CHAPTER

The delicate symmetrical appearance of this starfishobscures the fact that starfish are carnivores preyingon other forms of sea life. The small tunicate shown inthe inset is closely related to vertebrate animals, eventhough its appearance more closely resembles othersimpler animals.

Echinodermsand

InvertebrateChordates

hen most people see a starfish, they immediately think of the

ocean—an appropriate reaction, considering that starfish and their

relatives live only in the sea. Surprisingly not all of the members of

this phylum resemble stars. Some look like graceful long-stemmed

flowers; others, like a peculiar cross between a polka-dotted pickle

and a stalk of cauliflower. Some are as round and prickly as a

pincushion. Others are flat bristly disks with holes and notches

around their edges. Still others resemble armored feather dusters,

pentagon-shaped cookies, or the curlicues and flourishes beneath

ah old-fashioned signature.

What other animals belong to this phylum? How are they alladapted to the ocean world? Why do scientists consider them to be

closely related to the vertebrates? You will discover the answers to

these questions-'in the pages that follow.

GUIDE FOR READING

After you read the followingsections, you will be able to

29-1 Echinoderms• Relate the structure of

echinoderms to essential lifefunctions.

• Describe the characteristics ofthe classes of echinoderms.

• Explain how echinoderms fit intothe world.

29-2 Invertebrate Chordates• Name and discuss the three

distinguishing characteristics ofchordates.

• Describe the two subphyla ofinvertebrate chordates.

Journal ActivityYOU AND YOUR WORLD

Perhaps you have combed a beachfor treasures of shells, starfish, orsea-urchin spines, scrambled overgrassy dunes, swum in the ocean,or imagined what the seashore islike, In your journal, describe yourexperiences, whether they are realor imagined.

Figure 29-1 Echinoderm meansspiny skin, which as you can seefrom this sea urchin (bottom) isan appropriate name. The seacucumber (top) is also anechinoderm. Although its skin issmooth, it shows anothercharacteristic of echinoderms—five-part symmetry.

29-1 EchinodermsGuide For Reading

What are echinoderms?

How do echinoderms carry out essential functions?

¦ How are echinoderms classified?

¦ How do echinoderms affect other living things?

Nearly everyone who has visited the seashore has seenstarfish, sea urchins, sand dollars, or their remains washed upon the beach. These animals are members of the phylumEchinodermata (echino- means spiny; dermis means skin), aphylum that has a long and fascinating history stretching backto the beginning of the Cambrian Period, more than 580 millionyears ago.

What Is an Echinoderm?As their name indicates, echinoderms (ee-KlGH-noh-

dermz) are spiny-skinned animals. In addition to having aspiny skin, echinoderms are characterized by five-partradial symmetry, an internal skeleton, a water vascularsystem, and suction-cuplike structures called tube feet. Theinternal skeleton, or endoskeleton, is made up of hardenedplates of calcium carbonate, which are often bumpy or spiny.

The water vascular system, which you will learn more aboutshortly, consists of an internal network of fluid-filled canalsconnected to external appendages called tube feet. The watervascular system is involved with many essential life functionsin echinoderms, including feeding, respiration, internal trans¬port, elimination of waste products, and movement.

Some echinoderms, such as starfish and sand dollars, livein shallow water and are thus familiar to beach-goers. Otherechinoderms live only on coral reefs or on the floor of the deepocean. Although echinoderms possess certain characteristicsfound in no other animals, living or extinct, they share severalimportant features with members of our own phylum (Chor-data). For example, certain stages in the development of echin-oderm larvae are remarkably similar to stages seen in somemembers of the phylum Chordata. In addition, echinodermshave an internal skeleton (as do vertebrates) rather than an ex¬ternal skeleton (as do other invertebrates). For these reasons,biologists believe that among common invertebrates, echino¬derms are most closely related to humans.

