymph, and adult. The nymph form molts several timesbefore becoming an adult. Many insects—butterflies, beetles,ants, bees, moths, and flies—undergo complete metamorphosis,shown in Figure 17B. The stages of complete metamorphosisare egg, larva, pupa, and adult. Caterpillar is the common namefor the larva of a moth or butterfly. Other insect larvae are calledgrubs, maggots, or mealworms. Only larval forms molt.

When do grasshoppers molt?


Figure 17The two types of metamorphosis are shown here.

Observing MetamorphosisProcedure1. Place a 2-cm piece of ripe

banana in a jar and leave itopen.

2. Check the jar every day fortwo weeks. When you seefruit flies, cover the mouthof the jar with cheesecloth.

3. Identify, describe, and drawall the stages of metamor-phosis that you observe.

Analysis1. What type of metamorpho-

sis do fruit flies undergo?2. In which stages are the flies

the most active?

Eggs

NymphMolt

Molt

Nymph

Adult

Adult

Egg

LarvaPupa

Nymphs aresmaller versions oftheir parents.

Many insects gothrough completemetamorphosis.


Obtaining Food Insects feed on plants, the blood of animals,nectar, decaying materials, wood in houses, and clothes. Mouth-parts of insects, as shown in Figure 18, are as diverse as theinsects themselves. Grasshoppers and ants have large mandibles(MAN duh bulz) for chewing plant tissue. Butterflies and honey-bees are equipped with siphons for lapping up nectar in flowers.Aphids and cicadas pierce plant tissues and suck out plant fluids.Praying mantises and centipedes eat other animals. External par-asites, such as fleas and lice, drink the blood and body fluids ofother animals. Silverfish eat things that contain starch and somemoth larvae eat wool clothing.

Insect Success Because of their tough, flexible, waterproofexoskeletons; their ability to fly; rapid reproductive cycles; andsmall sizes, insects are extremely successful. Most insects haveshort life spans, so genetic traits can change more quickly in insectpopulations than in organisms that take longer to reproduce.Because insects generally are small, they can live in a wide range ofenvironments and avoid their enemies. Many species of insectscan live in the same area and not compete with one another forfood, because many are so specialized in what they eat.

Protective coloration, or camouflage, allows insects to blendin with their surroundings. Many moths resting on trees look liketree bark or bird droppings. Walking sticks and some caterpillarsresemble twigs. When a leaf butterfly folds its wings it looks like adead leaf.

Mosquitoes have mouthsthat are adapted for piercing skin and sucking blood.

Butterflies and other nectareaters have a long siphon thatenables them to drink nectarfrom flowers.

Grasshoppers have left andright mouthparts called mandiblesthat enable them to chew throughtough plant tissues.

Figure 18Feeding adaptations of insectsinclude different mouthparts.

Some insects may carry certain diseases to humans.Some species of mosquitoescan carry malaria or yellowfever. Research to learnabout one disease that iscarried by an insect. Build a bulletin board of all theinformation that you andyour classmates gather.

Math Skills Activity

Calculating Percent of Elasticity

For more help, refer to the Math Skill Handbook.

Practice Problem

A 40-cm strand of nylon can be stretched to a length of 46.5 cmbefore losing its elasticity. Calculate the percent of elasticity for nylonand compare it to that of spider’s silk.

ArachnidsSpiders, scorpions, mites, and ticks are examples of arachnids

(uh RAK nudz). They have two body regions—a head-chestregion called the cephalothorax (se fuh luh THOR aks) and anabdomen. Arachnids have four pairs of legs but no antennae.Many arachnids are adapted to kill prey with poison glands,stingers, or fangs. Others are parasites.

Scorpions Arachnids that have a sharp, poison-filled stinger atthe end of their abdomen are called scorpions. The venom fromthe stinger paralyzes the prey. Unlike other arachnids, scorpionshave a pair of well-developed appendages—pincers—with whichthey grab their prey. The sting of a scorpion is painful and can befatal to humans.


Example ProblemA strand of spider’s silk can be stretched from 65 cm to

85 cm before it loses its elasticity—the ability to snap backto its original length. Calculate the percent of elasticity ofspider’s silk.

