720

What You’ll Learn■ You will distinguish among

the classes of mollusks andsegmented worms.

■ You will compare and contrastthe adaptations of mollusksand segmented worms.

Why It’s ImportantMollusks are an important foodsource for many animals, includ-ing humans. Some mollusks arefilter feeders that clean impuri-ties out of their watery environ-ment. Earthworms turn, aerate,and fertilize the soil in whichthey live.

Mollusks and Segmented Worms

Mollusks and Segmented Worms

Nancy Rotenberg/Animals Animals

Visit to• study the entire chapter

online• access Web Links for more

information and activities onmollusks and segmentedworms

• review content with theInteractive Tutor and self-check quizzes

Mucus is an important adapta-tion for some mollusks. Thesesnails secrete a layer of mucus to reduce friction as they move along.

Understandingthe Photo

ca.bdol.glencoe.com

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Mollusks

What is a mollusk?Slugs, snails, squids, and some animals that live in shells in the ocean

or on the beach are all mollusks. These organisms belong to the phylumMollusca. Members of this phylum range from the slow moving slug tothe jet-propelled squid. Although most species live in the ocean, otherslive in freshwater and moist terrestrial habitats. Some aquatic mollusks,such as oysters, live much of their lives firmly attached to the ocean flooror to submerged docks or parts of boats. Others, such as the octopus,swim freely in the ocean. Land-dwelling slugs and snails can be foundslowly moving over leaves on the forest floor. Examples of mollusks areshown in Figure 27.1.

27.1 MOLLUSKS 721(bl)Bruce Watkins/Animals Animals, (br)Tom McHugh/Photo Researchers

Figure 27.1With at least 110 000described species, phylumMollusca is second in size onlyto insects and their relatives.

SECTION PREVIEWObjectivesIdentify the characteristicsof mollusks.Compare the adaptationsof gastropod, bivalve, andcephalopod mollusks intheir biomes.

Review Vocabularycoelom: a fluid-filled body

cavity completely sur-rounded by mesoderm(p. 684)

New Vocabularymantleradulaopen circulatory systemclosed circulatory system nephridia

27.1

Glue

Identify and Describe As you read Chapter 27, collect notes and information about gastropods, bivalves, and cephalopods on 3 � 5-inch index cards or quarter sheets of notebook paper. Slide your notes into the appropriate pocket for quick reference.

Mollusks Make the following Foldable to help you organize information on the three most common classes of mollusks.

Fold a vertical, 5-cm tab along the long edge of a sheet of paper.

Fold into thirds so the tab is on the inside.

Open the paper and glue the edges of the 5-cm tab to make a pocket. Label as shown.

Gastropods Bivalves Cephalopods

STEP 1

STEP 3

STEP 2

Predatory squids andoctopuses are mollusks thathave tentacles and do nothave an external shell.

B

Snails, slugs, their shell-lessrelatives, and other one-shelledanimals such as this limpet makeup the largest class of mollusks.

A

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Head

Head

MantleMantle

Foot

Arm

Tentacle

ShellGut

Gut

Reduced internal shellSnail

Squid

Visceral mass

Mantle

Shell

Foot

Figure 27.2A mollusk has a soft bodycomposed of a foot, a man-tle, and a visceral mass thatcontains internal organs.Some mollusks also have ashell. Evaluate Comparethe similarities and dif-ferences in the structuresof a snail and a squid.

Figure 27.3Look at the clam shellin this photo and locatethe small hole on itsedge. This tiny hole wasmade by the radula of asnail that ate the clam,leaving its shell behindto tell the tale of theclam’s fate.

Some mollusks have shells, and oth-ers, including slugs and squids, areadapted to life without a hard cover-ing. All mollusks have bilateral sym-metry, a coelom, a digestive tract withtwo openings, a muscular foot, and amantle. The mantle (MAN tuhl) is amembrane that surrounds the internalorgans of the mollusk. In shelled mol-lusks, the mantle secretes the shell.

Although mollusks look differentfrom one another on the outside, theyshare many similarities on the inside.You can see the similarities and thedifferences in these body areas inFigure 27.2 as you compare a snailand a squid.

Infer how the mantle protects mollusks.

How mollusks obtain foodHave you ever watched a snail clean

algae from the sides of an aquarium?Snails, like many mollusks, use a rasp-ing structure called a radula to obtainfood. A radula (RA juh luh), locatedwithin the mouth of a mollusk, is atonguelike organ with rows of teeth.The radula is used to drill, scrape,grate, or cut food. Figure 27.3 showsthe results of the use of a radula. Howdoes a radula cut food? Find out inFigure 27.4. Octopuses and squidsare predators that use their radulas totear up the food that they capture withtheir tentacles. Other mollusks aregrazers and some are filter feeders.Bivalves do not have radulas; they fil-ter food from the water.

Reproduction in mollusksMollusks reproduce sexually and

most have separate sexes. In mostaquatic species, eggs and sperm arereleased at the same time into thewater, where external fertilizationtakes place. Many gastropods that liveon land, and a few bivalves, are her-maphrodites and produce both eggsand sperm. Fertilization is internal.

722

mollusk from theLatin word molluscus, meaning“soft”; Mollusksare animals with adigestive tract withtwo openings, amuscular foot, anda mantle.