Echinoderm species vary greatly in appearance. Starfishexhibit a fragile beauty and perfection in shape that stirswonder in most observers. Some sea cucumber species fasci¬nate because their ugliness has a certain repulsive appeal.Regardless of appearance, however, echinoderms have adapta¬tions that make them successful survivors in the world of thesea. As you read this section, keep in mind that echinodermsare survivors of history. Their success is confirmed by the factthat some echinoderms alive today look much like their an¬cient ancestors who lived in the seas millions and millions ofyears ago.

Form and Function in EchinodermsAdult echinoderms have a body plan with five parts organ¬

ized symmetrically around a center. As a result of this bodyplan, adult echinoderms typically have neither an anterior nora posterior end and no brain. However, most echinoderms aretwo-sided. The side where the mouth is located is called theoral surface, and the opposite side is called the aboral surface.

All echinoderms have a unique system of internal tubescalled a water vascular system. The water vascular systemopens to the outside through a sievelike structure called themadreporite (ma-druh-POR-ight). In starfish, the madreporiteconnects to a tube called the ring canal that forms a circlearound the animal's digestive system. From the ring canal, fiveradial canals extend into each body segment. Attached to eachradial canal are hundreds of movable tube feet. The entirewater vascular system operates like a series of living hydraulicpumps that can propel water in or out of the tube feet. Whenwater is pushed into a tube foot, the tube foot expands. Whenwater is pulled out, the cup on the end of the tube foot shrinks,

Figure 29-2 Starfish, or sea star,species vary greatly. This bat starlives in the ocean off the coast ofsouthern California. Lacking the thinarms of other starfish, this speciesresembles a pentagon.

creating a partial vacuum that holds onto whatever the foot istouching. In this way, the tube feet act like living suction cups.A single tube foot alone cannot accomplish much, buthundreds acting together create enormous force. All echino¬derms "walk" with their tube feet, and some use their tube feet

for feeding.

FEEDING Echinoderms have several methods of feeding.Carnivores, such as many species of starfish, use their tube feetto pry open the shells of bivalve mollusks such as clams andscallops. Once the bivalve's shell is opened, the starfish flipsits stomach out of its mouth, pours out enzymes, and digests itsprey in the prey's own shell. When the starfish has finisheddining, it moves its stomach back into its mouth, leaving be¬hind an empty shell as the only evidence of its deed. Starfishalso eat snails, corals, and even other echinoderms. Herbi¬vores, such as sea urchins, scrape algae from rocks by usingtheir five-part jaw. Filter feeders, such as sea lilies, basketstars, and some brittle stars, use tube feet on flexible arms tocapture plankton that float by on ocean currents. Detritusfeeders, such as sea cucumbers, move much like a bulldozeracross the ocean floor, taking in a mixture of sand and detritus.Then, like an earthworm, they digest the organic material andpass the sand grains out in their feces.

RESPIRATION Echinoderms, like other animals, need toexchange carbon dioxide for oxygen. In most species the thin-walled tissue of the tube feet forms the main respiratory sur¬face. In some species small outgrowths called skin gills also

function in gas exchange.

INTERNAL TRANSPORT The functions of transportingoxygen, food, and wastes—which are performed by the circula¬tory system in many animals—are shared by different systems

Figure 29-3 The pressure exertedby the water vascular system of thestarfish (left) moves the animalalong the ocean bottom. Thedigestive system of a starfish (right)breaks down food, which is thentransported throughout the animal'sbody in the digestive glands and inthe fluid within the body cavity.

Upperpart ofstomach

Lowerpart ofstomach

639

Figure 29-4 Tube feel, located onthe underside of a starfish's arms,are used to perform many lifefunctions (left). Starfish use theirtube feet to open clams (right). Oncethe starfish has opened the clamshell, it flips its stomach out of itsmouth and begins to digest the clamright in the shell. No wonder starfishare not welcome in clam beds! EXCRETION In almost all echinoderms, solid wastes are

released through the anus in the form of feces. (The exceptionsare brittle stars, which lack an anus and thus release undi¬gested materials through the mouth.) Echinoderms, like manyother marine invertebrates, excrete nitrogen-containing cellu¬lar wastes primarily in the form of ammonia. Wastes seem to beexcreted in many of the same places around the body in whichgas exchange takes place—the tube feet and the skin gills.