Solution

This is what you know: original length of silk strand � 65 cmstretched length of silk strand � 85 cm

This is what you need to find: percent of elasticity

This is what you must do: Find the difference between the stretched and original length. 85 cm � 65 cm � 20 cm

Use the following equation to � 100 � % of elasticitymake your calculations:

� 100� 30.7 % of elasticity 20 cm�65 cm

difference in length��

original length

Heart

Book lungs

Air flowing in through spiracles

Blood flow between folds

Spiders Because spiders can’t chew theirfood, they release enzymes into their prey thathelp digest it. The spider then sucks the predi-gested liquid into its mouth.

Oxygen and carbon dioxide are exchangedin book lungs, illustrated in Figure 19. Open-ings on the abdomen allow these gases to moveinto and out of the book lungs.

Mites and Ticks Most mites are animal or plant para-sites. However, some are not parasites, like the mites that livein the follicles of human eyelashes. Most mites are so smallthat they look like tiny specs to the unaided eye. All ticks areanimal parasites. Ticks attach to their host’s skin and removeblood from their hosts through specialized mouthparts.Ticks often carry bacteria and viruses that cause disease inhumans and other animals. Diseases carried by ticks includeLyme disease and Rocky Mountain spotted fever.

Centipedes and Millipedes Two groups of arthropods—centipedes and millipedes—have

long bodies with many segments, exoskeletons, jointed legs,antennae, and simple eyes. They can be found in damp environ-ments, including in woodpiles, under vegetation, and in base-ments. Centipedes and millipedes reproduce sexually. They makenests for their eggs and stay with them until the eggs hatch.

Compare the centipede and millipede in Figure 20. Howmany pairs of legs does the centipede have per segment? Howmany pairs of legs does the millipede have per segment? Cen-tipedes hunt for their prey, which includes snails, slugs, andworms. They have a pair of poison claws that they use to injectvenom into their prey. Their pinches are painful to humans butusually aren’t fatal. Millipedes feed on plants and decayingmaterial and often are found under the damp plant material.


Figure 19Air circulates between the moistfolds of the book lungs bringingoxygen to the blood.

Figure 20Centipedes are predators—

they capture and eat other ani-mals. Millipedes eat plants or decaying plant material.

54 ◆ C

VISUALIZING ARTHROPOD DIVERSITY

LOBSTER Like crabs, lobsters arecrustaceans that belong to the groupcalled Decapoda, which means “tenlegs.” It’s the lobster’s tail, however,that interests most seafood lovers.

HISSING COCKROACHMost cockroaches are consid-ered to be pests by humans,but hissing cockroaches, suchas this one, are sometimes kept as pets.

Some 600 million years ago, the first arthropods lived in Earth’s ancient seas. Today, they inhabit nearly everyenvironment on Earth. Arthropods are the most abun-

dant and diverse group of animals on Earth. They range in sizefrom nearly microscopic mites to spindly, giant Japanese spidercrabs with legs spanning more than 3 m.

GRASS SPIDERGrass spiders spin fine, nearlyinvisible websjust above theground.

GOOSENECK BARNACLE Gooseneckbarnacles typically live attached toobjects that float in the ocean. They use their long, feathery setae to straintiny bits of food from the water.

CENTIPEDE One pair oflegs per segment distinguishesa centipede from a millipede,which has two pairs of legs per body segment.

Figure 21

MONARCH BUTTERFLY Monarchs are a common sight inmuch of the United States duringthe summer. In fall, they migratesouth to warmer climates.

▼

▼

▼

▼

▼

HORSESHOE CRAB Contraryto their name, horseshoe crabsare not crustaceans. They aremore closely related to spidersthan to crabs.

▼▼

CrustaceansCrabs, crayfish, shrimp, barnacles, pill bugs,

and water fleas are crustaceans. Crustaceansand other arthropods are shown in Figure 21.Crustaceans have one or two pairs of antennaeand mandibles, which are used for crushingfood. Most crustaceans live in water, but some,like the pill bugs shown in Figure 22A, live inmoist environments on land. Pill bugs are com-mon in gardens and around house founda-tions. They are harmless to humans.