Snails have a well-defined anddeveloped head area inaddition to a large foot.

A

The foot area of the squid appears to havebeen modified into arms and tentacles thatare used for capturing and holding prey.

B

Harry Rogers/Photo Researchers

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A Snail Figure 27.4Snails belong to the largest class of mollusks, the gas-tropods. Gastropods include periwinkles, conches, whelks, limpets, abalones, and slugs. A snail moves by gliding along on a thin layer of mucus secreted by a gland in the foot. You may have seen the silvery trails of a snail in a garden. Critical Thinking What other functions does mucus have for a snail?

27.1 MOLLUSKS 723Stephen Dalton/Animals Animals

Heart A snail has a two-chambered heart and anopen circulatory system. Somemollusks, such as squids, haveclosed circulatory systems.

BB

Shell The snail’s shell issecreted by the mantle andis attached to its body byone or more muscles. Asnail can protect its bodyby pulling its head andfoot inside the shell.

AA

Radula A snail obtainsfood by using its radula,a tonguelike organ withrows of teeth. The radulacan drill, scrape, grate, orcut food. As the anteriorend of the radula wearsdown, the posterior endcontinues to grow, pro-viding a continual supplyof new teeth.

DDFoot A snail’s foot is awell-developed, flat,muscular organ. Thesnail moves by contract-ing and expanding itsfoot to create a ripplingmotion which moves itforward.

FF

Lung In terrestrial species, thegill has been replaced by lungsthat can function in both waterand air.

EE

Tentacles A snail hastwo pairs of tentacles onits head. The eyes are onthe tip of the longer pair.The snail uses its shorterpair to smell and feel.

CC

Snail

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Figure 27.5One larval stage of mostmollusks resembles a spin-ning top with tufts of cilia.Most of these larvae arefree swimming before set-tling to the ocean floor foradult life. Mollusk larvaeare an important part ofmany food chains.

Juvenileclams fall off.Hook

GLOCHIDIUM

Glochidia become attached to fins and gill filaments.

Muscle

0.05mmGlochidia are expelled from female adult clam.

Female adult clam Juvenile

clam

Shell

Foot

724 MOLLUSKS AND SEGMENTED WORMS

Observe and InferHow do freshwater clams reproduce?

Solve the ProblemFreshwater clams are either male or female. Immature larvae,called glochidia, are formed following fertilization withinadult female clams’ reproductive systems. Glochidia eventuallyare released in the surrounding water.

Thinking Critically1. Infer What cell type must enter a female clam’s body in

order for glochidia to form? 2. Hypothesize Glochidia attach to and feed off of a specific

fish host. What happens to glochidia if no host is available?3. Describe How do glochidia change while attached to

their host?4. Infer It is estimated that a single clam can release over

1 000 000 glochidia. How might this be an adaptation to a life cycle that includes a parasitic stage?

Find out more about reproductionin mollusks by reading the Problem-Solving Lab on this page.

Although members of the phylumMollusca have different appearancesas adults, they all share similar devel-opmental patterns. One larval stage ofmost mollusks is similar, as you cansee in Figure 27.5.

Some marine mollusks have free-swimming larvae that propel them-selves by cilia. Most marine snails andbivalves have another developmentalstage called a veliger in which thebeginnings of a foot, shell, and mantlecan be seen.

Nervous control in mollusksMollusks have simple nervous sys-

tems that coordinate their movementand behavior. Some more advancedmollusks have a brain. Most molluskshave paired eyes that range from sim-ple cups that detect light to the com-plex eyes of octopuses that have irises,pupils, and retinas similar to the eyesof humans.

Circulation in mollusksMollusks have a well-developed cir-

culatory system that usually includes atwo- or three-chambered heart. Inmost mollusks, the heart pumps bloodthrough an open circulatory system. Inan open circulatory system, theblood moves through vessels and intoopen spaces around the body organs.This adaptation exposes body organsdirectly to blood that contains nutri-ents and oxygen, and removes meta-bolic wastes. Some mollusks, such asoctopuses, move nutrients and oxygenthrough a closed circulatory system. Ina closed circulatory system, bloodmoves through the body enclosedentirely in a series of blood vessels. Aclosed system provides an efficientmeans of gas exchange within the body.

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27.1 MOLLUSKS 725(tl)M.C. Chamberlain/DRK Photo, (tr)Wayne & Karen Brown, (b)Lawson Wood/CORBIS

Figure 27.6Shelled gastropodsvary from petite,thin-shelled speciesto large animals with thick shells.

Figure 27.7Sea slugs such as thisChromodoris specieslive in the ocean. They eat hydras, seaanemones, and seasquirts. Infer Howdoes color benefitthis sea slug?

The pink conchis a largegastropod witha thick shell.

A

The smooth dove shellis a small, delicategastropod. Theseorganisms can be foundin the Florida Keys andWest Indies.

B

Respiration in mollusksMost mollusks have respiratory

structures called gills. Gills are special-ized parts of the mantle that consist ofa system of filamentous projectionsthat contain a rich supply of blood forthe transport of gases. Gills increasethe surface area through which gasescan diffuse. In land snails and slugs, themantle cavity appears to have evolvedinto a primitive lung.