RESPONSE As you might expect in animals that have nohead, echinoderms have primitive nervous systems. Mostechinoderms have a nerve ring that surrounds the mouth andradial nerves that connect the ring with the body sections.Scattered sensory cells that are sensitive to chemicals releasedby potential food are also characteristic of most species. Star¬fish additionally have up to 200 light-sensitive cells clusteredin eyespots at the tip of each arm. Although these structureshave lenses, they do little more than tell the animal whether itis light or dark. Some echinoderms also possess statocysts(simple organs for balance that tell an organism whether it isright side up).

Although you might think that the tough, spiny skins ofechinoderms protect them from predators, spines actually offerprotection for only a few species—the crown-of-thorns starfish,for example." Many predators have learned ways around the

in echinoderms. Because respiration (gas exchange) and theremoval of metabolic wastes occur through skin gills and tubefeet located all over the body, a system to deliver oxygen andcarry away carbon dioxide and other wastes is not essential.The distribution of nutrients is performed primarily by the di¬gestive glands and the fluid within the body cavity.

spiny defenses of echinoderms. For example, basket stars,feather stars, and spiny sea urchins are very slow moving.Clever fishes (and you will meet some clever fishes in Chapter31) have learned to turn these animals over and attack themthrough their unprotected underside. For this reason, manyechinoderms hide under rocks and in crevices by day, comingout to feed at night, when most predators are asleep.

MOVEMENT Most echinoderms use tube feet and thinlayers of muscle fibers attached to the plates of the endoskele-ton to move. An echinoderm's mobility is determined in partby the structure of its endoskeleton, In sand dollars and sea ur¬chins, the plates are fused together to form a rigid box that en¬closes the animal's internal organs. These animals usually havemovable spines attached to their endoskeleton, which they usealong with their tube feet to creep from one place to another orto burrow in the sand. In starfish, brittle stars, and featherstars, the skeletal plates move around a series of flexible joints,enabling these echinoderms to use their arms for locomotion.Feather stars can swim for short distances by flapping theirarms like wings, but starfish and brittle stars are only able tocrawl. In sea cucumbers, the plates are reduced to tiny vestigesinside a soft, muscular body wall. The loss of the plates makesthe body of sea cucumbers very flexible. Some species are ableto crawl along the ocean floor like large, fat worms by contract¬ing the muscles of the body wall.

REPRODUCTION Most echinoderms are either male orfemale, although some are hermaphrodites. In starfish, thesperm or eggs are produced in testes or ovaries, respectively,which fill the arms during the reproductive season. The ani¬mals shed their sperm and eggs into the water. Individual star¬fish detect gametes of their own species in the water, and theyrespond to that stimulus by releasing their own gametes. Fertil¬ization takes place in open water, and larvae swim around forsome time as members of the huge community of plankton thatswarm in the ocean. Eventually the larvae, which have bilateralsymmetry, swim to the ocean bottom, where they mature andmetamorphose into adults that have radial symmetry.

Many starfish have incredible abilities to repair themselveswhen damaged. In fact, if a starfish is pulled into pieces, eachpiece can grow into a new animal as long as it contains a por¬tion of the central part of the body. This ability of a starfish toregenerate itself has caused a great deal of trouble to peoplewho earn their living fishing for bivalves (two-shelled mol-lusks). In the past, angry shellfishermen who were aware thatstarfish ate bivalves would tear the animals into two or threepieces and toss them overboard. Imagine their surprise whenthey noticed even more starfish in their bivalve beds. Today,shellfishermen know that starfish have the ability to regenerateand that every piece of torn starfish they throw back could de-

Figure 29-5 The basket starspreads its branching arms to filterparticles of food from the water.When disturbed, the basket star curlsup these arms and exposes thearmored surface for protection.

Figure 29-6 Unlike adults, whichmostly crawl along the oceanbottom, echinoderm larvae are free-swimming. These larvae resembleclosely the free-swimming larvae ofinvertebrate chordates.