Crustaceans, like the blue crab shown inFigure 22B, have five pairs of legs. The first pairof legs are claws that catch and hold food. Theother four pairs are walking legs. They alsohave five pairs of appendages on the abdomencalled swimmerets. They help the crustaceanmove and are used in reproduction. In addi-tion, the swimmerets force water over thefeathery gills where the oxygen and carbondioxide are exchanged. If a crustacean loses anappendage, it will grow back, or regenerate.

Value of ArthropodsArthropods play several roles in the environment. They are a

source of food for many animals, including humans. Somehumans consider shrimp, crab, crayfish, and lobster as food del-icacies. In Africa and Asia, many people eat insect larvae andinsects such as grasshoppers, termites, and ants, which are excel-lent sources of protein.

Agriculture would be impossible without bees, butterflies,moths, and flies that pollinate crops. Bees manufacture honey,and silkworms produce silk. Many insects and spiders are preda-tors of harmful animal species, such as stableflies. Useful chemi-cals are obtained from some arthropods. For example, beevenom is used to treat rheumatic arthritis.

Not all arthropods are useful to humans. Almost every culti-vated crop has some insect pest that feeds on it. Many arthro-pods—mosquitoes, tsetse flies, fleas, and ticks—carry humanand other animal diseases. In addition, weevils, cockroaches,carpenter ants, clothes moths, termites, and carpet beetlesdestroy food, clothing, and property.

Insects are an important part of the ecological communitiesin which humans live. Removing all of the insects would causemore harm than good.


Figure 22Pill bugs—also called roly

polys—are crustaceans that liveon land. How are they similar tocentipedes and millipedes? Thesegments in some crustaceansaren’t obvious because they arecovered by a shieldlike structure.

Controlling Insects One common way to control probleminsects is by insecticides. However, many insecticides kill helpfulinsects as well as harmful ones. Because of their rapid life cycles,many insects have developed resistance to insecticides. Anotherproblem is that many toxic substances that have been used tokill insects remain in the environment and accumulate in thebodies of animals that eat them. As other animals eat the con-taminated animals, the insecticides can find their way intohuman food. Humans also are harmed by these toxins.

Several types of biological controls have been developed andare being tested. Different types of bacteria, fungi, and virusesare being used to control some insect pests. Natural predatorsand parasites of insect pests have been somewhat successful incontrolling certain pests. Other biological controls involve usingmales that can’t reproduce or naturally occurring chemicals thatinterfere with the reproduction or behavior of insect pests.

Origin of Arthropods Because of their hard body parts,arthropod fossils like the one in Figure 23 are among the oldestand best-preserved fossils of many-celled animals. Some arthro-pod fossils are more than 500 million years old. Recall thatearthworms and leeches have individual body segments. Becauseof this, scientists hypothesize that arthropods probably evolvedfrom an ancestor of segmented worms. Over time, groups ofbody segments fused and became adapted for locomotion, feed-ing, and sensing the environment. The hard exoskeleton andwalking legs allowed arthropods to be among the first animalsto live successfully on land.


Section Assessment

1. What are three characteristics of all arthropods?

2. What are the advantages and disadvan-tages of an exoskeleton?

3. Compare and contrast insects with arachnids.

4. Which stages of complete and incompletemetamorphosis are different?

5. Think Critically Choose an insect you arefamiliar with and explain how it is adaptedto its environment.

6. Concept Mapping Make an events-chain concept map of complete metamorphosis andone of incomplete metamorphosis. For morehelp, refer to the Science Skill Handbook.

7. Making and Using Graphs Of the majorarthropod groups, 88% are insects, 7% arearachnids, 3% are crustaceans, 1% are cen-tipedes and millipedes, and all others make up1%. Show these data in a circle graph. For morehelp, refer to the Science Skill Handbook.

Figure 23More than 15,000 species of trilo-bites have been classified. Theyare one of the most recognizedtypes of fossils.

ACTIVITY C ◆ 57

Compare your observations with those ofother students in your class. For more help,refer to the Science Skill Handbook.

Observing a Crayfish

3. Gently touch the crayfish with the stirrer. Howdoes the body feel?

4. Observe how the crayfish moves in the water.

5. Observe the compound eyes. On which bodyregion are they located?

6. Drop a small piece of ground beef into theaquarium. Observe the crayfish’s reaction.Wash your hands.

7. Return the aquarium to its proper place.

Conclude and Apply1. Infer how the location of the eyes is an

advantage for the crayfish.