Excretion in mollusksMollusks are the oldest known ani-

mals to have evolved excretory struc-tures called nephridia. Nephridia(nih FRIH dee uh) are organs thatremove metabolic wastes from ananimal’s body. Mollusks have one ortwo nephridia that collect wastesfrom the coelom, which is locatedaround the heart only. Wastes aredischarged into the mantle cavity,and expelled from the body by thepumping of the gills.

Diversity of MollusksPhylum Mollusca is large and

diverse. Three mollusk classes—Gastropoda, Bivalvia, and Cephalo-poda—include the most common andwell-known species.

Gastropods: One-shelled mollusks

The largest class of mollusks isGastropoda, or the stomach-footedmollusks. The name comes from theway the animal’s large foot is posi-tioned under the rest of its body.Most species of gastropods have ashell. Other gastropod species, suchas slugs, have no shell.

Shelled gastropods include snails,abalones, conches, periwinkles, whelks,limpets, cowries, and cones. They canbe found in freshwater, saltwater, ormoist terrestrial habitats. Shelled gas-tropods may be plant eaters, predators,or parasites. Figure 27.6 shows twoexamples of shelled gastropods.

Instead of being protected by a shell,the body of a slug is protected by athick layer of mucus. Colorful sea slugs,also called nudibranchs, are protectedin another way. When certain species ofsea slugs feed on jellyfishes, they incor-porate the poisonous nematocysts ofthe jellyfish into their own tissues with-out causing these cells to discharge.Any fishes trying to eat the sea slugs arerepelled when the nematocysts dis-charge into the unlucky predator. Thebright colors of these gastropods warnpredators of the potential danger, asshown in Figure 27.7.

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726 MOLLUSKS AND SEGMENTED WORMSNancy Sefton/Photo Researchers

Figure 27.8In bivalves, the mantle forms two siphons, one forincoming water and one for water that is excreted.

Bivalves: Two-shelled mollusksTwo-shelled mollusks such as clams,

oysters, and scallops belong to the classBivalvia, illustrated in Figure 27.8.Most bivalves are marine, but a fewspecies live in freshwater habitats.Bivalves occur in a range of sizes. Someare less than 1 mm in length and others,such as the tropical giant clam, may be1.5 m long. Bivalves have no distincthead or radula. Most use their large,muscular foot for burrowing in themud or sand at the bottom of theocean or a lake. A ligament, like ahinge, connects their two shells, calledvalves; strong muscles allow the valvesto open and close over the soft body.See if you can identify the shells pic-tured in the MiniLab by using thedichotomous key given.

One of the main differencesbetween gastropods and bivalves isthat bivalves are filter feeders thatobtain food by filtering small particlesfrom the surrounding water. Bivalvemollusks have several adaptations forfilter feeding, including gill cilia thatbeat to draw water in through anincurrent siphon. As water moves overthe gills, food and sediments becometrapped in mucus. Cilia that line thegills push food particles to the mouth.Cilia also act as sorting devices. Largeparticles, sediment, and anything elsethat is rejected is transported to themantle where it is expelled through

Compare and ContrastIdentifying Mollusks Have you ever collected shells from abeach and wondered what they were? Use the followingdichotomous key to determine the names of the shells.

Procedure! To use a dichotomous key, begin with a choice from the

first pair of descriptions.@ Follow the instructions for the next choice. Notice that

either a scientific name can be found at the end of eachdescription, or directions will tell you to go on to anothernumbered set of choices.1A One shell . . . . . . . . . . . . . . . . . . . . . . Gastropods see 21B Two shells. . . . . . . . . . . . . . . . . . . . . . . . . Bivalves see 52A Shelf inside shell . . . . . . . . . . Common Atlantic slipper:

Crepidula fornicata2B No shelf inside shell . . . . . . . . . . . . . . . . . . . . . . . see 33A Flat coil . . . . . . . . Sundial shell: Architectonica nobilis3B Thick coil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . see 44A Spotted surface . . . . . Junonia shell: Scaphella junonia4B Lined surface . . Banded tulip shell: Fasciolaria hunteria5A Polished surface . . Sunray shell: Macrocallista nimbosa5B Rough surface. . . Lion’s paw shell: Lyropecten nodosus

Analysis1. Infer How is a dichotomous key used to identify a variety

of organisms?2. Evaluate What shell features were easy to pick out using

the key? What features were more difficult?

AB

E F

C

D

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PhysicalScience

Connection

Newton’s thirdlaw For everyforce applied byan object, there isan equal butopposite forceapplied on theobject. A squidexpels water bypushing on thewater. The waterthen pushes on thesquid, propelling itin the directionopposite the waterstream. Compareand contrast thisexample ofNewton’s third lawwith what happensduring a rocketlaunch.

27.1 MOLLUSKS 727(l)Doug Perrine/DRK Photo, (c)Ken Lucas/Visuals Unlimited, (r)Douglas Faulkner/Photo Researchers

Figure 27.9The class Cephalopoda includessquids (A) and octopuses (B). Thegenus Nautilus (C) is theonly remaining livingexample of acephalopodwith an exter-nal shell. Allother mem-bers of thisgroup areextinct.