641

The almost 6000 species of living echinoderms are found inalmost every ocean in the world. However, no echinodermshave ever entered fresh water, and they cannot survive for longon land. Although they share certain characteristics, echino-derm species are remarkably diverse in appearance. The fol¬lowing descriptions of echinoderm classes will provide a briefintroduction to these animals.

STARFISH This class contains the common starfish,which are also known as sea stars. Starfish occur in manycolors, and you may be surprised to learn that many specieshave more than five arms. Starfish creep slowly along theocean bottom. Most are carnivorous, preying upon the bivalvesthey encounter as they move. Some species of starfish are im¬portant predators in rocky areas along the coast.

Figure 29-7 The brittle star gets itsname from the fact that it can shedits arms when it is threatened (left).This distracts predators so that thebrittle star can escape. In time, itwdl re grow the missing arm. Somestarfish, such as the sun star, havemore than hue arms (right).

BRITTLE STARS These animals live in tropical seas,especially on coral reefs. They look much like a common star¬fish, but they have longer, more flexible arms and are thus ableto move much more rapidly. In addition to using speed for pro¬tection, brittle stars protect themselves by shedding one ormore of their arms when attacked. The detached parts keepwriggling violently, distracting predators, while the rest of theanimal escapes. Brittle stars are filter and detritus feeders thathide by day and wander around in search of food only underthe cover of night.

The Echinoderm Classes

SEA CUCUMBERS As their name implies, these echino¬derms look like warty moving pickles with a mouth at one endand an anus at the other. Most sea cucumbers are detritusfeeders. Although these animals are not numerous in shallowwater, herds containing hundreds of thousands of them oftencover areas of the sea floor at great depths. A few species of seacucumbers expel sticky substances that attach to a predator.The predator, in all probability an attacking crustacean, is im¬mobilized as it is glued into a helpless ball.

Figure 29-8 The slate urchin(top, right) has thick, strong spinesthat were once harvested for useas implements for writing onslateboards—thus, its name. Seaurchins have a lanternlike set ofbony plates inside their body thatpower their jaws (inset). The sanddollar (top, left) gets its namefrom its flattened, coin-shapedappearance.

SEA URCHINS AND SAND DOLLARS This class includesdisk-shaped sand dollars, oval heart urchins, and round sea ur¬chins. Many of these animals, which are found in marine envi¬ronments all over the world, are grazers that eat largequantities of algae. Others are detritus feeders. Heart urchinsand sand dollars live hidden in burrows that they dig in sandor mud. Most sea urchins wedge themselves in crevices in rockduring the day and come out only at night. However, many seaurchins have formidable defenses in the form of long, sharpspines. One type of sea urchin even has small blue poison sacscovering the tips of each spine, ensuring that wounds it inflictswill be painful!

SEA LILIES AND FEATHER STARS These filter feeders,which have 50 or more long, feathery arms, comprise the mostancient class of echinoderms. Although sea lilies and featherstars are not common today, a rich fossil record indicates that

643

Figure 29-9 Acting much like aliving vacuum cleaner, this seacucumber (top) moves along theocean bottom swallowing organicmaterial along with sand. Sea lilies(center) and feather stars (bottom)feed by filtering floating organicmaterial from the water. The namesreflect the delicate beauty of theseanimals.

they were once widely distributed. Sea lilies are sessile animalsthat are attached to the ocean bottom by a long, stemlike stalk.Modern sea lilies live at depths of 100 meters or more. Manyfeather stars live on coral reefs, where they perch on top ofrocks at night and use their tube feet to catch floating plankton.

How Echinoderms Fit into the WorldEchinoderms are numerous in most marine habitats. In

many areas, starfish are important carnivores that control thepopulations of other animals. Sometimes their numbers rise orfall suddenly, causing major changes in the numbers of otherforms of marine life. For example, several years ago the coral-eating crown-of-thorns starfish suddenly appeared in greatnumbers over wide areas of the Pacific Ocean. Within a shortspan of time, these starfish caused extensive damage to manycoral reefs. The extent of their damage surprised and alarmedmarine biologists, many of whom took drastic action to kill thestarfish by injecting them with poisonous chemicals. We still donot know what caused this population explosion in the crown-of-thorns starfish or what will be its long-term effects on coralreefs.

