chapter 29: echinoderms and invertebrate...

Echinoderms andInvertebrate Chordates

What You’ll Learn■ You will compare and

contrast the adaptations of echinoderms.

■ You will distinguish the fea-tures of chordates by examin-ing invertebrate chordates.

Why It’s ImportantBy studying how echinodermsand invertebrate chordatesfunction, you will enhance yourunderstanding of evolutionaryrelationships between thesetwo groups.

Scan the chapter, examiningthe illustrations and readingthe captions. As you read, writedown the key idea illustratedin each figure.

To find out more about echino-derms and invertebrate chor-dates, visit the Glencoe ScienceWeb site.science.glencoe.com

READING BIOLOGYREADING BIOLOGY

29ChapterChapter

A sea star extends its stom-ach from its mouth andengulfs a sea urchin. Hourslater, the sea star draws its stomach back in andmoves away. All that’s leftof the urchin is the bumpyglobe you see here. Even itsspines are gone.

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BIOLOGY

http://science.glencoe.com

Section

What Is an Echinoderm?Members of the phylum Echino-

dermata have a number of unusualcharacteristics that easily distinguishthem from members of any otheranimal phylum. Echinoderms moveby means of hundreds of hydraulic,suction cup-tipped appendages andhave skin covered with tiny, jawlikepincers. Echinoderms (ih KI nuhdurmz) are found in all the oceans ofthe world.

Echinoderms have endoskeletonsIf you were to examine the skin of

several different echinoderms, youwould find that they all have a hard,spiny, or bumpy endoskeleton cov-ered by a thin epidermis. The long,pointed spines on a sea urchin areobvious. Sea stars, sometimes called

starfishes, may not appear spiny atfirst glance, but a close look revealsthat their long, tapering arms, calledrays, are covered with short, roundedspines. The spiny skin of a seacucumber consists of soft tissueembedded with small, platelike struc-tures that barely resemble spines.The endoskeleton of all echinodermsis made primarily of calcium carbon-ate, the compound that makes uplimestone.

Some of the spines found on seastars and sea urchins have becomemodified into pincerlike appendagescalled pedicellariae (PED ih sihl AHR

ee ay). An echinoderm uses its jawlikepedicellariae for protection and forcleaning the surface of its body. Youcan examine these structures in theMiniLab on the following page.

29.1 ECHINODERMS 787

Think about what the best defensemight be for a small animal thatmoves slowly in tide pools on the

seashore. Did you think of armor, spines, orperhaps poison as methods of protection?Sea urchins are masters of defense—someuse all three methods. The sea urchin looksdifferent from the feather star and fromthe sea star on the facing page, yetall three belong to the same phy-lum. What characteristics dothey have in common? Whatfeatures determine whether ananimal is an echinoderm?

SECTION PREVIEW

ObjectivesCompare similaritiesand differences amongthe classes of echino-derms.Interpret the evidencebiologists have fordetermining that echinoderms are closerelatives of chordates.

Vocabularyraypedicellariatube feetampullawater vascular systemmadreporite

29.1 Echinoderms

Feather star(above) and seaurchin (inset)

OriginWORDWORD

echinodermFrom the Greekwords echinos,meaning “spiny,”and derma, meaning“skin.” Echinodermsare spiny-skinnedanimals.

pedicellariaeFrom the Latinword pediculus,meaning “littlefoot.” Pedicellariaeresemble little feet.

Examining Pedicellariae Echinodermsmove by tube feet. They also have tinypincers on their skin called pedicellariae.

Procedure! Observe a slide of sea star pedicellar-

iae under low-power magnification.CAUTION: Use caution when work-ing with a microscope and slides.

@ Record the general appearance of one pedicellaria. Whatdoes it look like?

# Make a diagram of one pedicellaria under low-powermagnification.

$ Record the size of one pedicellaria in micrometers.

Analysis1. Describe the general appearance of one pedicellaria.2. What is the function of this structure?3. Explain how the structure of pedicellariae assists in their

function.

MiniLab 29-1MiniLab 29-1

Echinoderms have radial symmetryYou may remember that radial sym-

metry is an advantage to animals thatare stationary or move slowly. Radialsymmetry enables these animals tosense potential food, predators, and

other aspects of their environmentfrom all directions. Observe the radialsymmetry, as well as the various sizesand shapes of spines, of each echino-derm pictured in Figure 29.1.

The water vascular systemAnother characteristic unique to

echinoderms is the water vascular sys-tem that enables them to move,exchange gases, capture food, andexcrete wastes. Look at the close-upof the underside of a sea star inFigure 29.2. You can see that groovesfilled with tube feet run from the areaof the sea star’s mouth to the tip ofeach ray. Tube feet are hollow, thin-walled tubes that end in a suction cup.Tube feet look somewhat like minia-ture droppers. The round, muscularstructure called the ampulla (AM puhlah) works something like the bulb ofa dropper. The end of a tube footworks like a tiny suction cup. Eachtube foot works independently of theothers, and the animal moves alongslowly by alternately pushing out andpulling in its tube feet. You can learnmore about the operation of thewater vascular system in the Physics

Figure 29.1All echinoderms have radial symmetryas adults and an endoskeleton com-posed primarily of calcium carbonate.

Pedicellariae

Observing and Inferring Magnification: 22�

A living sand dollarhas a solid, immov-able skeleton com-posed of flattenedplates that are fusedtogether.

BB

A sea lily’s featheryrays are composed of calcified skeletalplates covered with an epidermis.

CC

A brittle star’s long, snakelikerays are composed of overlap-ping, calcified plates coveredwith a thin layer of skin cells.

AA

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Connection at the end of this chapter.The water vascular system is a

hydraulic system that operates underwater pressure. Water enters andleaves the water vascular system of asea star through the madreporite(MAH dray pohr ite), a sievelike, disk-shaped opening on the upper surfaceof the echinoderm’s body. You canthink of this disk as being like the lit-tle strainer that fits into the drain in asink and keeps large particles out ofthe pipes. You can find out how a seastar eliminates wastes by reading theInside Story on the next page.

Finally, tube feet function in gasexchange and excretion. Gases areexchanged and wastes are eliminatedby diffusion through the thin walls ofthe tube feet.

Echinoderms have varied nutritionAll echinoderms have a mouth,

stomach, and intestines, but theirmethods of obtaining food vary. Seastars are carnivorous and prey onworms or on mollusks such as clams.Most sea urchins are herbivores andgraze on algae. Brittle stars, sea lilies,and sea cucumbers feed on dead anddecaying matter that drifts down tothe ocean floor. Sea lilies capture thissuspended organic matter with theirtentaclelike tube feet and move it totheir mouths.

Echinoderms have a simple nervous system

Echinoderms have no head orbrain, but they do have a centralnerve ring that surrounds the mouth.Nerves extend from the nerve ringdown each ray. Each radial nervethen branches into a nerve net thatprovides sensory information to theanimal. Echinoderms have cells thatdetect light and touch, but most donot have sensory organs. Sea stars arean exception. A sea star’s body con-

sists of long, tapering rays thatextend from the animal’s central disk.At the tip of each ray, on the under-side, is an eyespot, a sensory organconsisting of a cluster of light-detect-ing cells. When walking, sea starscurve up the tips of their rays so thatthe eyespots are turned up and out-ward. This enables a sea star to de-tect the intensity of light coming fromevery direction.