2. How does the structure of the pincers aid ingetting food?

3. What can you infer about the exoskeleton andprotection?

Acrayfish has a segmented body and a fusedhead and thorax. It has a snout and eyes on

movable eyestalks. Most crayfish have pincers.What are the pincers for?

What You’ll InvestigateHow does a crayfish use its appendages?

Materials crayfish in a small aquariumuncooked ground beefstirrer

Goals■ Observe a crayfish.■ Determine the function of pincers.

Safety PrecautionsWARNING: Use care when working with live animals.

Procedure 1. Copy the data table and use it to record all of

your observations during this activity.

2. Your teacher will provide you with a crayfishin an aquarium. Leave the crayfish in theaquarium while you do the activity. Drawyour crayfish.

Body Number of FunctionRegion Appendages

Head

Thorax

Abdomen

Crayfish Observations

Tube feet

Sieve plate

Ring canal

Echinoderm CharacteristicsEchinoderms are found in oceans all over the world. The term

echinoderm is from the Greek words echinos meaning “spiny” andderma meaning “skin.” Echinoderms have a hard endoskeletoncovered by a thin, bumpy or spiny epidermis. They are radiallysymmetrical, which allows them to sense food, predators, andother things in their environment from all directions.

All echinoderms have a mouth, stomach, and intestines. Theyfeed on a variety of plants and animals. For example, sea starsfeed on worms and mollusks, and sea urchins feed on algae. Oth-ers feed on dead and decaying matter called detritus (de TRI tus)found on the ocean floor.

Echinoderms have no head or brain, but they do have a nervering that surrounds the mouth. They also have cells that respondto light and touch.

Water-Vascular System A characteristic unique to echino-derms is their water vascular system. It allows them to move,exchange carbon dioxide and oxygen, capture food, and releasewastes. The water-vascular system, as shown in Figure 24, is anetwork of water-filled canals with thousands of tube feet con-nected to it. Tube feet are hollow, thin-walled tubes that each end

in a suction cup. As the pressure in thetube feet changes, the animal is able tomove along by pushing out and pullingin its tube feet.


S E C T I O N

Echinoderms

■ List the characteristics of echinoderms.

■ Explain how sea stars obtain and digest food.

■ Discuss the importance of echinoderms.

Vocabularywater-vascular systemtube feet

Echinoderms are a group of animals that affect oceans andcoastal areas.

Figure 24Sea stars alternately extend and withdraw their tube feet,enabling them to move.

Types of EchinodermsApproximately 6,000 species of echinoderms (ih KI nuh

durmz) are living today. Of those, more than one-third are seastars. The other groups include brittle stars, sea urchins, sanddollars, and sea cucumbers.

Sea Stars Echinoderms with at least five arms arrangedaround a central point are called sea stars. The arms are linedwith thousands of tube feet. Sea stars use their tube feet to openthe shells of their prey. When the shell is open slightly, the seastar pushes its stomach through its mouth and into its prey. Thesea star’s stomach surrounds the soft body of its prey andsecretes enzymes that help digest it. When the meal is over, thesea star pulls its stomach back into its own body.

What is unusual about the way that sea stars eat their prey?

Sea stars reproduce sexually when females release eggs andmales release sperm into the water. Females can produce twomillion eggs in one season.

Sea stars also can repair themselves by regeneration. If a seastar loses an arm, it can grow a new one. If enough of the centerdisk is left attached to a severed arm, a whole new sea star cangrow from that arm.

Brittle Stars Like theone in Figure 25, brittlestars have fragile, slender,branched arms that breakoff easily. This adaptationhelps a brittle star surviveattacks by predators.While the predator is eat-ing a broken arm, thebrittle star escapes. Brittlestars quickly regeneratelost parts. They live hid-den under rocks or in lit-ter on the ocean floor.Brittle stars use their flex-ible arms for movementinstead of their tube feet.Their tube feet are usedto move particles of foodinto their mouth.

SECTION 4 Echinoderms C ◆ 59

Modeling theStrength of Tube FeetProcedure1. Hold your arm straight out,

palm up.2. Place a heavy book on your

hand.3. Have your partner time how

long you can hold your armup with the book on it.