In many coastal areas, sea urchins are important becausethey control the distribution of algae. However, if present inlarge numbers, they can threaten to literally "eat out of houseand home" the other dwellers that share this habitat.

In various parts of the world, some echinoderms—for ex¬ample, sea urchin eggs and sea cucumbers—are considereddelicacies by some people. Many more echinoderms, however,are useful as research subjects and as possible sources of medi¬cine. Several chemicals extracted from starfish and sea cucum¬bers are currently being studied as potential anti-cancer andanti-viral drugs. Sea urchins have been the subject of pioneer¬ing studies in embryology. These animals are easy to study be¬cause they produce large eggs that are fertilized externally anddevelop in plain sea water. Sea urchin embryos also make ex¬cellent subjects for testing the effects of drugs on cell divisionand development.

SECTION REVIEW

1. What is an echinoderm? Name five kinds ofechinoderms.

2. How do tube feet help echinoderms to carry out theiressential life functions?

3. How do starfish move? How do starfish open bivalves?

4. Connection—Ecology Why is tearing a starfish apartand throwing it back into the water not a good way tolimit a starjish population?

644

29-2 Invertebrate ChordatesThe phylum Chordata. to which fishes, frogs, birds, snakes

dogs cows and humans belong, will be the subject of many ofthe chapters to come. Most of the chordatesvertebrates, which means that they have backbones, so theyare placed in the subphylum Vertebrata. But there are alsosome invertebrate chortates. The ¦"-rtebrate chordates aredivided into two subphyla-tumcates and lanceleta Becausethev show possible links between vertebrates and the rest othe animal kingdom, the invertebrate chordates are of great

evolutionary interest.

What Is a Chordate?Members of the phylum Chordata are chorda^'

Chordates are animals that are characterized by a noto-chord a hollow dorsal nerve cord, and pharyngeal (thro )slits Some chordates possess these distinguishing characteris¬es as adults; Others, only as embryos. However, all chorda esdisplay these three characteristics at some stage of their lite.

The first chordate characteristic, the notochord, m a lonshflexible supporting rod that runs through at least part of thebody usually along the dorsal surface just beneath the nervecord' Most chordates have a notochord only during the early

part of embryonic life. In most vertebrates, the notochord is

-^VtSrirracteristic, the hollow do.alnerve cord runs along the dorsal surface just above the noto¬chord. Remember that in most invertebrates, nerve cords runalong ventral surfaces. In most chordates, the front end of thisnerve cord develops into a large brain. Nerves 'his cordat regular intervals along the length of the animal and connectto internal organs, muscles, and sense organs.

The third chordate characteristic, pharyngeal slits are

paired structures in the pharyngeal (fuh-RmH-jee-uhO othroat, region of the body, (Remember that pharynx anotheword for throat.) In aquatic chordates such as lancelets andfishes the pharyngeal slits are gill slits that connect the pha-rvngeal cavUy ^th the outside. The location of gills is very im¬portant Many invertebrates have gills of some sort in variouplaces but only chordates have pharyngeal gills. In terrestnachordates that use lungs for respiration, pharyngeal slits arepresent for only a brief lime during the development of he em-brvo These slits soon close up as the embryo develops Inchordates such as humans, pouches form in the pharyngeal re-

ion but never open up to form slits, ^r this reason some sci¬entists regard pharyngeal pouches, not slits, as the

chordate characteristic.

TUNICATES Tunicates are small marine chordates thateat plankton they filter from the water. They get their name

Guide For ReadingWhat are chordates? What areinvertebrate chordates?What are the distinguishingcharacteristics of chordates?Why are invertebrate chordatesimportant to evolutionarybiologists?

Figure 29-10 Although it seemslike a simple animal, the tunicate isa chordate. It is, therefore, a relatweof ours—although a very distantone.

Figure 29-11 Sea squirts aretunicates. /Ls' adults, these organismsare sessile, living firmly attached toone place. However, the larvae ofthese animals, like the larvae ofechinoderms, are free-swimming.