Echinoderms have bilaterally symmetrical larvae

If you examine the larval stages ofechinoderms, you will find that theyhave bilateral symmetry, a featuremore common to chordates. The cil-iated larva that develops from thefertilized egg of an echinoderm isshown in Figure 29.3. Throughmetamorphosis, the free-swimming larvae make dramatic changes in bothbody parts and in symmetry.The bilateral symmetry ofechinoderm larvae indicatesthat echinoderm ancestorsalso may have had bilateralsymmetry, suggesting aclose relationship to thechordates. You can observesea urchin development inthe BioLab at the end of this chapter.


Figure 29.2Tube feet enable seastars and other echin-oderms to creepalong the ocean bot-tom or to pry openthe shells of bivalves.

Figure 29.3These sea urchin lar-vae are only 1 mm insize. The larval stageof echinoderms isbilateral, eventhough the adultstage has radial symmetry.

Ray

Nerve ring

Mouth

Reproductiveorgan

Endoskeletalplates

Radialnerve Ampula

Radialcanal

Ringcanal

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A Sea Star

If you ever tried to pull a sea star from a rock where it isattached, you would be impressed by how unyielding and

rigid the animal seems to be. Yet at other times, the animalshows great flexibility, such as when it rights itself after beingturned upside down.

Critical Thinking How is radial symmetry useful to a sea star? Blood sea star

INSIDESSTORTORYY

INSIDE

Endoskeleton A sea star can maintain arigid structure or be flexible because it hasan endoskeleton in the form of calciumcarbonate plates just under its epidermis.The plates are connected by bands of soft tissue and muscle. When the muscles are contracted, the body becomes firm and rigid. When the muscles are relaxed, the body becomes flexible.

11

Madreporite Water flowsin and out of the watervascular system throughthe madreporite.

22

Tube feet The suctionaction of tube feet,caused by the contractionand relaxation of theampulla, is so strong that the sea star’s muscles can open a clam or oyster shell.

33

Eyespots When moving, a sea star curvesup the tips of its rays so that the eyespotsare turned up and outward. Echinodermeyespots distinguish between light anddark but do not form images.

44Digestive gland Thedigestive gland gives offchemicals for digestion.

55

Anus Wasteproducts of diges-tion are eliminatedthrough the anus.

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Pedicellariae The pincerlikepedicellariae on the rays of the

sea star will pinch any animalthat tries to crawl over it.

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Stomach To eat, a seastar pushes its stomachout of its mouth andspreads the stomach over the food. Powerfulenzymes secreted by thedigestive gland turn solidfood into a soupy liquidthat the stomach caneasily absorb. Then thesea star pulls the stom-ach back into its body.

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Pedicellaria

Diversity ofEchinoderms

Approximately 6000 species ofechinoderms exist today. More thanone-third of these species are in theclass Asteroidea (AS tuh royd ee uh), towhich the sea stars belong. The fourother classes of living echinodermsare Ophiuroidea (OH fee uh royd eeuh), the brittle stars; Echinoidea (ehkihn OYD ee uh), the sea urchins andsand dollars; Holothuroidea (HOH

loh thuh royd ee uh), the sea cucum-bers; and Crinoidea (cry NOYD eeuh), the sea lilies and feather stars.

Sea starsMost species of sea stars have five

rays, but some have more. Somespecies may have more than 40 rays.The rays are tapered and extend fromthe central disk. You have alreadyread about the characteristics of seastars that make them a typical exam-ple of echinoderms.

Brittle starsAs their name implies, brittle stars

are extremely fragile, Figure 29.4. Ifyou try to pick up a brittle star, partsof its rays will break off in your hand.This is an adaptation that helps thebrittle star survive an attack by apredator. While the predator is busywith the broken-off ray, the brittlestar can escape. A new ray will regen-erate within weeks.

Brittle stars do not use their tubefeet for locomotion. Instead, theypropel themselves with the snakelike,slithering motion of their flexiblerays. They use their tube feet to passparticles of food along the rays andinto the mouth in the central disk.

Sea urchins and sand dollarsSea urchins and sand dollars are

globe- or disk-shaped animals covered

with spines, as Figure 29.4 shows.They do not have rays. The circular,flat skeletons of sand dollars have afive-petaled flower pattern on thesurface. A living sand dollar is cov-ered with minute, hairlike spines thatare lost when the animal dies. A sanddollar has tube feet that protrudefrom the petal-like markings on itsupper surface. These tube feet aremodified into gills. Tube feet on theanimal’s bottom surface aid in bring-ing food particles to the mouth.

Sea urchins look like living pin-cushions, bristling with long, usually

Figure 29.4Echinoderms areadapted to life in avariety of habitats.

Basket stars, akind of brittle star,live on the softsubstrate foundbelow deep oceanwaters.

AA

Sea urchins oftenburrow intorocks to protectthemselvesfrom preda-tors andrough water.

BB


Sand dollars bur-row into the sandyocean bottom.They feed on tinyorganic particlesfound in the sand.

CC

What makes sea cucumbersrelease gametes? The orangesea cucumber lives in groups of100 or more per square meter.In the spring, these sea cucum-bers produce large numbers ofgametes (eggs and sperm), whichthey shed in the water all at thesame time. The adaptive value ofsuch behavior is that fertilization of many eggs is assured.When one male releases sperm, the other sea cucumbers inthe population, both male and female, also release theirgametes. Biologists do not know whether the sea cucumbersrelease their gametes in response to a seasonal cue, such asincreasing day length or increasing water temperature, orwhether they do this in response to the release of sperm byone sea cucumber.

AnalysisDesign an experiment that will help to determine

whether sea cucumbers release eggs and sperm in responseto the release of sperm from one individual or in response toa seasonal cue.

Thinking CriticallyIf you find that female sea cucumbers release 200 eggs in

the presence of male sperm and ten eggs in the presence ofwater that is warmer than the surrounding water, whatwould you do in your next experiment?

Problem-Solving Lab 29-1Problem-Solving Lab 29-1 Designing anExperiment

pointed spines. They have long, slen-der tube feet that, along with thespines, aid the animal in locomotion.

The sea urchin’s spines protect itfrom predators. In some species, sacslocated near the tips of the spinescontain a poisonous fluid that isinjected into an attacker, further pro-tecting the urchin. The spines alsoaid in locomotion and in burrowing.Burrowing species move their spinesin a circular motion that grinds awaythe rock beneath them. This action,which is aided by a chewing action ofthe mouth, forms a depression in therock that helps protect the urchinfrom predators and from wave actionthat could wash it out to sea.

Sea cucumbersSea cucumbers are so called because

of their vegetablelike appearance,shown in Figure 29.5. Their leatherycovering allows them to be moreflexible than other echinoderms; theypull themselves along the ocean floorusing tentacles and tube feet. Whensea cucumbers are threatened, theyexhibit a curious behavior. They mayexpel a tangled, sticky mass of tubesthrough the anus, or they may rup-ture, releasing some internal organsthat are regenerated in a few weeks.These actions confuse their predators,giving the sea cucumber an opportu-nity to move away. Sea cucumbersreproduce by shedding eggs andsperm into the water, where fertiliza-tion occurs. You can find out moreabout sea cucumber reproduction inthe Problem-Solving Lab on this page.