Analysis1. Describe how your arm feels

after a few minutes.2. If the book models the sea

star and your arm modelsthe clam, infer how a seastar successfully overcomesa clam to obtain food.

Figure 25A brittle star’s arms are so flexible that theywave back and forth in the ocean currents.They are called brittlestars because theirarms break off easily if they are grabbed bya predator.

Sand dollarslive on ocean floorswhere they can burrowinto the sand.

Sea Urchins and Sand Dollars Another group of echino-derms includes sea urchins, sea biscuits, and sand dollars. Theyare disk- or globe-shaped animals covered with spines. They donot have arms, but sand dollars have a five-pointed pattern ontheir surface. Living sand dollars, like those in Figure 26A, arecovered with stiff, hairlike spines. Sea urchins, like those in Figure 26B, have long, pointed spines that protect them frompredators. Some have sacs near the end of the spines that containpoisonous fluid that is injected into predators. The spines alsohelp in movement and burrowing. Sea urchins have five tooth-like structures around their mouth.

Sea Cucumbers The animal shown in Figure 27 is a seacucumber. Sea cucumbers are soft-bodied echinoderms thathave a leathery covering. They have tentacles around theirmouth and rows of tube feet on their upper and lower surfaces.When threatened, sea cucumbers may expel their internalorgans. These organs regenerate in a few weeks. Some seacucumbers eat detritus, and others eat plankton.

60 ◆ C

Figure 27Sea cucumbers have short tubefeet, which they use to movearound.

Research Visit the GlencoeScience Web site atscience.glencoe.com forinformation about how echinoderms are used byhumans.Communicate toyour class what you learn.

Figure 26Like all echinoderms,sand dollars and seaurchins are radiallysymmetrical.

Sea urchins use tube feetand their spines to move aroundon the bottom of the ocean.


Value of EchinodermsEchinoderms are important to the

marine environment because they feed ondead organisms and help recycle materials.Sea urchins control the growth of algae incoastal areas. Sea urchin eggs and seacucumbers are used for food in someplaces. Many echinoderms are used inresearch and some might be possiblesources of medicines. Sea stars are impor-tant predators that control populations ofother animals. However, because sea starsfeed on oysters and clams, they alsodestroy millions of dollars’ worth of mol-lusks each year.

Origin of Echinoderms Like the example in Figure 28, agood fossil record exists for echinoderms. Echinoderms dateback more than 400 million years. The earliest echinodermsmight have had bilateral symmetry as adults and may have beenattached to the ocean floor by stalks. Many larval forms of mod-ern echinoderms are bilaterally symmetrical.

Scientists hypothesize that echinoderms more closely resem-ble animals with backbones than any other group of inverte-brates. This is because echinoderms have complex body systemsand an embryo that develops the same way that the embryos ofanimals with backbones develop.

SECTION 4 Echinoderms C ◆ 61

Figure 28Ophiopinna elegans was a brittlestar that lived about 165 millionyears ago.

Section Assessment

1. What characteristics do all echinodermshave in common?

2. How do echinoderms move and get theirfood?

3. How are sea urchins beneficial?

4. What methods of defense do echinodermshave to protect themselves from predators?

5. Think Critically Why would the ability to regenerate lost body parts be an impor-tant adaptation for sea stars, brittle stars,and other echinoderms?

6. Forming Hypotheses In your Science Journal,write a hypothesis about why echinoderms liveon the ocean floor. For more help, refer to theScience Skill Handbook.

7. Communicating Choose an echinoderm thatis discussed in this section and write about it in your Science Journal. Describe the following:its appearance, how it gets food, where it lives,and other interesting facts. For more help,refer to the Science Skill Handbook.

What do worms eat?


Earthworms are valuable because they improve the soil in which they live.There can be 50,000 earthworms living in one acre. Their tunnels increase air

movement through the soil and improve water drainage. As they eat the decayingmaterial in soil, their wastes can enrich the soil. Other than decaying material,what else do earthworms eat? Do they have favorite foods?

What You’ll Investigate What types of foods do earthworms eat?

Goals■ Construct five earthworm

habitats.■ Test different foods to determine

which ones earthworms eat.

Safety

WARNING: Do not handle earth-worms with dry hands. Do not eat anymaterials used in this activity.