Figure 29-12 Lancelets are smallfish/ike creatures that often live withtheir body half buried in sand. Theyfilter food particles from the water.

from a special body covering called the tunic. Only the tad¬pole-shaped larvae of tunicates have a notochord and a dorsalnerve chord. When most tunicate larvae mature, they undergometamorphosis and become sessile adults that grow into colo-

aTltsirf h0 3 ^ SUrfaCe- B0th 'arval tunicates andadults filter feed and breathe at the same time through a pha¬ryngeal basket pierced by gill slits.

LANCELETS The small fishlike creatures called lanceletsive in the sandy bottom of shallow tropical oceans. Unlike tun¬

icates, adult lancelets have a definite head. They have a mouth

nair.Tf^n1".-? a, g Pharyn§eaI region with up to a hundredpairs of gill slits. Lancelets feed by passing water through theirpharynx, where food particles are caught in a sticky mucus.

is mucus is swallowed into a digestive tract that starts at onend of the pharynx and continues straight through the animal

to the anus, near the tail.

^aVe 3 simP,e' P"mitive heart that pumps bloodlanrf»i t Ve*seis In a closed circulatory system. Additionally,ancelets show evidence of segmentation in the arrangement of

n nerves and muscles. A lancelet's muscles are organizedto V-shaped units that are paired on either side of the body

Each muscle unit receives a branch from the main nerve cordA similar segmented nerve and muscle organization is found inall living vertebrates. Unlike most vertebrates, lancelets haveno jaw. Their mouth is composed entirely of soft tissues. Lan-ceJets also lack appendages and can move only by bendingtheir bodies back and forth. 8

How Invertebrate Chordates Fitinto the World

In some ways, studying invertebrate chordates is like usinga time machine to study the ancestors of our own subphylum. Itis important to remember that living vertebrates did not evolvefrom hying lancelets or tunicates. Both these subphyla haveevolved oyer time. However, similarities in structure and em-bryolog'cal development indicate that vertebrates and inverte-b ate chordates evolved from common ancestors manymillions of years ago.

1. What characteristics are found in a chordate?

2. What characteristics of tunicates and lancelets makethem seem like close relatives of vertebrates?

3. Critical Thinking—Making Comparisons Whichcharacteristics of tunicates and lancelets are unlikevertebrate cbaracteristics? Salps are free-swimming invertebrate chordates found in the open ocean.

The Secret Life of SalpsSometimes a remarkably simple change in

the techniques biologists use to study theworld causes us to alter our ideas about theway the living world works. For example, theinvention of the microscope opened up theworld of "unseen life." Recently, new methodshave contributed to our understanding of theonce mysterious open-water tunicates known

as salps.Salps are free-swimming animals that live

in the open sea. Biologists have known abouttheir existence for many years, but they knewlittle about their importance. This was due tothe fact that research vessels had no way ofcollecting and identifying salps. These beauti¬ful animals are so fragile that they literally fallto pieces if they are handled roughly. And thatwas exactly what happened in the collectingnets marine biologists used to gather plankton.Any salps that entered the net were squeezedinto a clear, featureless mush.

Once scuba-diving scientists became suffi¬ciently experienced (and sufficiently brave) tohop off their boats in the middle of the ocean,however, our knowledge of salps increaseddramatically. It became clear that salps areeverywhere. Giant herds of salps drift just be¬neath the surface. Certain species form hugesnakelike colonies that stretch for manymeters.

By collecting salps carefully, researchershave learned how they live. In many places,salps form important links in the ecology ofthe open sea. Salps eat certain plankton andare themselves food for other plankton, seaturtles, and certain fishes. This new knowledgehas come to light because of a simple changein research techniques. Although these vitalcreatures have been nearly ignored for dec¬ades, marine biologists can now study themclosely and discover how they fit into the webof life.

647

PROBLEM

How can echinoderms be identified?