Sea lilies and feather starsSea lilies and feather stars resem-

ble plants in some ways. Sea lilies arethe only sessile echinoderms. Featherstars are sessile only in larval form.The adult feather star uses its feath-ery arms to swim from place to place.

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Figure 29.5Sea lilies and featherstars use their featheryrays to capture down-ward-drifting organicparticles (a). Seacucumbers trap organicparticles by sweepingtheir mucous-coveredtentacles over theocean bottom (b).

a

b

Orange sea cucumber

Crinoids600 species

Asteroids1500 species

Echinoids950 species

Ophiuroids2000 species

Holothuroids1500 species

PRESENTCENOZOICPALEOZOICPRECAMBRIAN MESOZOIC

Origins of EchinodermsThe earliest echinoderms may

have been bilaterally symmetrical asadults, and probably were attached tothe ocean floor by stalks. Anotherview of the earliest echinoderms isthat they were bilateral and free-swimming. The development ofbilateral larvae is one piece of evi-dence biologists have for placingechinoderms as the closest inverte-brate relatives of the chordates.


Section AssessmentSection AssessmentUnderstanding Main Ideas1. How does a sea star move? Explain in terms of

the water vascular system of echinoderms.2. Describe the differences in symmetry between

larval echinoderms and adult echinoderms.3. How are sea cucumbers different from other

echinoderms?4. Compare how sea urchins and sea cucumbers

obtain food.

Thinking Critically5. How do the various defense mechanisms among

the echinoderm classes help deter predators?

6. Classifying Prepare a key that distinguishesamong classes of echinoderms. Include informa-tion on features you may find significant. Formore help, refer to Organizing Information inthe Skill Handbook.

SKILL REVIEWSKILL REVIEW

Figure 29.6Most echinoderms have been foundas fossils from the early Paleozoicera. Fossils of brittle stars are found beginning at a later period.

Recall that most invertebrates showprotostome development, whereasdeuterostome development appearsmainly in chordates. The echino-derms represent the only major groupof deuterostome invertebrates.

Because the endoskeletons ofechinoderms easily fossilize, there isa good record of this phylum.Echinoderms, as a group, date fromthe Paleozoic era, as shown in Figure29.6. More than 13 000 fossil specieshave been identified.

ANIMALS

Section

794 ECHINODERMS AND INVERTEBRATE CHORDATES

What Is an InvertebrateChordate?

The chordates most familiar to youare the vertebrate chordates—chor-dates that have backbones, such asbirds, fishes, and mammals, includinghumans. But the phylum Chordata(kor DAHT uh) includes three sub-phyla: Urochordata, the tunicates

(sea squirts); Cephalochordata, thelancelets; and Vertebrata, the verte-brates. In this section you will exam-ine the tunicates and lancelets—invertebrate chordates that have nobackbones. You will study the verte-brate chordates in the next unit.

Invertebrate chordates may not lookmuch like fishes, reptiles, or humans,but like all other chordates, they havea notochord, a dorsal hollow nervecord, gill slits, and muscle blocks atsome time during their development.In addition, all chordates have bilateralsymmetry, a well-developed coelom,and segmentation. The features sharedby invertebrate and vertebrate chor-dates are illustrated in Figure 29.7.You can observe these features ininvertebrate chordates in the Problem-Solving Lab later in this section.

The brightly colored object pictured hereis a sea squirt. As one of your closestinvertebrate relatives, it is placed,

along with humans, in the phylum Chordata.At first glance, this sea squirt may seem toresemble a sponge more than its fellow chor-dates. It is sessile, and it filters foodparticles from water it takes inthrough the opening at the topof its body. What characteris-tics could a human—or a fishor a lizard, for that mat-ter—share with this colorful,ocean-dwelling organism?

SECTION PREVIEW

ObjectivesSummarize the charac-teristics of chordates.Explain how inverte-brate chordates arerelated to vertebrates.Distinguish betweensea squirts and lancelets.

Vocabularynotochorddorsal hollow

nerve cordgill slit

29.2 InvertebrateChordates

Figure 29.7Chordate characteris-tics—the notochord,dorsal hollow nervecord, gill slits, andmuscle blocks—areshared by inverte-brate as well as ver-tebrate chordates.

Mouth

Dorsal hollownerve cord Notochord

Gill slitsMuscle blocks

Anus Tail

Sea squirt and ahuman (inset)

All chordates have a notochordAll chordate embryos have a noto-

chord (NOHT uh kord)—a long,semirigid, rodlike structure locatedbetween the digestive system and thedorsal hollow nerve cord. The noto-chord is made up of large, fluid-filledcells held within stiff, fibrous tissues.In invertebrate chordates, the noto-chord is retained into adulthood. Butin vertebrate chordates, this structureis replaced by a backbone. Inverte-brate chordates do not develop abackbone.

The notochord develops just afterthe formation of a gastrula frommesoderm on what will be the dorsalside of the embryo. The physical sup-port provided by a notochord enablesinvertebrate chordates to make pow-erful side-to-side movements of

the body. These movements propelthe animal through the water at agreat speed.

All chordates have a dorsal hollow nerve cord

The dorsal hollow nerve cord inchordates develops from a plate ofectoderm that rolls into a hollowtube. This occurs at the same time asthe development of the notochord.The sequence of development of thedorsal hollow nerve cord is illustratedin Figure 29.8. This tube is composedof cells surrounding a fluid-filledcanal that lies above the notochord.In most adult chordates, the cells inthe posterior portion of the dorsalhollow nerve cord develop into thespinal cord. The cells in the anteriorportion develop into a brain. A pair

29.2 INVERTEBRATE CHORDATES 795

Figure 29.8After gastrulation, organs beginto form in a chordate embryo.

The notochord forms frommesoderm on the dorsal sideof a developing embryo.

AA

The dorsal hollow nerve cordoriginates as a plate of dorsalectoderm just above thedeveloping notochord.

BB The edges of this plate ofectoderm fold inward, eventu-ally meeting to form a hollowtube surrounded by cells. Thedorsal hollow nerve cordpinches off from the ectodermand develops into the centralnervous system of the animal.

CC

Cells migrate from the meeting margins of the neural tube andeventually form other organs,including bones and muscles.

DD

Notochord

Neuralfold

Neuralplate

Ectoderm

Outer layerof ectoderm

Dorsal hollownerve cord

MesodermEndoderm

Neuralfold

Neuralplate



Cells that formbones andmuscle

Notochord

OriginWORDWORD

chordataFrom the Latinword chorda, mean-ing “cord.” Thephylum Chordataconsists of animalswith notochords.

of nerves connects the nerve cord toeach block of muscles.