Materialsorange peelsapple peelsbanana skinkiwi fruit skinwatermelon rind*skins of five

different fruitswidemouthed jars (5)potting soil

waterhumus*peat mossearthwormsblack construction

paper (5 sheets)masking tapemarkerrubber bands (5)*Alternate materials

Procedure

Conclude and Apply

7. Wrap a sheet of black construc-tion paper around each jar and secure it with a rubberband.

8. Using the masking tape andmarker, label each jar with thetype of fruit it contains.

9. Place all of your jars in the samecool, dark place. Observe yourjars every other day for a weekand record your observationsin your data table.

1. Copy the data table below in yourScience Journal.

2. Pour equal amounts of soil into each of the jars. Do not pack the soil. Leave several centimeters of space at the top of each jar.

3. Sprinkle equal amounts of waterinto each jar to moisten the soil.Avoid pouring too much water into the jars.

4. Pour humus into each of your jars to a depth of 2 cm. The humusshould be loose.

5. Add watermelon rinds to thefirst jar, orange peels to the sec-ond, apple peels to the third,kiwi fruit skins to the fourth,and a banana peel to the fifthjar. Each jar should have 2 cm offruit skins on top of the layer ofhumus.

6. Add five earthworms to each jar.

1. Compare the amount of fruit skinsleft in each jar.

2. Infer the type of food favored byearthworms.

3. Infer why some of the fruit skinswere not eaten by the earthworms.

4. Identify a food source in each jarother than the fruit skins.

5. Predict what would happen in the jars over the next month if youcontinued the experiment.

ACTIVITY C ◆ 63

Use the results of your experiment and infor-mation from your reading to help you write a recipe for an appetizing dinner that wormswould enjoy. Based on the results of yourexperiment, add other fruit skins or foods toyour menu you think worms would enjoy.

Date Watermelon Orange Apple Kiwi Bananarind peels peels skins peels

Fruit Wastes

nucleus organelle marsupial mantle element ibosome primate sea otter hypertension jet icior

wanomeudog

ntumcaiha

ppuatimou

digco

aganiheenc

breoz

opopoioem

nucleus organelle marsupial mantle element ibosome primate sea otter hypertension jet icior

wanomeudog

ntumcaiha

ppuatimou

digco

aganiheenc

breoz

opopoioem

Science & Language ArtsandandandScience Language Arts

Respond to the ReadingRespond to the Reading When I stayed for aweek in NewOrleans… I had an

apartment with a balcony…But when I first stepped out onit, the first thing I saw was ahuge beetle. It lay on its back directly under the light fix-ture. I thought it was dead, then saw its legs twitch andtwitch again. Big insects horrify me. As a child I fearedmoths and spiders, but adolescence cured me, as if thosefears evaporated in the stew of hormones. But I never gotenough hormones to make me easy with the large, hard-shelled insects: wood roaches, June bugs, mantises, cicadas.This beetle was a couple of inches long; its abdomen wasribbed, its legs long and jointed; it was dull reddish brown;it was dying. I felt a little sick seeing it lie there twitching,enough to keep me from sitting out on the balcony thatfirst day…And if I had any courage or common sense, Ikept telling myself, I’d… put it out of its misery. We don’tknow what a beetle may or may not suffer…

from “The Creatures on My Mind”by Ursula K. Le Guin

1. How do you suppose the beetle injured itself?

2. What kind of insectsdoes the author fear?

3. From the author’sdescription, in whatstage of development is the beetle?


migration compound dichotomous greenhouexoskeleton permafrost magma isotopes plat

cellgymmaionthekidproignparzonmoorpetalalloomogeproacctabnoleathtemsolahavdim

migration compound dichotomous greenhouexoskeleton permafrost magma isotopes plat

cellgymmaionthekidproignparzonmoorpetaalloomogeproacctabnoleattemsolhavdim

Writing a Personal ExperienceNarrative Write about a per-sonal experience. Use the experi-ence to think about an importantquestion or topic in your life.For example, you might writeabout an accident you had. Theaccident might have made youconsider the importance of good health.

andandandLinking Science Linking Science

Edward B. Lewis was part of a genetic research team that won the1995 Nobel Prize in Physiology or Medicine. They investigated howgenes controlled the development of a fruit fly, Drosophila, throughits various stages of development. Lewis investigated how genes ofthe fruit fly could control the development of specific regions intoorgans. Lewis also discovered that the genes were arranged on thechromosomes in the same order as the body parts that they con-trolled. For instance, the genes on one end of a complex strand con-trolled the development of the head, the genes in the middlecontrolled the development of the abdominal section, and the geneson the end controlled the development of the tail region. Thesegenetic discoveries were a significant breakthrough because many ofthe same principles discovered in the fruit fly were found to apply inhumans as well.