BB

AA

ca.bdol.glencoe.com/self_check_quiz

the excurrent siphon, or to the foot,where it is eliminated from the ani-mal’s body.

Cephalopods: Head-footed mollusks

The head-footed mollusks aremarine organisms in the class Cepha-lopoda. This class includes the octopus,squid, cuttlefish, and chambered nau-tilus, as shown in Figure 27.9. Theonly cephalopod with a shell is thechambered nautilus, but some species,such as the cuttlefish, have a reducedinternal shell. Scientists consider thecephalopods to have the most complexstructures and to be the most recentlyevolved of all mollusks.

In cephalopods, the foot has evolvedinto tentacles with suckers, hooks, or adhesive structures. Cephalopods

swim or walk over the ocean floor inpursuit of their prey, capturing it withtheir tentacles. Once tentacles havecaptured prey, it is brought to themouth and bitten with beaklike jaws.Then the food is torn and pulled intothe mouth by the radula.

Like bivalves, cephalopods havesiphons that expel water. These mol-lusks can expel water forcefully inany direction, and move quickly byjet propulsion. Squids can attainspeeds of 20 m per second using thissystem of movement. You may beaware that cephalopods use jetpropulsion to escape from danger.Squids and octopuses also canrelease a dark fluid to cloud thewater. This “ink” helps to confusetheir predators so they can make aquick escape.

Understanding Main Ideas1. Describe how mucus is important to some

mollusks.

2. What adaptations make cephalopods effectivepredators in an aquatic biome?

3. Compare and contrast filter feeding with obtaining food by using a radula in an aquaticbiome.

4. Compare how squids and sea slugs protect themselves in the marine biome.

Thinking Critically5. How are the methods of movement for the snail,

clam, and squid related to the structure of eachone’s foot?

6. Get the Big Picture Develop a classification key to identify the similarities and differences of the three classes of mollusks discussed. Formore help, refer to Get the Big Picture in the Skill Handbook.

SKILL REVIEWSKILL REVIEW

CC

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27.2SECTION PREVIEWObjectivesDescribe the characteris-tics of segmented wormsand their importance tothe survival of theseorganisms.Compare and contrastthe classes of segmentedworms.

Review Vocabularyparasitism: symbiotic rela-

tionship in which oneorganism benefits at theexpense of the otherspecies (p. 44)

New Vocabularysetaegizzard

What is a segmented worm?Segmented worms are classified in the phylum Annelida. They include

leeches and bristleworms, shown in Figure 27.10, as well as earthworms.Segmented worms are bilaterally symmetrical and have a coelom and twobody openings. Some have a larval stage that is similar to the larval stagesof certain mollusks, suggesting a common ancestor.

The basic body plan of segmented worms is a tube within a tube. Theinternal tube, suspended within the coelom, is the digestive tract. Food istaken in by the mouth, an opening in the anterior end of the worm, andwastes are released through the anus, an opening at the posterior end.

Most segmented worms have tiny bristles called setae (SEE tee) on eachsegment. The setae help segmented worms move by providing a way toanchor their bodies in the soil so each segment can move the animal along.

Segmented worms can be found in most environments, except in thefrozen soil of the polar regions and the dry sand and soil of the deserts.

(t)O

.S.F

./Ani

mal

s A

nim

als,

(bl

)Ste

ve A

ustin

, P

apili

o/C

OR

BIS

, (b

r)R

on S

efto

n/B

ruce

Col

eman

, In

c.

Figure 27.10The phylum Annelidacontains about 15 000species.

Leeches live inmarine, freshwater, orterrestrial habitats.Suckers are found onone or both ends ofthe body.

A

Bristleworms havedistinct heads, eyes,and tentacles. They aremostly marine animals.

B

728

Worms Are Nature’s GardenersFinding Main Ideas As you read throughthe section about segmented worms, answerthe following questions on a separate sheetof paper.1. What is the basic body plan of a

segmented worm?2. Describe the digestive process of an

earthworm.3. Why are some leeches considered

parasites?

Segmented Worms

Earthworm

0728-0739 C27S2 BDOL-829900 8/4/04 7:00 PM Page 728

27.2 SEGMENTED WORMS 729Bruce Davidson/Animals Animals

Figure 27.11Segmentation is easilyseen in earthworms.The giant earthwormof Australia can bemore than 3 m long.Infer Why is segmen-tation an importantadaptation?

Data TableAnimal Characteristics

A Externally segmented body, no internal segments

B No coelom, but has internal segments

C Lives in water, has two body openings, sexes areseparate

D Backbone present, has digestive, circulatory,excretory systems

E Both male and female reproductive organs present

F Externally segmented body, has internal segments

Earthworms are just one of about15 000 species of segmented wormsthat live in soil, freshwater, and thesea. Can you identify a segmentedworm? Find out by reading theProblem-Solving Lab on this page.

Segmentation supports diversified functions

The most distinguishing character-istic of segmented worms is theircylindrical bodies that are dividedinto ringed segments, as seen in theworm in Figure 27.11. In mostspecies, this segmentation continuesinternally as each segment is sepa-rated from the others by a body parti-tion. Segmentation is an importantadaptation for movement becauseeach segment has its own muscles,allowing shortening and lengtheningof the body.