All chordates have gill slitsThe gill slits of a chordate are

paired openings located in the phar-ynx, behind the mouth. Many chor-dates have several pairs of gill slitsonly during embryonic development.Invertebrate chordates that have gillslits as adults use these structures tostrain food from the water. In somevertebrates, especially the fishes, thegill slits develop into internal gillsthat are adapted to exchange gasesduring respiration.

All chordates have muscle blocksMuscle blocks are modified body

segments that consist of stacked mus-cle layers. You have probably seenmuscle blocks when you ate a cookedfish. The blocks of muscle cause the

meat to separate easily into flakes.Muscle blocks are anchored by thenotochord, which gives the muscles afirm structure to pull against. As aresult, chordates tend to be moremuscular than members of otherphyla.

Muscle blocks also aid in move-ment of the tail. At some point indevelopment, all chordates have amuscular tail. As you know, humansare chordates, and during the earlydevelopment of the human embryo,there is a muscular tail that disap-pears as development continues. Inmost animals that have tails, thedigestive system extends to the tip ofthe tail, where the anus is located.Chordates, however, usually have atail that extends beyond the anus.You can observe many of the chor-date traits in a lancelet in theMiniLab on the next page.

Diversity ofInvertebrate Chordates

The invertebrate chordates belongto two subphyla of the phylum chordata: subphylum Urochordata,the tunicates (TEW nuh kaytz), alsocalled sea squirts, and subphylumCephalochordata, the lancelets.

Tunicates are sea squirtsMembers of the subphylum Uro-

chordata are commonly called tuni-cates, or sea squirts. Although adulttunicates do not appear to have anyshared chordate features, the larvalstage, as shown in Figure 29.9, has atail that makes it look similar to atadpole. Tunicate larvae do not feed,and are free swimming only for a fewdays after hatching. Then they settleand attach themselves with a suckerto boats, rocks, and the ocean bot-tom. Many adult tunicates secrete a

Figure 29.9Tunicate larvae areabout 1 cm long andare able to swim freelythrough the water (a).As adults, tunicatesbecome sessile filterfeeders enclosed in atough, baglike layer oftissue called a tunic (b).

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a

b

Examining a Lancelet Branchiostomacaliforniense is a small, sea-dwellinglancelet. At first glance, it appears to bea fish. However, its structural parts andappearance are quite different.

Procedure! Place the lancelet onto a glass slide.

CAUTION: Wear disposable latex gloves and handle pre-served material with forceps.

@ Use a dissecting microscope to examine the animal. CAUTION: Use care when working with a micro-scope and slides.

# Prepare a data table that will allow you to record the fol-lowing: General body shape, Length in mm, Head regionpresent, Fins and tail present, Nature of body covering,Sense organs such as eyes present, Habitat, Segmented body.

$ Indicate on your data table if the following can easily beobserved: gill slits, notochord, dorsal hollow nerve cord.

Analysis 1. How does Branchiostoma differ structurally from a fish?

How are its general appearance and habitat similar tothose of a fish?

2. Explain why you were not able to see gills, notochord, anda dorsal hollow nerve cord.

3. Using its scientific name as a guide, where might the habi-tat of this species be located?

MiniLab 29-2MiniLab 29-2tunic, a tough sac made of cellulose,around their bodies. Colonies oftunicates sometimes secrete just onebig tunic that has a common openingto the outside. You can find out howtunicates eat in the Inside Story on thenext page.

Only the gill slits in adult tunicatesindicate their chordate relationship.Adult tunicates are small, tubular ani-mals that range in size from micro-scopic to several centimeters long,about as big as a large potato. If youremove a tunicate from its sea home,it might squirt out a jet of water forprotection—hence the name seasquirt.

Lancelets are similar to fishesLancelets belong to the subphylum

Cephalochordata. They are small,streamlined, and common marineanimals, usually about 5 cm long, asFigure 29.10 shows. They spendmost of their time buried in the sandwith only their heads sticking out.Like tunicates, lancelets are filterfeeders. Unlike tunicates, however,lancelets retain all their chordate fea-tures throughout life.

Mouth


NotochordGill slitsin pharynx

Muscle blocksIntestine

Anus

Oral hood with tentaclesFigure 29.10Lancelets usually spend most of theirtime buried in the sand with only theirheads sticking out so they can filter tinymorsels of food from the water (a). Thelancelet’s body looks very much like atypical chordate embryo (b).

Observing

ba

Water

Mouth

Gill slits

Stomach

Anus

Intestine

Reproductiveorgans

Esophagus


A Tunicate

Tunicates, or sea squirts, are a group of about 1250 speciesthat live in the ocean. They may live near the shore or at

great depths. They may live individually, or several animals mayshare a tunic to form a colony.

Critical Thinking In what ways are sponges and tunicates alike?

Purple bell tunicate

INSIDESSTORTORYY

INSIDE

Excurrent siphon Water leavesthe body of the animal throughthe excurrent siphon. When atunicate is disturbed, it mayforcefully spout water from itsmouth and excurrent siphonsimultaneously.

11

Ciliated grooveDuring filter feeding,food is trapped bymucus secreted in aciliated groove. Thefood and mucus aredigested in the ani-mal’s intestine.

33

Heart The heart of the tunicate is unusualbecause it pumps bloodin one direction for several minutes andthen reverses direction.

44

Tunic Tunicates are covered with a layer of tissue called atunic. Some tunics are thick and tough, and others are thinand translucent. All protect the animal from predators.

55

Pharynx The pharynxis lined with gill slitsand cilia. The beatingof the cilia causes a cur-rent of water to movethrough the animal.Food is filtered out,and dissolved oxygen isremoved from thewater in the pharynx.

66

Incurrent siphon Water comes into the animal through the incurrentsiphon, the animal’s mouth.

22

What does a slice through an invertebrate chordateshow? Why are tunicates and lancelets important? Beinginvertebrate chordates, they show three major structures thatare present at some time during all chordate development.

Analysis The diagram at

right shows a cross section of an inver-tebrate chordate. Your task is to determine what the various structures marked A-F are.

Thinking Critically1. What three structures are present in all chordates at

some time during their development? Does the cross-section diagram of the lancelet confirm your answer?Explain.

2. How would you know that the cross section was not from an echinoderm?

3. How might the cross section differ if it were taken from an adult tunicate? A young developing tunicate?

Problem-Solving Lab 29-2Problem-Solving Lab 29-2 Interpreting ScientificIllustrations

Although lancelets look somewhatsimilar to fishes, they have only onelayer of skin, with no pigment and noscales. Lancelets do not have a dis-tinct head, but they do have lightsensitive cells on the anterior end.They also have a hood that coversthe mouth and the sensory tentaclessurrounding it. The tentacles directthe water current and food particlestoward the animal’s mouth.

Origins of InvertebrateChordates

Because sea squirts and lanceletshave no bones, shells, or other hardparts, their fossil record is incom-plete. Biologists are not sure wheresea squirts and lancelets fit in thephylogeny of chordates. According toone hypothesis, echinoderms, inver-tebrate chordates, and vertebrates allarose from ancestral sessile animalsthat fed by capturing food in tenta-cles. Modern vertebrates probablyarose from the free-swimming larvalstages of ancestral invertebrate chor-dates. Recent discoveries of fossilforms of organisms that are similar toliving lancelets in rocks 550 millionyears old show that invertebratechordates probably existed beforevertebrate chordates.