To learn more about careers in genetics, visit theGlencoe Science Web site at science.glencoe.com.

Animal Geneticist�

CareerCareer ConnectionConnection

Understanding LiteratureLegends and Oral Traditions In the passage you justread, the author uses her personal experience to con-sider her connection to other living things. A personalexperience narrative is one in which the author tells astory using his or her own experience. In this piece, theauthor recounts a minor event in her life when she hap-pens upon a dying beetle. The experience allows theauthor to pose some important questions about anotherspecies and to think about how beetles might feel whenthey die. How do you think the beetle is feeling?

Science Connection The author names several arthro-pod species in the passage, including insects and anarachnid. Beetles, June bugs, mantises, cicadas, andmoths are all insects. The spider is an arachnid. Of thearthropods the author names, can you tell which onesgo through a complete metamorphosis?

Legends and Oral Traditions

SCIENCE AND LANGUAGE ARTS C ◆ 65


Section 1 Mollusks1. Mollusks are soft-bodied invertebrates that

usually are covered by a hard shell. Theymove using a muscular foot.

2. Mollusks with one shell are gastropods.Bivalves havetwo shells.Cephalopodshave an inter-nal shell and a foot that isdivided into tentacles. What group ofmollusks does this animal belong to?

Section 2 Segmented Worms

1. Segmented wormshave tube-shapedbodies divided intosections, a body cav-ity that holds theinternal organs, andbristlelike structurescalled setae to helpthem move. How dothe tube worms in thephoto use their setae?

2. An earthworm’s digestive system has amouth, crop, gizzard, intestine, and anus.

3. Leeches are parasites that attach to animalsand feed on their blood.

Section 3 Arthropods

1. More than a million species of arthropodsexist, which is more than any other group ofanimals. Most arthropods are insects.

66 ◆ C CHAPTER STUDY GUIDE

2. Arthropods are groupedby number of body seg-ments and appendages.How would the arthropodshown here be grouped?

3. Exoskeletons cover, pro-tect, and support arthro-pod bodies.

4. Young arthropods develop either by com-plete metamorphosis or incomplete meta-morphosis.

Section 4 Echinoderms1. Echinoderms have a hard, spiny exoskeleton

covered by a thin epidermis. Most have awater-vascular system that enables them tomove, exchange carbon dioxide and oxygen,capture food, and give off wastes.

2. Brittle stars have slender, branched armsthat are fragile. Sea urchins and sand dollarsare disk-shaped animals covered with spines.Sea cucumbers are soft-bodied animals thathave a leathery covering. How does the seacucumber in the photograph move?

Study GuideChapter 22

On the front of your Foldable, list characteris-tics of each group of

invertebrates and examples of each group.

After You ReadFOLDABLESReading & StudySkills

FOLDABLESReading & Study Skills

CHAPTER STUDY GUIDE C ◆ 67

Vocabulary Words a. appendage i. moltingb. closed circulatory j. open circulatory

system systemc. crop k. radulad. exoskeleton l. setaee. gill m. spiraclef. gizzard n. tube feetg. mantle o. water-vascularh. metamorphosis system

Using Vocabulary Using the vocabulary words, replace

the underlined words with the correct science term.

1. Mollusk shells are secreted by the crop.

2. As earthworms move through soil usingtheir appendages, they take in soil, which isstored in the gizzard.

3. The endoskeleton covers and protectsarthropod bodies.

4. Insects exchange oxygen and carbon diox-ide through book lungs.

5. Spines act like suction cups and help seastars move and feed.

6. Snails use a muscular foot to get food.

7. The blood of mollusks moves in a spiracle.

Study GuideChapter 22

After each day’s lesson, make a practicequiz for yourself. Later, when you’re study-ing for the test, take the practice quizzesthat you created.