Segmentation also allows for spe-cialization of body tissues. Groups ofsegments may be adapted for a partic-ular function. Certain segments havemodifications for functions such assensing and reproduction.

Nervous systemSegmented worms have simple

nervous systems in which organs inanterior segments have become modi-fied for sensing the environment.Some sensory organs are sensitive tolight, and eyes with lenses and retinashave evolved in certain species. Insome species there is a brain located inan anterior segment. Nerve cords con-nect the brain to nerve centers calledganglia, located in each segment. Youcan find out how earthworms respondto their environment in the BioLab atthe end of this chapter.

Circulation and respiration Segmented worms have a closed cir-

culatory system. Blood carrying oxygento and carbon dioxide from body cells

ClassifyWhen is it an annelid? You are on a zoological researchexpedition to South America. As the invertebrate specialist,you are asked by your fellow scientists to classify a number ofanimals.

Solve the Problem

Thinking CriticallyAnalyze Which are annelids, which are not, and which requiremore study to decide? Explain your answer for each animal.

0728-0739 C27S2 BDOL-829900 8/4/04 7:02 PM Page 729

flows through vessels to reach all partsof the body. Segmented worms mustlive in water or in wet areas on landbecause they also exchange gasesdirectly through their moist skin.

Infer how dry condi-tions affect segmented worms.

Digestion and excretionSegmented worms have a complete

internal digestive tract that runs thelength of the body. Food and soil takenin by the mouth eventually pass to thegizzard. In the gizzard, a muscular sacand hard particles help grind soil andfood before they pass into the intestine.Undigested material and solid wastespass out the worm’s body through the

anus. Segmented worms have twonephridia in almost every segment thatcollect waste products and transportthem through the coelom and out ofthe body. Find out what an earthwormeats in Figure 27.12.

Reproduction in segmentedworms

Earthworms and leeches are her-maphrodites, producing both eggsand sperm. During mating, twoworms exchange sperm. Each wormforms a capsule for the eggs andsperm. The eggs are fertilized in thecapsule, then the capsule slips off theworm and is left behind in the soil. Intwo to three weeks, young wormsemerge from the eggs.

Bristleworms and their relativeshave separate sexes and reproducesexually. Usually eggs and sperm arereleased into the seawater, where fer-tilization takes place. Bristleworm lar-vae hatch in the sea and become partof the plankton. Once segment devel-opment begins, the worm settles tothe bottom.

Diversity of Segmented Worms

The phylum Annelida includesthree classes: class Oligochaeta,earthworms; class Polychaeta, bristle-worms and their relatives; and classHirudinea, leeches.

Earthworms Earthworms are the most well-

known annelids because they can beseen easily by most people. Althoughearthworms have a definite anteriorand posterior section, they do nothave a distinct head. Earthwormshave only a few setae on each seg-ment. What does an earthworm looklike internally? You can find out in theMiniLab on this page.

730 MOLLUSKS AND SEGMENTED WORMS

Interpret ScientificDiagramsA Different View of an Earthworm What does an earthworm look like internally? You could look at it many different ways—from the dorsal or ventral side, along the length of the animal (a longitudinal view), or in cross section through a segment.

Procedure! Diagram A illustrates a

longitudinal dorsal view ofthe internal organs of anearthworm. Note that thesegments are numbered.

@ Use Diagram B as a guideto how a cross sectionslice appears through segment 9.

AnalysisDescribe Make your own cross-section diagrams of segments8 and 14. Label all the parts shown in your diagrams.

Brain34

56789

10111213141516

12

Pharynx

Esophagus

Bloodvessel

Crop

Nephridia

Blood vessel

Esophagus

Muscle layers

Nephridia

Ventral nervecord

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An EarthwormFigure 27.12As an earthworm burrows through soil, it loosens, aerates, andfertilizes the soil. Burrows provide passageways for plant rootsand improve drainage of the soil. Critical Thinking In what wayis segmentation an important advantage in earthwormmovement?

Mouth Anearthworm takessoil into its mouth,the beginning ofthe digestive tract.

AA

Crop The cropis a sac that holdssoil temporarilybefore it ispassed into the gizzard.

BB

Gizzard The gizzardgrinds the organic matter,or food, into small piecesso that the nutrients inthe food can be absorbedas it passes through theintestine. Undigestedfood and any remainingsoil are eliminatedthrough the anus.

CC

Nervous system Anearthworm has a system ofnerve fibers in each segment.These are connected byventral nerve cords to a simplebrain located near the mouth.

EE

27.2 SEGMENTED WORMS 731

Earthworm

Setae An earthwormalternately contracts sets of longitudinal and circularmuscles to move. First itcontracts its longitudinalmuscles on severalsegments, which bunch up.This causes tiny setae toprotrude, anchoring theworm in the soil. Then theearthworm’s circular musclescontract, the setae arewithdrawn, and the wormmoves forward.

GG

Nephridia Nephridiaare excretory structuresthat eliminate metabolicwastes from nearly everysegment.

FF

Circulatory system The closedcirculatory system consists ofenlarged blood vessels that areheavily muscled. When thesemuscles contract, they help pumpblood through the system, much asa heart does in other animals.