Section AssessmentSection AssessmentUnderstanding Main Ideas1. Describe the four features of chordates.2. How are invertebrate chordates different from

vertebrates?3. Compare the physical features of sea squirts and

lancelets.4. How do sea squirts and lancelets protect them-

selves?

Thinking Critically5. What features of chordates suggest that you are

more closely related to invertebrate chordatesthan to echinoderms?

6. Designing an Experiment Assume that youhave found some tadpolelike animals in the waternear the seashore and that you can raise them in a laboratory. Design an experiment in which youwill determine whether the animals are larvae oradults. For more help, refer to Practicing ScientificMethods in the Skill Handbook.

SKILL REVIEWSKILL REVIEW

A

B

C

D

E

F

S ea urchins are typical of most echinoderms in thattheir sexes are separate, fertilization is external,

and development of a fertilized egg is quite rapid.Thus, these animals are excellent choices for studyinggametes, watching fertilization, and observingchanges occurring in a fertilized egg.

INVESTIGATEINVESTIGATE

ProblemHow can you induce a sea urchin

to release its gametes?

Objectives In this BioLab, you will:■ Induce sea urchins to release their

gamete cells. ■ Observe living sperm and egg cells

under the microscope. ■ Observe developmental changes in

a fertilized sea urchin egg.

Materialslive sea urchins beakerssea water petri dishglass slides and dropper

cover slips microscopesyringe filled with test tube

potassium chloride

Safety PrecautionsAlways wear goggles in the lab.

Skill HandbookUse the Skill Handbook if you need

additional help with this lab.

PREPARATIONPREPARATION

1. Fill a small beaker (250 mL) withsea water.

2. Obtain a live sea urchin fromyour teacher and locate an area ofsoft tissue next to its mouth.

3. Using a syringe, your teacher will insert the needle into thissoft tissue and inject the syringecontents into the sea urchin.

4. Turn your animal so that itsmouth is facing up and place it in a petri dish. CAUTION: Usecare in handling live animals.

5. Wait a minute or two, then check the petri dish. If the sea

urchin is male, a milky whitemass of sperm will be present inthe dish. If it is female, a yelloworange mass of eggs will be seen.

6. If you have a female sea urchin,hold her upside down directlyover the seawater-filled beakerand allow the eggs to fall directlyinto the water.

7. If your urchin is male, use a drop-per to add several drops of spermfrom the petri dish to your beakerof sea water.

8. Check with your classmates to seewho has a male and who has a

PROCEDUREPROCEDURE


Observing Sea Urchin Gametesand Egg Development

Red sea urchin

Going FurtherGoing Further

1. Compare and Contrast Compareeggs and sperm, noting numbersreleased, numbers observedunder low power, size, and abilityto move.

2. Predicting Based on the patternof fertilization, predict the reasonfor the large number of gametesreleased in nature.

3. Observing Describe the behaviorof sperm when they first come incontact with an egg.

4. Observing How does an unfertil-ized egg differ in appearance

from a fertilized egg? Draw botheggs in your data table.

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

Project Continue to observe fertilized eggsand note the stages of development. Keep arecord of time after fertilization and corre-sponding changes in development.

To find out more about echinoderm develop-

ment, visit the Glencoe Science Web site.science.glencoe.com

female sea urchin. Share gametecells.

9. Use a clean dropper to transfer adrop of sperm from the beaker to amicroscope slide. Observe underlow power without a cover slip.

10. Add a cover slip and observeunder high power. Note themovement of sperm. Draw severalsperm cells and indicate their sizein µm. Note the approximatenumber of sperm cells present.

11. Repeat steps 9 and 10 for eggcells. In step 10, use only lowpower to observe egg cells.

12. For this step, work with a partner.While one partner transfers some

sperm to the slide with egg cellsusing a clean dropper, the otherpartner should observe under lowpower.

13. Observe the process of fertiliza-tion and note any changes thatoccur to the egg. Record yourobservations in a data table.

14. When fertilization has beenaccomplished, place the fertilizedeggs in a test tube filled with 10mL of seawater. Label your tubeand observe the eggs 24 hourslater under low power. Record anychanges that you see. CAUTION:Wash your hands immediatelyafter working with animals.


BIOLOGY


Many organisms use hydraulic systems to supplyfood and oxygen to, and remove wastes from, cellslying deep within the body. Hydraulics is a branch

of science that is concerned with the practicalapplications of liquids in motion. In living sys-

tems, hydraulics is usually concerned with the useof water to operate systems that help organisms

find food and move from place to place.

The sea star uses a unique hydraulic mechanismcalled the water vascular system for move-

ment and for obtaining food. The water vascularsystem provides the water pressure that operatesthe tube feet of sea stars and other echinoderms.

The water vascular system On the upper surface of a sea star is a sievelike disk, the madre-porite, which opens into a fluid-filled ring.Extending from the ring are long radial canalsrunning along a groove on the underside of eachof the sea star’s rays. Many small lateral canalsbranch off from the sides of the radial canals.Each lateral canal ends in a hollow tube foot.The tube foot has a small muscular bulb at oneend, the ampulla, and a short, thin-walled tube at the other end that is usually flattened into asucker. Each ray of the sea star has many tubefeet arranged in two or four rows on the bottomside of the ray. The tube feet are extended orretracted by hydraulic pressure in the water vas-cular system.

Mechanics of the water vascular system Theentire water vascular system is filled with waterand acts as a hydraulic system, allowing the seastar to move. The muscular ampulla contractsand relaxes with an action similar to the squeez-ing of a dropper bulb. When the muscles in thewall of the ampulla contract, a valve between thelateral canal and the ampulla closes so that waterdoes not flow backwards into the radial canal.The pressure from the walls of the ampulla acts

on the water, forcing it into the tube foot’s suckerend, causing it to extend.

When the extended tube foot touches a rockor a mollusk shell, the center of the foot isretracted slightly. This creates a vacuum,enabling the tube foot to adhere to the rock orshell. The tip of the tube foot also secretes asticky substance that helps it adhere. To moveforward, muscles in the ampulla relax, and mus-cles in the tube foot wall contract. These actionsshorten the tube foot and pull the sea star for-ward. Water is forced back into the relaxedampulla. When the muscles in the ampulla con-tract, the tube foot extends again. This pattern ofextension and retraction of tube feet results incontinuous movement. It is the coordinatedmovement of many tube feet that enable the seastar to move slowly along the ocean floor.


ConnectionPhysicsPhysics

Connection Hydraulics of Sea Stars

In what way do scallops and earthworms also usehydraulic pressure for locomotion?

To find out more about hydraulic pressure systems,

visit the Glencoe Science Web site.science.glencoe.com

CONNECTION TO BIOLOGYCONNECTION TO BIOLOGY

Sea star opening a mollusk to feed

BIOLOGY


Chapter 29 AssessmentChapter 29 Assessment

SUMMARYSUMMARY

Section 29.1

Section 29.2

Main Ideas■ Echinoderms have spines or bumps on their

endoskeletons, radial symmetry, and water vas-cular systems. Most move by means of the suc-tion action of tube feet.