Study Tip

Complete the following concept map about insects.

Insects

Three bodyregions

have

which are

Mouthparts

has

including

Legs

Wings

has

AssessmentChapter

Choose the word or phrase that best answersthe question.

1. What structure covers organs of mollusks?A) gills C) mantleB) food D) visceral mass

2. What structures do echinoderms use tomove and to open shells of mollusks?A) mantle C) spinesB) calcium plates D) tube feet

3. Which organism has a closed circulatorysystem?A) earthworm C) slugB) octopus D) snail

4. What evidence suggests that arthropodsmight have evolved from annelids?A) Arthropods and annelids have gills.B) Both groups have species that live in salt

water.C) Segmentation is present in both groups.D) All segmented worms have setae.

5. Which of the following characteristics istypical of echinoderms?A) an endoskeletonB) a mantleC) a segmented bodyD) a water-vascular system

6. How do millipedes differ from centipedes?A) Millipedes are terrestrial and segmented.B) Millipedes eat plants.C) Millipedes have only one pair of legs on

each segment.D) Millipedes have poison fangs.

7. Of the following organisms, which have twobody regions and four pairs of legs?A) annelids C) insectsB) arachnids D) mollusks

8. Which is an example of an annelid?A) earthworm C) slugB) octopus D) snail

9. Which group of animals is the largest?A) annelids C) echinodermsB) arthropods D) mollusks

10. Which sequence shows incomplete metamorphosis?A) egg—larvae—adultB) egg—nymph—adultC) larva—pupa—adultD) nymph—pupa—adult

11. Describe how the slug inthe photo obtains food.

12. Compare the ability ofclams, oysters, scallops, andsquid to protect themselves.

13. Compare an earthworm gizzard to teeth inother animals.

14. What evidence suggests that mollusks andannelids share a common ancestor?

15. After molting but before the new exoskele-ton hardens, an arthropod causes its bodyto swell by taking in extra water or air. Howdoes this behavior help the arthropod?

16. Classifying Place the following animalsinto arthropod groups: spider, pill bug,crayfish, grasshopper, crab, silverfish, cricket,wasp, scorpion, shrimp, barnacle, tick, andbutterfly.

17. Recognizing Cause and Effect If all theearthworms were removed from a hectare ofsoil, what would happen to the soil? Why?

68 ◆ C CHAPTER ASSESSMENT

22

CHAPTER ASSESSMENT C ◆ 69

18. Interpreting Scientific Illustrations Usingthe illustrations in Section 1, infer why gas-tropods are sometimes called univalves.

19. Researching Information The suffixing -ptera means “wings.” Research the meaningof the prefix of each insect group listedbelow and give an example of a member ofeach group.

Diptera HomopteraOrthoptera HemipteraColeoptera

20. Comparing and Contrasting Complete thisVenn diagram to compare arthropods toannelids. What characteristics do they havein common? How are they different?

21. Construct Choose an arthropod that devel-ops through complete metamorphosis andconstruct a three-dimensional model foreach of the four stages. Share your con-struction with your class.

Antonio researched mollusks andmade the following circle graph, whichseparates mollusk species into seven living groups.

Study the circle graph and answer thefollowing questions.

1. According to this information,approximately what percentage ofmollusk species belongs to the group Gastropoda?A) 40 percent C) 80 percentB) 60 percent D) 95 percent

2. One of these groups was thought to beextinct until 1952, when marine biolo-gists discovered some organisms livingoff the coast of Costa Rica. Accordingto the graph, the group thought to beextinct is most likely the _____ .F) BivalviaG) CephalopodaH) GastropodaJ) Monoplacophora

Test Practice

Gastropoda40,000

Polyplacophora850

Bivalvia7,000

Cephalopoda1,000

Scaphopoda350

Monoplacophora11

Aplacophora320

AssessmentChapter 22

Go to the Glencoe Science Web site at science.glencoe.com or use the Glencoe Science CD-ROM for additional chapter assessment.

TECHNOLOGY

Annelids Arthropods

Bilateralsymmetry

Closedcirculatory

system

Opencirculatory

system


C: Chapter 2: Mollusks, Worms, Arthropods, and Echinoderms

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