136 Unit 1: Diversity of Living Things

VOCABULARYmollusk p. 136gill p. 137lung p. 137echinoderm p. 139

BEFORE, you learned

• Body shape affects how animals move and behave

• Cnidarians have radial symme-try and simple body systems

• Worms have bilateral symmetryand segmented body systems

NOW, you will learn

• About different types of mollusks and their features

• About different types of echinoderms and their features

KEY CONCEPT

Most mollusks have shells, and echinoderms have spinyskeletons.

THINK ABOUT

How does a snail move?

Snails belong to a group of molluskscalled gastropods. The name means“belly foot.” Snails are often put intoaquariums to clean up the algae thatcan build up along the walls of thetank. If you get a chance, look at asnail moving along the glass walls ofan aquarium, observe how it uses itsfoot to move. How would you describethe action of the snail’s foot?

Mollusks are soft-bodied animals. One characteristic that is shared by all is a soft body. Manyof these invertebrate animals also have an outer shell to protect theirbody. Oysters, clams, snails, and mussels are all mollusks. So are octo-puses, squids, and slugs. Mollusks live on land and in freshwater andsaltwater environments. You will read about three groups of mollusks:bivalves, gastropods, and cephalopods.

Most mollusks have well-developed organ systems. They havemuscles, a digestive system, a respiratory system, a circulatory system,and a nervous system with sensory organs. Mollusks reproduce sexu-ally, and in most species there are distinct male and female organisms.Two adaptations distinguish mollusks as a group. First, all molluskshave a muscular foot. A mollusk’s head is actually attached to its foot.Second, all mollusks have a mantle, a layer of folded skin that protectsits internal organs.

mollusks

VOCABULARYChoose a strategy from earlier chapters, such as aword triangle, or one ofyour own to take notes onthe term mollusk.

foot

Chapter 4: Invertebrate Animals 137

BivalvesBivalves are named for a hard shell that is made up oftwo matching halves. Clams, mussels, scallops, and oys-ters are all bivalves. The shell, when it is closed,completely encloses the body. If you’ve ever seen a rawoyster, you know that a bivalve’s body looks like a massof tissue. Bivalves do not have a distinct head, but theydo have a mouth and sensory organs. The scallopshown in the photograph has light-sensitive organs thatlook like tiny eyes.

Bivalves are filter feeders, they filter food from thesurrounding water. To move, a bivalve balances upright,opens its shell, and extends its foot. The animal movesby pushing the foot in and out. The foot is also used forburrowing, digging down into the sand.

The invertebrates you’ve studied so far—sponges, cnidarians, andworms—take in oxygen all along the surface of their bodies. A bivalvetakes in oxygen through a pair of gills. A is an organ that filtersdissolved oxygen from water. The gill is an adaptation that allows anorganism to take in a lot of oxygen in just one area of its body. It ismade up of many folds of tissue that create a large surface area. Bloodpicks up the oxygen and moves it to the rest of the animal’s body. Inmost bivalves, the gills also filter food from the water.

Check Your Reading What are the two functions of gills and how do those functionsrelate to where bivalves live?

Gastropods Gastropods are the most diverse group of mollusks.Some, such as snails and slugs, live on land. Many live in water, for example, conches, whelks, and periwinkles. Many gastropods are protected by a spiral-shaped shell. To protect itself, a gastropod withdraws into the shell.

The gastropod’s head is located at the end of itsfoot. The head has eyes and specialized tentacles forsensing. Many gastropods have a cutting mouth part,called a radula, that shreds their food. Some gastropodseat animals, but most feed on plants and algae.Gastropods that live in water have gills. Some gas-tropods that live on land have lungs. A is an organthat absorbs oxygen from the air. Like gills, lungs have a large surface area.

lung

gill

Most of this blue-eyed scallop’s body is inside its two-part shell.

Bivalve

This brown-lipped snail extends most ofits body out of its shell when it moves.

Gastropod

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138 Unit 1: Diversity of Living Things

CephalopodsCephalopods (SEHF-uh-luh-PAHDZ) live in saltwater environments.Octopuses, squids, and chambered nautiluses are cephalopods. Amongmollusks, cephalopods have the most well-developed body systems.

Cephalopods have a brain and well-developed nerves.They have a pair of eyes near their mouth. The foot, whichsurrounds the mouth, has tentacles for capturing prey. Themantle is adapted to push water forcefully through a tube-shaped structure called a siphon. This produces a jet ofwater that moves the animal. Gills take in oxygen, which ispicked up by blood vessels and pumped through the bodyby three hearts.

Octopuses and squids do not have protective shells.They do have protective behaviors, however. Some canchange body color to match their surroundings. Somerelease dark clouds of inklike fluid into the water, to con-fuse their predators. The lack of a shell lets them movefreely through the water.

The nautilus is the only cephalopod that has a shell. The shell ismade up of separate compartments, or chambers. The nautilus itselflives in the outermost chamber. The inner chambers are filled withgas, which makes the animal better able to float. The chambered shellalso provides the soft-bodied nautilus with protection from predators.