■ Echinoderms include sea stars, sea urchins, sanddollars, sea cucumbers, sea lilies, and featherstars.

■ Deuterostome development, an internal skele-ton, and bilaterally symmetrical larvae are indi-cators of the close phylogenetic relationshipbetween echinoderms and chordates.

Vocabularyampulla (p. 788)madreporite (p. 789)pedicellaria (p. 787)ray (p. 787)tube feet (p. 788)water vascular system

(p. 789)

Echinoderms

Main Ideas■ Chordates have a dorsal hollow nerve cord, a

notochord, muscle blocks, gill slits, and a tail atsome stage during development.

■ Sea squirts and lancelets areinvertebrate chordates.

■ Vertebrate chordates may have evolved from larval stagesof ancestral invertebrate chordates.

Vocabularydorsal hollow nerve

cord (p. 795)gill slit (p. 796)notochord (p. 795)

InvertebrateChordates

CHAPTER 29 ASSESSMENT 803

1. Sea stars, sea urchins, sand dollars, seacucumbers, sea lilies, and feather stars areexamples of echinoderms that all have________.a. exoskeletonsb. jointed appendagesc. tube feetd. larvae with radial symmetry

2. Of the following, which is NOT a character-istic of chordates?a. dorsal hollow nerve cordb. notochordc. pedicellariaed. muscle blocks

UNDERSTANDING MAIN IDEASUNDERSTANDING MAIN IDEAS 3. When a sea star loses a ray, it is replaced bythe process of ________.a. regeneration c. metamorphosisb. reproduction d. parthenogenesis

4. Animals that have spines or bumps on theirendoskeletons, radial symmetry, and watervascular systems are ________.a. invertebrate chordatesb. chordatesc. vertebratesd. echinoderms

5. A close phylogenetic relationship betweenechinoderms and some chordates is indicatedby the fact that both have similar ________.a. habitats c. sizesb. larvae d. gills


6. Spines on sea stars and sea urchins are modi-fied into pedicellariae used for ________.a. feeding c. breathingb. protection d. reproduction

7. The water vascular system operates the tubefeet of sea stars and other echinoderms bymeans of ________.a. water pressure c. water pumps b. water exchange d. water filtering

8. Tube feet, in addition to functioning in loco-motion, also function in ________.a. gas exchange and digestionb. digestion and circulationc. gas exchange and excretiond. excretion and digestion

9. Water enters and leaves the water vascularsystem of a sea star through the ________.a. radial canal c. tube feetb. ampulla d. madreporite

10. Sea squirts and lancelets are invertebratechordates that have ________.a. pedicellariaeb. exoskeletonsc. tube feetd. larvae with bilateral symmetry

11. When a sea cucumber is threatened, it can________ its internal organs.

12. The ________ is a semirigid, rodlike structurecommon to all members of the phylumChordata.

13. When a sea star lifts up the tips of its rays, itis detecting ________.

14. Muscle blocks attached to the notochordenable chordates to be more ________.

15. Examine the diagram below. From whichgroup did brittle stars most likely evolve?

16. Tunicates and lancelets get food by ________.17. Most echinoderms flourished in the

Paleozoic era. Brittle stars require habitatsimilar to other echinoderms, but they didnot flourish during the Paleozoic becausethey most likely ________.

18. A ________ is a flat, disc-shaped echinodermwithout rays, and only minute hairlike spines.

19. Sea stars are more likely to leave a fossilrecord than ________ such as tunicates andlancelets.

20. The ________of this larva shows its close rela-tionship to chordates.

21. If you were an oyster farmer, why would yoube advised not to break apart and throw backany sea stars that were destroying the oysterbeds?

22. How does a sessile animal such as a sea squirtprotect itself?

APPLYING MAIN IDEASAPPLYING MAIN IDEAS

804 CHAPTER 29 ASSESSMENT

TEST–TAKING TIPTEST–TAKING TIP

All or None When filling in answer ovals, remember to fill inthe whole oval. A computer will be scoring youranswers. Don’t give the right answer for a prob-lem only to lose points on it because the computercouldn’t read your oval.


CHAPTER 29 ASSESSMENT 805

23. Relate the various functions of the water vas-cular system to the environment in whichechinoderms live.

24. How is the ability of echinoderms to regen-erate an adaptive advantage to these animals?

25. Explain how a sea squirt maintains homeosta-sis.

26. Observing and Inferring Explain why thetube feet of a sand dollar are located on itsupper surface as well as on its bottom surface.

27. Comparing and Contrasting Compare thepedicellariae of echinoderms with the nema-tocysts of cnidarians.

28. Concept Mapping Complete the conceptmap by using the following vocabulary terms:ampulla, madreporite, tube feet, water vascu-lar system.

THINKING CRITICALLYTHINKING CRITICALLY

ASSESSING KNOWLEDGE & SKILLSASSESSING KNOWLEDGE & SKILLS

The diagrams below represent cross sectionsof larvae. The intestines are shown in redand the nerve cords are shown in blue.

Interpreting Scientific Illustrations Use thediagram to answer the following questions.1. Which of the diagrams shows a cross

section of a lancelet?a. Ab. Bc. Cd. none of the diagrams

2. Which of the diagrams would representsegmented worms and echinoderms?a. Ab. Bc. Cd. none of the diagrams

3. What does the yellow, solid area repre-sent?a. nerve cord c. notochordb. intestines d. spinal cord

4. What is wrong with diagram C if it rep-resents an invertebrate chordate?a. The notochord is ventral.b. The nerve cord is ventral and there is

no notochord.c. It is too flat.d. The intestine should be round.

5. Interpreting Scientific IllustrationsUsing the same color code and the samethree organs, draw a diagram of a crosssection of a larval sea squirt, sea star, and earthworm.

For additional review, use the assessmentoptions for this chapter found on the Biology: TheDynamics of Life Interactive CD-ROM and on theGlencoe Science Web site.science.glencoe.com

CD-ROM

echinoderms

All

a sievelike

have a

radial and lateral canals

connected to

4.

1.

that includes

2.

3.

AA BB CC


Direction ofwater flow

CnidariansLike sponges, cnidarians are made up of two

cell layers and have only one body opening. Thecell layers of a cnidarian, however, are organizedinto tissues with different functions. Cnidariansare named for stinging cells that contain nemato-cysts that are used to capture food. Jellyfishes,corals, sea anemones, and hydras belong to phy-lum Cnidaria.

806

InvertebratesHow are jellyfishes, earthworms, sea stars, and butterflies alike?

All of these animals are invertebrates—animals without back-bones. The ancestors of all modern invertebrates had simple bodyplans. They lived in water and obtained food, oxygen, and other mate-rials directly from their surroundings, just like present-day sponges,jellyfishes, and worms. Some invertebrates have external coveringssuch as shells and exoskeletons that provide protection and support.

For a preview of the invertebrate unit, study this BioDigest before you read the chapters.After you have studied the invertebrate chapters, you can use the BioDigest to review the unit.

BIODIGESTBIODIGEST

Sponges are filter feeders. A sponge takes inwater through pores in the sides of its body,filters out food, and releases the waterthrough the opening at the top.