DD

Alvin E. Staffan/Photo Researchers

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Microsurgeon

Would you like to beable to reattach

an accident victim’shand? Then you mightconsider a career as amicrosurgeon.

Skills for the JobMicrosurgeons use high-

powered microscopes andthree-dimensional computer tech-nology to see and repair tiny nerves and blood vessels. Amicrosurgeon in ophthalmology might repair a retina,while other microsurgeons remove tumors deep within abrain, or transplant organs. Microsurgeons who reattachhands, feet, and ears often use leeches after surgery toimprove blood flow through the reattached body part.Microsurgeons must complete four years of college, fouryears of medical school, three to five years of a residencyprogram, and special training in microsurgery. They mustalso pass an examination to become certified.

For more careers in related fields, visitca.bdol.glencoe.com/careers

parapodia fromthe Greek wordspara, meaning“before,” andpodion, meaning“foot”; Polychaeteworms move usingfleshy, paddlelikeflaps called parapodia.

(t)Deep Light Productions/Science Photo Library/Photo Researchers, (b)Larry Lipsky/DRK Photo

Figure 27.13The fanworm traps food in the mucus on its“fans.” Disturbances in the water, such as achange in the direction of the current or thepassing by of an organism, cause theseworms to quickly withdraw into their tubes.

Earthworms eat their way throughsoil. As they eat, they create spaces forair and water to flow through soil. Assoil passes through their digestive tracts, nutrients are extracted and un-digested materials pass out of theworms. Castings, the wastes of an earth-worm, help fertilize soil.

Bristleworms and their relativesThe class Polychaeta includes

bristleworms and their relatives—fanworms, shown in Figure 27.13,lug worms, plumed worms, and seamice. Polychaetes are primarily ma-rine organisms. Most body segmentsof a polychaete have many setae,hence the name. Polychaete means“many bristles.” Most body segmentsof a polychaete also have a pair ofappendages called parapodia, whichcan be used for swimming or crawlingover corals and the bottom of the sea.Parapodia also function in gasexchange. A polychaete has a headwith well-developed sense organs,including eyes.

LeechesLeeches are segmented worms with

flattened bodies and usually no setae.Most leeches live in freshwaterstreams or rivers. Unlike earthworms,many species are parasites that suckblood or other body fluids from thebodies of their hosts, which includeducks, turtles, fishes, and humans.Front and rear suckers enable leechesto attach themselves to their hosts.

You may cringe at the thought ofbeing bitten by a leech, but the bite isnot painful. This is because the salivaof the leech contains chemicals thatact as an anesthetic. Other chemicalsprevent the blood from clotting. Aleech can ingest two to five times itsown weight in one meal. Once fed, aleech will drop off its host. It may noteat again for months.

732 MOLLUSKS AND SEGMENTED WORMS

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Understanding Main Ideas1. What is the most distinguishing characteristic of

annelids? Why is it important?2. Describe how bristleworms reproduce.3. How do earthworms improve soil fertility?4. Why are leeches classified in phylum Annelida?

Thinking Critically5. Polychaetes actively swim, burrow, and crawl.

How do parapodia support the active life thatmost polychaetes pursue?

6. Interpret Scientific Illustrations Using Figure 27.12, interpret the function of the mus-cular system in the earthworm as the animal movesthrough the soil. For more help, refer to InterpretScientific Illustrations in the Skill Handbook.

SKILL REVIEWSKILL REVIEW

27.2 SEGMENTED WORMS 733

Bivalves10 000 species

Gastropods40 000 species

Annelids15 000 species

Nematodes12 000 species

Cestodes5000 species

Trematodes10 000 species

Turbellarians3000 species

Cephalopods600 species

Species numbers are approximate and subject to change pending discoveries or extinctions.

Figure 27.14Mollusks and segmentedworms are closely related.

ANIMALS

Origins of Mollusksand Segmented Worms

Fossil records show that molluskslived in great numbers as long as 500 million years ago. Gastropod,bivalve, and cephalopod fossils havebeen found in Precambrian deposits.Some species, such as the chamberednautilus, appear to have changed verylittle from related species that livedlong ago. Find out how fossil mol-lusks are used to determine the age of

rocks in the Connection to EarthScience at the end of this chapter.

Annelids probably evolved in thesea, perhaps from larvae of ancestralflatworms. The fossil record for seg-mented worms is limited because seg-mented worms have almost no hardbody parts. Tubes constructed bypolychaetes are the most commonfossils of this phylum. Some of thesetubes appear in the fossil record asearly as 540 million years ago, as youcan see in Figure 27.14.

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734 A VIEW OF THE CELL

Before YouBegin

An earthworm spends its time eating its waythrough soil, digestingorganic matter, and pass-ing inorganic matterthrough the digestive sys-tem and out of its body.Earthworms are depend-ent on soil for food andshelter. They respond tostimuli in a way that willensure a continuous supplyof food and a safe place inwhich to live. In thisBioLab, you will design anexperiment to determinethe responses of earth-worms to various stimuli.

How do earthwormsrespond to theirenvironment?

ProblemHow do earthworms respond to light, different surfaces,moist and dry environments, and warm and cold environments?