Check Your Reading How is the foot of a cephalopod adapted for hunting?

How do mollusk shells compare with echinoderm skeletons?PROCEDURE

Closely observe the mollusk shells and skeletons of sea stars and sand dollars you are given.

Examine the shape and texture of each. Sort them by their characteristics.

WHAT DO YOU THINK?How are the shells and skeletons the same? How are they different?

CHALLENGE Based on your observations, what can you infer about the bodies of living mollusks, sea stars, and sand dollars?

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SKILL FOCUSObserving

MATERIALSSelection of mollusk shells, sea stars, and sand dollars

TIME15 minutes

Mollusks and EchinodermsMollusks and Echinoderms

VISUALIZATIONCLASSZONE.COM

Watch how differentcephalopods move.

This Maori octopus has a well-developed head attached to a footwith eight tentacles.

Cephalopods

Mollusks show a range of adaptations.You might not think that a clam would belong to the samegroup as an octopus. These organisms look very different fromone another. They also interact with the environment in differ-ent ways. The great variety of mollusks on Earth today providesa good example of how adaptations within a group can lead togreat diversity. A good example of this is the range of adapta-tions shown in the shape and function of a mollusk’s foot.

The foot of the bivalve is a simple muscular structure thatmoves in and out of its shell. The foot allows a bivalve to crawlalong the ocean floor and to bury itself in the sand.Gastropods have a head at the end of the foot, which runs thelength of the body. Muscles in the foot produce ripples thatallow the gastropod to glide over a surface as it searches for food.In cephalopods, the foot has tentacles to pull food into its mouth.The tentacles also help some cephalopods move along the ocean floor.

Echinoderms have unusual adaptations.are a group of invertebrates that live in the ocean. In

their adult form, their bodies have radial symmetry. Sea stars, seaurchins, sea cucumbers, and sand dollars belong to this group.Echinoderms feed off the ocean floor as they move along. An echino-derm’s mouth is located at the center of the body, on the underside.Some echinoderms, such as sea urchins and sand dollars, filter foodfrom their surroundings. Others, such as sea stars, are active predatorsthat feed on clams, snails, and even other echinoderms.

Spines and SkeletonsEchinoderm means “spiny-skinned.” Some of the more familiar echinoderms have long, sharp spines, like the sea urchin in the photograph at the bottom of this page. However, some echinodermspecies, such as sea cucumbers, have spines that are very small.

One unusual adaptation that echinoderms have is a typeof skeleton. Remember that echinoderms are inverte-brates, they have no bone tissue. The echinodermskeleton is made up of a network of stiff, hardplates. The plates lie just under the surface of theechinoderm’s skin. Some echinoderms, such assea stars, have skeletons with loosely connectedplates and flexible arms. In other echinoderms, suchas sand dollars, the plates grow close together, sothe skeleton does not allow for much flexibility.

Echinoderms

COMPARE How does thefoot of this clam comparewith the foot of an octopus?

COMBINATION NOTESRemember to take notesand make sketches forthe main idea:Echinoderms have unusu-al adaptations.

This purple sea urchin hasvery obvious spines.

foot

139

Water Vascular System and Tube FeetAnother adaptation that is unique to echinoderms is a water vascularsystem. This system is made up of water-filled tubes that radiate outfrom the center of the echinoderm’s body. Tiny openings along theupper surface of the echinoderm’s body feed water into these tubes.At the base of the tubes is a series of tube feet.

Muscles attached to the top of each tube can close the tube off,producing suction at the base of the tube. The tube feet stick to theocean floor, allowing the echinoderm to pull itself along. The tube feetcan also be used for hunting prey. For example, a sea star can sur-round a clam or oyster with its body, as shown in the photograph onthe left. The tube feet pull the shell open. Then, the sea star’s stomachis pushed out through its mouth and into the bivalve’s shell, where itbegins to digest the bivalve’s body. Not all echinoderms eat other ani-mals. Some, like sea urchins, feed off algae on the ocean floor.

KEY CONCEPTS1. What two features do all

mollusks have?

2. What are two features allechinoderms have?

3. What are two functions oftube feet in echinoderms?

CRITICAL THINKING4. Analyze For mollusks and

echinoderms, what are theadvantages and disadvantagesof having a shell or spiny skeleton?

5. Compare and ContrastCompare the foot of molluskswith the tube feet of echino-derms.

CHALLENGE6. Analyze Animals with lungs

or gills can be larger than animals that take in oxygenthrough their skin. What fea-ture do both lungs and gillshave that affect the amount of oxygen they can absorb?What role does the circulatorysystem play?

140 Unit 1: Diversity of Living Things

This sea star has captured a bivalve and is using itstube feet to open the shell.

This sea star’s arms have been pulled up to show howits tube feet are attached to the bivalve’s shell.

tube feet