VITAL STATISTICSVITAL STATISTICS

CnidariansSize ranges: Smallest: Haliclystus salpinx, jel-lyfish, diameter, 25 mm; largest: giant jellyfishmedusa, diameter, 2 m; largest coral colony:Great Barrier Reef, length, 2027 kmMost poisonous: The sting of an Australianbox jelly can kill a human within minutes.Distribution: Worldwide in marine, brackish,and freshwater habitats.Numbers of species:Phylum Cnidaria

Class Hydrozoa—hydroids, 2700 speciesClass Scyphozoa—jellyfishes, 200 speciesClass Anthozoa—sea anemones and corals,

6200 species

SpongesSponges, phylum Porifera, are invertebrates

made up of two cell layers. Most sponges areasymmetrical. They have no tissues, organs, ororgan systems. Most adult sponges do not movefrom place to place.

RoundwormsRoundworms, phylum Nematoda, have a

pseudocoelom and a tubelike digestive systemwith two body openings. Most roundworms arefree-living, but many plants and animals areaffected by parasitic roundworms.

FlatwormsFlatworms, phylum Platyhelminthes, include

free-living planarians, parasitic tapeworms, andparasitic flukes. Flatworms are bilaterally symmet-rical animals with flattened solid bodies and nobody cavities. Flatworms have one body openingthrough which food enters and wastes leave.

Invertebrates

807

BIODIGESTBIODIGEST

Jellyfishes and othercnidarians have nemato-cysts on their tentacles.

Free-living flatworms have a head end with organs that sensethe environment. Flatworms candetect light, chemicals, food, andmovements in their surroundings.

Parasitic roundworms such asthis Trichinella are contractedby eating undercooked pork.Other roundworms can be con-tracted by walking barefoot oncontaminated soil.

Prey

Nematocyst VITAL STATISTICSVITAL STATISTICS

FlatwormsSize ranges: Largest, beef tapeworm,length, 30 mDistribution: Worldwide in soil, marine,brackish, and freshwater habitatsNumbers of species:Phylum Platyhelminthes:

Class Turbellaria—free-living planarians,3000 species

Class Cestoda—parasitic tapeworms, 3500 species

Class Trematoda—parasitic flukes, 8000 species

MollusksSlugs, snails, clams, squids, and octopuses are

members of phylum Mollusca. All mollusks arebilaterally symmetrical and have a coelom, twobody openings, a muscular foot for movement,and a mantle, which is a thin membrane that sur-rounds the internal organs. In shelled mollusks,the mantle secretes the shell.

Classes of MollusksThe three major classes of mollusks are gas-

tropods with one shell or no shell; bivalves withtwo hinged shells; and cephalopods. Cephalopodsinclude octopuses, squids, and shelled nautilusesthat all have muscular tentacles and are capableof swimming by jet propulsion. All mollusks,except bivalves, have a rough, tongue-like organ called a radula usedfor obtaining food.

FOCUS ON ADAPTATIONSFOCUS ON ADAPTATIONS

The type of body cavity an ani-mal has determines how large

it can grow and how it takes in food and eliminates wastes. Acoelo-mate animals, such as planarians,have no body cavity. Water anddigested food particles travelthrough a solid body by the processof diffusion.

Animals such as roundwormshave a fluid-filled body cavity calleda pseudocoelom that is partly linedwith mesoderm. Mesoderm is a layerof cells between the ectoderm andendoderm that differentiates intomuscles, circulatory vessels, andreproductive organs. The pseudo-coelom provides support for the

Body Cavities

Invertebrates

808

BIODIGESTBIODIGEST

Gastropods, such as snails,use their radulas to scrapealgae from rock surfaces.


MollusksSize ranges: Largest: tropical giant clam,length, 1.5 m; North Atlantic giant squid,length, 18 m; Pacific giant octopus, length, 10m; smallest: seed clam, length, less than 1 mmDistribution: Worldwide in salt-, fresh-, andbrackish water, and on land in moist temper-ate and tropical habitats.Numbers of species:Phylum Mollusca

Class Gastropoda—snails and slugs, 80 000species

Class Bivalvia—bivalves, 10 000 speciesClass Cephalopoda—octopuses, squids, and

nautiluses, 600 species

Marine flatworm

Bivalves, such as clams,strain food from waterby filtering it throughtheir gills.

Cephalopods, such asoctopuses, are predators.They capture prey usingthe suckers on their longtentacles.

Segmented WormsBristleworms, earthworms, and leeches are

members of phylum Annelida, the segmentedworms. Segmented worms are bilaterally symmet-rical, coelomate animals that have segmented,cylindrical bodies with two body openings. Mostannelids have setae, bristlelike hairs that extendfrom body segments, that help the worms move.

Segmentation is an adaptation that providesthese animals with great flexibility. Each segmenthas its own muscles. Groups of segments have dif-ferent functions, such as digestion or reproduction.

Classes of Segmented WormsPhylum Annelida has three classes: Hirudinae,

the leeches; Oligochaeta, the earthworms; andPolychaeta, the bristleworms.

Invertebrates

attachment of muscles,making movement moreefficient. Earthworms havea coelom, a body cavity sur-rounded by mesoderm inwhich internal organs aresuspended. The coelom actsas a watery skeleton againstwhich muscles can work.

809

BIODIGESTBIODIGEST

An acoelomateflatworm

A coelomatesegmented worm

A pseudocoelomate roundworm

Digestive tract

Most bristleworms have a distincthead and a body with many setae.

Leeches have flat-tened bodies withno setae. Mostspecies are parasitesthat suck blood andbody fluids fromducks, turtles, fishes,and mammals.


Segmented WormsSize ranges: Largest: giant tropical earth-worm, length, 4 m; smallest: freshwaterworm, Aeolosoma, length, 0.5 mmDistribution: Terrestrial and marine, brack-ish, and freshwater habitats worldwide,except polar regions and deserts.Numbers of species:Phylum Annelida

Class Hirudinea—leeches, 500 speciesClass Oligochaeta—earthworms,

3100 speciesClass Polychaeta—bristleworms,

8000 species

ArthropodsArthropods are bilaterally symmetrical, coelo-

mate invertebrates with tough outer coveringscalled exoskeletons and jointed appendages thatare used for walking, sensing, feeding, and mat-ing. Exoskeletons protect and support their softinternal tissues and organs. Jointed appendagesallow for powerful and efficient movements.

Arthropod DiversityTwo out of three animals on Earth today are

arthropods. The success of arthropods can beattributed to adaptations that provide efficientgas exchange, acute senses, and varied types of

mouthparts for feeding. Arthropods includeorganisms such as spiders, crabs, lobsters,shrimps, crayfishes, centipedes, millipedes,and the enormously diverse group ofinsects.

FOCUS ON ADAPTATIONSFOCUS ON ADAPTATIONS

Insects have many adaptations thathave led to their success in the air,

on land, in freshwater, and in saltwater. For example, insects havecomplex mouthparts that are welladapted for chewing, sucking, pierc-ing, biting, or lapping. Differentspecies have mouthparts adapted toeating a variety of foods.