MATERIALS (per group)

assorted echinoderms(pictures or preserved specimens)

PROCEDURE

1. On a separate piece of paper, draw a data tablesimilar to the one shown here.

2. Your teacher will provide either pictures orpreserved echinoderm specimens. Each speci¬men will be numbered.

3. Use the key to identify each numbered speci¬men. Start at step 1 and read descriptions Aand B. Only one of the descriptions correctlyapplies to the specimen you are examining. Atthe end of a description is the identity of thespecimen or directions to proceed to anotherstep. Continue to follow the directions step bystep until you identify the specimen.

4. After you identify the specimen, write its namenext to its identification number in the datatable. Then proceed to the next numberedspecimen.

G ECHINODERMS

SpecimenIdentificationNumber

Identity of Specimen(from Identification Key)

1

2

34

L^- —- OBSERVATIONS

1. What feature did all of the echinoderms youexamined have in common?

2. How did the echinoderms you examineddiffer?

3. Did any echinoderms with visible differenceshave the same identity?

ANALYSIS AND CONCLUSIONS

1. How is the use of an identification key similarto the process of classification?

2. Why is it possible for two organisms that lookdifferent to have the same identity based onthe key used in this investigation?

Identification Key

1 A. Has obvious radial symmetryB. Appears to have bilateral symmetry Go to 2A. Has arms or branches CucumberB. Spherical, oval, or disk shaped Co to 3

3 A. Arms in multiples of five Go to 5B. Arms are branched and feathery Go to 4

4 A. Arms are long, slender, and flexible Go to 7B, Arms are thick and less flexible Star

o A. Spherical; covered with spines StarfishB. Not spherical Sea Urchin6 A- Oval Go to 6B. Flattened disk Urchin

7 A. Has a long stalk !. Sand DollarB. Stalk short or absent Sea Lily

' ' ""' Feather Star

648

•v 1r u d E imjS 1r ii ii ¥6 I % &

SUMMARIZING THE CONCEPTS

The key concepts in each section of this chapter are listed below to help youreview the chapter content. Make sure you understand each concept and itsrelationship to other concepts and to the theme of this chapter.

29-1 Echinoderms

• Echinoderms are spiny-skinned animals withfive-part radial symmetry, an internal skele¬ton, a water vascular system, and suction-

cuplike structures called tube feet.

• Echinoderms are marine animals; no echino¬derms live in fresh water or on land. Certainstages in the development of echinodermlarvae are similar to stages seen in membersof the phylum Chordata.

• All echinoderms have a water vascular sys¬tem that opens to the outside through asievelike structure called the madreporite.

• In a starfish, the madreporite connects to atube that leads to the ring canal, a part of thewater vascular system, which forms a circlearound the starfish's digestive system. Nu¬trients are moved around the animal in thedigestive glands and the body cavity.

• Starfish reproduce externally by pouringeggs and sperm into the ocean water. Thelarvae that result from a fertilized egg float aspart of the plankton.

• In spite of their fragile appearance, echino¬

derms are important predators in manyenvironments.

29-2 Invertebrate Chordates

• Most chordates are vertebrates, whichmeans they have backbones. However, a fewchordate species are invertebrates. Thereare two subphyla of invertebrate, chordates—tunicates and lancelets.

• At some stage of their life, all chordates pos¬sess these distinguishing characteristics: anotochord, a hollow dorsal nerve cord, andpharyngeal (throat) slits.

• Tunicates are small chordates that live in theocean. Tunicate larvae resemble tadpolesand can move around in the water.

• Lancelets are small fishlike creatures thatlive in sandy ocean bottoms. Adult lanceletshave a definite head.

9 Invertebrate chordates are important be¬cause they indicate that vertebrate and in¬vertebrate chordates evolved from commonancestors many millions of years ago.

REVIEWING KEY TERMS

Vocabulary terms are important to your understanding of biology. The key termslisted below are those you should be especially familiar with. Review these termsand their meanings. Then use each term in a complete sentence. If you are notsure of a term's meaning, return to the appropriate section and review its definition.

29-1 Echinodermsechinodermwater vascular systemtube feet

29-2 InvertebrateChordateschordatenotochord

hollow dorsalnerve cord

pharyngeal slit

649

CONTENT REVIEW

Multiple Choice

Choose the letter of the answer that best completes each statement.