HypothesesPlace your worm in a tray with some moist soil. Watch yourworm for about 5 minutes, and record what you observe.Make a hypothesis based on your observations about whatthe worm might do under conditions of light and dark,rough and smooth surfaces, moist and dry surfaces, andwarm and cold conditions. Limit your investigation as timerequires.

ObjectivesIn this BioLab, you will:■ Measure the sensitivity of earthworms to different stimuli,

including light, water, and temperature.■ Interpret earthworm responses according to terms of adap-

tations that promote their survival.

Possible Materialslive earthworms sandpaperglass pan warm tap waterculture dishes waterthermometer or penlight

temperature probe rulerdropper cotton swabsice hand lens or stereomicroscopeblack paperpaper towels

Safety PrecautionsCAUTION: Be sure to treat the earthworm in a humane man-ner at all times. Wet your hands before handling earth-worms. Always wear goggles in the lab.

Skill HandbookIf you need help with this lab, refer to the Skill Handbook.

PREPARATIONPREPARATION

Matt Meadows

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27.2 SEGMENTED WORMS 735

1. As a group, make a list of possible ways you might test yourhypothesis. Keep the available materials in mind as you plan your procedure.

2. Be sure to design an experiment that will test one variable at atime. Plan to collect quantitative data. Make sure to incorporate a control.

3. Record your procedure and list materials and amounts you willneed. Design and construct a data table for recording your findings.

Check the PlanDiscuss the following points with other group members.

1. What data will you collect, and how will they be recorded?2. Does each test have one variable and a control? What are they?3. Each test should include measurements of some kind. What are

you measuring in each test?4. How many trials will you run for each test?5. Make sure your teacher has approved your experimental plan

before you proceed further.6. Carry out your experiment. 7. Return earthworms to the container your

teacher has provided. Wash your hands thoroughly with soap andwater. Make wise choices about whether other lab materials shouldbe disposed of or cleaned for reuse.

CLEANUP AND DISPOSAL

PLAN THE EXPERIMENTPLAN THE EXPERIMENT

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

1. Check Your Hypothesis Which surface didthe worm prefer? Explain.

2. Interpret Observations At which tempera-ture was the worm most active? Explain.

3. Observe and Infer How did the earthwormrespond to light? How did it respond to dryand moist environments? Of what survivalvalue are these behaviors?

4. Draw Conclusions In general, what condi-tions do earthworms prefer?

5. Analyze where errors mayhave occurred in your experiment.ERROR ANALYSIS

Project Based on your experiment, designanother experiment that would help toanswer a question that arose from yourwork. You might want to try other variablessimilar to the ones you used, or you mightchoose to investigate a completely differentvariable.

Web Links To find out more about segmented worms, visit ca.bdol.glencoe.com/segmented_worms

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Mollusks asIndicators

Although a few species of mollusks live onland, most mollusks are marine or fresh-

water organisms. How is it, then, that on one ofhis journeys to South America, Charles Darwinfound aquatic mollusk shells thousands of feetabove sea level? This observation by the famousnaturalist helped to support Darwin’s hypothesisthat Earth has changed over time.

Mollusks once ruled Earth Mollusks first appear in Earth’s fossil record more than 500 million years ago. By 30 million years later,these shelled creatures had become the dominantlife form on Earth. Thousands of species of mol-lusks evolved to fill available niches. Yet, numerousspecies of mollusks became extinct at the close of

the Mesozoic Era about 65 million years ago.Today, the number of mollusk species is estimatedat 110 000.

The present is the key to the pastBecause mollusks are generally well preserved inthe fossil record, abundant, easy to recognize,and widely distributed geographically, they areexcellent index fossils. Index fossils, together withtheir modern relatives, can be used to hypothe-size about ancient climates and environments.

Mollusk shells can also provide informationabout the biotic, physical, and chemical changesthat occur in an ecosystem. Modern mollusks, forexample, have been used to determine the sourceand distribution of various aquatic pollutants.

Mollusks as timekeepers Mollusks can alsobe thought of as marine timekeepers. A molluskshell grows only along one edge. The pigmentedpatterns produced by the animal along this grow-ing edge rarely change. Thus, the pattern pro-duced is not only specific to the species but also is a space and time record of the shell-producingprocess of that particular organism.

Mollusk shells also can be used to estimate anage because these structures contain the elementstrontium. By measuring the amounts of differ-ent isotopes of strontium in the shell, scientistsare able to closely estimate the age of the shell,and, by extension, the exact age of the rockscontaining the shell.

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Research Ammonites were early mollusks thatlived from about 230 million years ago to about 65 million years ago. Ammonites are now extinct.Research to find out if these mollusks are goodindex fossils. Explain your answer.

To find out more about mollusks and other index fossils, visit ca.bdol.glencoe.com/earth_science

Fossilized mollusk shells

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Section 27.1Key Concepts■ Mollusks have bilateral symmetry, a

coelom, and a digestive tract with twoopenings. Many also have shells.

■ Most gastropods, such as snails, have ashell, mantle, radula, an open circulatorysystem, gills, and nephridia. Gastropodswithout shells, such as slugs, are protectedby a covering of mucus.

■ Bivalve mollusks have paired shells, calledvalves, and are filter feeders. They have noradula. Clams and scallops are bivalves.