If you have ever been bitten by amosquito, you know that mosquitoeshave piercing mouthparts that cut

through your skin to suck up blood.In contrast, butterflies and mothshave long, coiled tongues that theyextend deep into tubular flowers tosip nectar. Grasshoppers and manybeetles have hard, sharp mandiblesthey use to cut off and chew leaves.But the heavy mandibles of staghornbeetles no longer function as jaws;instead, they have become defensiveweapons used for competition andmating purposes.

Insects

Invertebrates

810

BIODIGESTBIODIGEST

The evolution of jointedappendages with many differentfunctions probably led to the suc-cess of the arthropods as a group.

Like other membersof class Arachnida,the black widow spider has four pairsof jointed legs and chelicerae, a pair of biting appendages nearthe mouth.

Staghorn beetle

Lobsters, class Crustacea, have antennae and two compound eyes on movable stalks. Theirmandibles move from side to side to seize prey.

Arthropod OriginsArthropods most likely evolved from seg-

mented worms; they both show segmentation.However, an arthropod’s segments are fused and

have a greater complexity of structure than thoseof segmented worms. Because arthropods haveexoskeletons, fossil arthropods are frequentlyfound, and consequently more is known abouttheir origins than about the phylogeny of worms.

Invertebrates

Different Foods for Different Stages

Because insects undergo metamorphosis,they often utilize different food sources atdifferent times of the year. For example,monarch butterfly larvae feed on milkweedleaves, whereas the adults feed on milkweedflower nectar. Apple blossom weevil larvaefeed on the stamens and pistils of unopenedflower buds, but the adult weevils eat appleleaves. Some adult insects, such as mayflies,do not eat at all! Instead, they rely on foodstored in the larval stage for energy to mateand lay eggs. 811

BIODIGESTBIODIGEST

Mosquitomouthparts

Grasshoppermouthparts

Butterflymouthparts

Members of classInsecta, the insects,such as this lunamoth, have threepairs of jointed legsand one pair ofantennae for sensingtheir environments.

Millipedes, class Diplopoda, areherbivores. Millipedes have up to100 body segments, and each seg-ment has two pairs of legs.


ArthropodsSize ranges: Largest insects: tropical stickinsect, length, 33 cm; Goliath beetle, mass,100 g; smallest insect: fairyfly wasp, length,0.21 mmDistribution: All habitats worldwide.Numbers of species:Phylum Arthropoda

Class Arachnida—spiders and their rela-tives, 57 000 species

Class Crustacea—crabs, shrimps, lobsters,crayfishes, 35 000 species

Class Merostomata—horseshoe crabs, 4 species

Class Chilopoda—centipedes, 2500 speciesClass Diplopoda—millipedes,

10 000 speciesClass Insecta—insects, 750 000 species

EchinodermsEchinoderms, phylum Echinodermata, are

radially symmetrical, coelomate animals withhard, bumpy, spiny endoskeletons covered by athin epidermis. The endoskeleton is comprised ofcalcium carbonate. Echinoderms move using aunique water vascular system with tiny, suction-cuplike tube feet. Some echinoderms have longspines also used in locomotion.

Echinoderm DiversityThere are five major classes of echinoderms.

They include sea stars, brittle stars, sea urchins,sea cucumbers, sand dollars, sea lilies, and featherstars.

Echinoderms have bilaterally symmetrical lar-vae, a feature that suggests a close relationship tothe chordates.

Invertebrate ChordatesAll chordates have, at one stage of their life

cycles, a notochord, a dorsal hollow nerve cord,gill slits, and muscle blocks. A notochord is a long,semirigid, rodlike structure along the dorsal sideof these animals. The dorsal hollow nerve cord is

Invertebrates

812

BIODIGESTBIODIGEST

The tube feet of a sea star operateby means of a hydraulic water vas-

cular system. Sea stars movealong slowly by alternately

pushing out and pullingin their tube feet.

Sea cucumbers have aleathery skin and areflexible. Like mostechinoderms, theymove using tube feet.


EchinodermsSize ranges: Largest: sea urchin, diameter,19 cm; longest: sea cucumber, length, 60 cmDistribution: Marine habitats worldwide.Numbers of species:Phylum Echinodermata

Class Asteroidea—sea stars, 1500 speciesClass Crinoidea—sea lilies and feather

stars, 600 speciesClass Ophiuroidea—brittle stars,

2000 speciesClass Echinoidea—sea urchins and sand

dollars, 950 speciesClass Holothuroidea—sea cucumbers,

1500 species

The long, thin arms of brittle stars are fragile and break easily, but they grow back. Brittlestars use their arms to walkalong the ocean bottom.

Invertebrates

813

BIODIGESTBIODIGEST

The lancelet is anexample of aninvertebrate chor-date. Notice thatthe lancelet’s bodyis shaped like thatof a fish eventhough it is a burrowing filterfeeder.

BIODIGEST ASSESSMENTBIODIGEST ASSESSMENT

Understanding Main Ideas1. An animal that is a filter feeder, takes in

water through pores in the sides of itsbody, and releases water from the top is a________.a. roundworm c. spongeb. gastropod d. lancelet

2. Nematocysts are unique to ________.a. sponges c. annelidsb. mollusks d. cnidarians

3. An example of a free-living flatworm is a________.a. planarian c. nematodeb. tapeworm d. vinegar-eel

4. Which of the following is used by seg-mented worms for movement?a. cheliceraeb. nematocystsc. setaed. water vascular system

5. Which of the following are invertebratechordates?a. sea anemones c. bivalvesb. lancelets d. squid

6. Parasitism is a way of life for most________.a. flukes c. cnidariansb. sponges d. annelids

7. An example of an animal with no bodycavity is a(n) ________.a. sea star c. earthwormb. flatworm d. clam

8. An octopus belongs to phylum Molluscabecause it has a mantle, bilateral symme-try, two body openings, and ________.a. an external shellb. a muscular footc. a pseudocoelomd. segmentation

9. Leeches feed by ________.a. grazing on aquatic plantsb. stinging preyc. filter feedingd. sucking blood

10. Which of the following characteristics isunique to arthropods?a. nematocycts c. filter feedingb. jointed appendages d. tube feet

Thinking Critically1. A radula is to a snail as a(n) ________ is

to a jellyfish. Explain your answer.

2. Why is more known about animals withhard parts than is known about animalswith only soft parts?

3. In what ways are echinoderms more similar to vertebrates than to other inver-tebrates?

4. You are examining a free-living animalthat had a thin, solid body with two sur-faces. Into what phylum is this organismclassified? Explain.

5. In what two ways are spiders differentfrom insects?

a fluid-filled canal lying above the notochord. Gillslits are paired openings in the pharynx that, insome invertebrate chordates, are used to strainfood from the water. In other chordates, gill slitsdevelop into internal gills used for gas exchange.Muscle blocks are modified body segments con-sisting of stacked muscle layers. Muscle blocks areanchored by the notochord.

Invertebrate chordates have all of these fea-tures at some point in their life cycles. The inver-tebrate chordates include the lancelets and thetunicates, also known as sea squirts.

chapter 29: echinoderms and invertebrate...

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