1. A kind of echinoderm that is eaten by somepeople is aa. sea urchin. c. starfish.b. sea lily. (j lancelet.

2. To open a clam, a starfish uses itsa. tube feet. c. madreporite.b. brain. d stomach.

3. Echinoderms havea. a backbone.

b. a long history on Earth.c. lungs.

d. smooth skin.4. Echinoderms show

a. bilateral symmetry.b. top and bottom symmetry.c. radial symmetry.d. no symmetry.

True or False

Determine whether each statement is true or false If itis false, change the underlined word or words to make

1. All echinoderms have bilateral symmetrv. fi2. In echinoderms. tube feet and skin gills

used in respiration and excretion. 73. Lancelets have a primitive heart.4. Echinoderms have nerves attached to 8

plates in their endoskeleton.5. A sea cucumber is a herbivore.

5. Digested nutrients are moved around thebody of a starfish in itsa. skin gills. c. bony plates.b. digestive glands, d. water vascular

system.6. Tunicates and lancelets are examples of

a. vertebrates. c. echinoderms.

^ ^sh- d. chordates.7. The side of an echinoderm where the

mouth is located is called thea. aboral surface. c. oral surface.

b. tunicate. d. vascular surface.8. Invertebrate chordates lack a

a. larva. c. nerve cord.

b. notochord. d. backbone.

is true, write "true." If itthe statement true.

Tube feet are able to create suction whenair is pumped out of them.Some echinoderms have madreporites thattell them whether they are right side up.If a piece of a starfish contains a portion ofthe central part of the body, the piece isable to regenerate.

Word Relationships

A. An analogy is a relationship between two pairs of words or phrases senera/lvwrmen ,n the fo lowing manner: abed. The symbol: is read Cto'andthe

anTa'l aj^ls toplan,^^ ^ * -

1. starfish:echinoderm::tunicate: 2. sea cucumberrdetritus feeder::feather smr ~ 3. eyespots:light::statocysts:4. madreporite.aboral surface::mouth:__

B. In each of the following sets of terms, three of the terms are related. One termdoes not belong. Determine the characteristic common to three of the terms andthen identify the term that does not belong.

5. starfish, sea lily, lancelet, sea urchin6. ring canal, radial canal, tube feet, skin7. notochord, hollow dorsal nerve cord, pharyngeal slits, vertebrae8. tube feet, brain, water vascular system, madreporite

CONCEPT MASTERY

Use your understanding of the concepts developedof the following in a brief paragraph.

1. What is radial symmetry? Name an animalthat shows this kind of symmetry.

2. Briefly explain how a starfish eats a clam.3. How does the water vascular system of a

starfish help this animal to move?4. What structures on a starfish tell this

animal about its environment?

in the chapter to answer each

5. How do starfish reproduce?6. How does a sea cucumber feed?7. What characteristics does a lancelet share

with vertebrate chordates?8. Why is it not a good idea to break up a

starfish and throw the pieces back into thewater, especially if you fish for oysters?

CRITICAL AND CREATIVE THINKING

Discuss each of the following in a brief paragraph.

1. Making predictions Suppose that you areliving alone on a small tropical island inthe Pacific Ocean. This island is protectedby a coral reef that surrounds it. One daywhile you are skin diving, you noticeseveral crown-of-thorns starfish eating someof the coral animals that are part of yourreef. Predict what might happen if thecrown-of-thorns starfish increase in number.

2. Making comparisons Compare the formand function of a starfish and a seacucumber. Describe the animals'adaptations for movement and feeding.

3. Applying concepts Explain why manyfertilized starfish eggs never develop intoadult starfish.

4. Designing an experiment Your friend tellsyou that starfish can regenerate themselvesfrom even a small portion of an arm. Youchallenge this assumption. Design anexperiment to prove who is correct.

5. Using the writing process Suppose thathumans had the ability to regeneratethemselves. For example, an arm might beable to grow a whole new body. Write ascience fiction story that describes how thisprocess might work for a person who wasseverely injured in an automobile accident.