■ Cephalopods have tentacles with suckers, beaklike jaws, a mouth with a radula, and a closed circulatory system. Cephalopodsinclude the octopus, squid, and chamberednautilus.

Vocabularyclosed circulatory system

(p. 724)mantle (p. 722)nephridia (p. 725)open circulatory system

(p. 724)radula (p. 722)

Mollusks

Vocabularygizzard (p. 730)setae (p. 728)Segmented

Worms

STUDY GUIDESTUDY GUIDE

CHAPTER 27 ASSESSMENT 737(bl)Bruce Davidson/Animals Animals, (br)Steve Austin, Papilio/CORBIS

To help you review thediversity of mollusks, use the Organiza-tional Study Fold on page 721.

Section 27.2Key Concepts■ The phylum Annelida includes the earth-

worms, bristleworms and their relatives,and leeches. Annelids are bilaterally sym-metrical and have a coelom and two bodyopenings; some have larvae that look likethe larvae of mollusks. Their bodies arecylindrical and segmented.

■ Earthworms have complex digestive, excretory, muscular, and circulatory systems.

■ Bristleworms and their relatives are mostlymarine species. They have many setae andparapodia that are used for crawlingalong.

■ Leeches are flattened, segmentedworms. Most are aquatic parasites.

■ Fossil remains of mollusks show thatthey first lived over 500 million years ago.Fossil records show that segmented wormsfirst appeared 540 million years ago.

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Review the Chapter 27 vocabulary words listed inthe Study Guide on page 737. Match the wordswith the definitions below.

1. membrane that surrounds the internalorgans of a mollusk

2. sac with muscular walls and hard particles thatgrind soil before it passes into the intestine

3. in some mollusks, the rasping, tonguelikeorgan used to drill, scrape, grate, or cut food

4. organs that remove metabolic wastes froman animal’s body

5. system where blood moves through vesselsinto open spaces around the body organs

6. When an earthworm passes soil through itsdigestive tract, the soil does NOT gothrough the ________.A. stomach C. gizzardB. nephridia D. crop

7. Which of the following animals have setae?A. snailsB. clamsC. earthwormsD. squids

8. Which of the following is a gastropod?

A. C.

B. D.

9. Which of the following word pairs are mostclosely related?A. filter feeding—radulaB. scraping algae—siphonC. predation—tentacleD. nephridia—gizzard

10. Which of the following does NOT use aradula for feeding?A. snail C. oysterB. slug D. squid

11. Leeches that suck blood or other body fluidsfrom their hosts are considered to be _______.A. parasites C. filter feedersB. grazers D. predators

12. Open Ended Compare the protective adap-tations of gastropods and cephalopods.

13. Open Ended Interpret the function of abristleworm’s reproductive system and relateit to its life in the aquatic biome.

14. Open Ended Compare the adaptations forsurvival in two different mollusks: a snail living on land and an octopus living in theocean.

15. Analyze Fill in the table with key adapta-tions of the animals.

16. Natural pearlsare made by oysters in response to foreignmaterial, such as a grain of sand lodginginside the shell. Pearls have been used as jew-elry for centuries. Find out about the historyof pearls and present-day pearl culture at

. Visit a jeweler andask to see and photograph marine and fresh-water pearls. Prepare a poster or a multi-media presentation with your findings andpresent it to your class.

REAL WORLD BIOCHALLENGE

738 CHAPTER 27 ASSESSMENT

Data Table

Gastropods Bivalves Cephalopods

Getting food

Circulation

Excretion

Protection

Locomotion

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Multiple ChoiceUse the map to answer questions 19–21.

19. The zebra mussel, a non-native mollusk, wasfirst discovered in North America (Michiganand Ohio) in 1988. By 2001, zebra musselshad spread as far south as ________.A. Florida C. GeorgiaB. Louisiana D. Texas

20. By 2001, zebra mussels from Michigan andOhio had invaded _______ additional states.A. 12 C. 14B. 8 D. 18

21. In 2001, you would NOT find zebra musselsin _______.A. KansasB. OklahomaC. West VirginiaD. Wisconsin

Study the diagram and answer question 22.

22. Nerve cells send the chemical messages thatproduce pain. In the diagram, the chemicalpain messages are squares activated by cal-cium, the round figures. Scientists studyingthe venom from sea slugs have found that insmall quantities, the venom stops the pain sig-nal from being sent. You might infer that thevenom interrupts the signal at ________.A. calcium’s entranceB. the pain message exitC. the receiving cellD. all of the above

Venomcompound

Sensorynerve cell

Calcium

Calcium'sentrance

Chemical messages

Painmessage

Receiving cell

States with zebra mussels ininland and adjacent waters, 2001

Constructed Response/Grid InRecord your answers or fill in the bubbles on your answer document using the correct place value.

23. Open Ended When temperatures increase and soil becomes drier, what happens to earthworms?24. Grid In If a farmer’s field has 75 earthworms per square meter, how many earthworms are in

4.5 square meters?

17. Infer A leech’s saliva contains chemicals thatact as an anesthetic. How does this benefitthe leech?

18. Hypothesize What would happen to afarmer’s field if all the earthworms wereremoved?

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