UNIT

3MARINEINVERTEBRATES

If you take a walk along the shore at low tide,

you may observe mussels attached to rocks, crabs

scurrying about in shallow water, or snails moving

slowly over rocks, grazing on the algae-coated

surfaces. If you dig in the sand, you may unearth

some sandworms. When you look into a tide pool,

you may find colorful sea anemones with tentacles

outstretched, waiting to catch a meal. What do all

these animals have in common? All of them are

invertebrates—that is, they have no backbones.

Marine invertebrates are divided into several

phylums. Your study of marine invertebrates actu-

ally began in Chapter 6, with an examination of

the rotifer, sponge, and bryozoan. In this unit, you

will learn more about invertebrates and the adap-

tations they have that enable them to survive in

the marine environment.

CHAPTER 7 Cnidarians

CHAPTER 8 Marine Worms

CHAPTER 9 Mollusks

CHAPTER 10 Crustaceans

CHAPTER 11 Echinoderms

171

When you have completed this chapter, you should be able to:

DISCUSS the characteristics and life cycle of the jellyfish.

COMPARE and CONTRAST the corals and sea anemones.

EXPLAIN how coral reefs are built up over time.

DESCRIBE some important features of the hydroids.

A swimming race in the ocean is canceled when the competitorscomplain of painful stings; a boat sinks into the waters off the FloridaKeys when its hull is ripped open by an underwater hazard. What dothese two mishaps have in common? The swimmers in the race werestung by jellyfish; and the ship sank after it hit a coral reef. Both inci-dents were caused by animals that have stinging tentacles.

Animals with stinging tentacles are classified in the phylumCnidaria. Besides jellyfish and corals, the phylum Cnidaria includes sea anemones (shown above), hydras, and other similar animals.There are more than 9000 species in this phylum, divided into threeclasses—Scyphozoa, Anthozoa, and Hydrozoa. In this chapter, you willfind out how these unusual animals carry out their life functions.

172

7.1Jellyfish

7.2Sea Anemones

7.3Corals

7.4Hydroids

Cnidarians77

7.1 JELLYFISH

Jellyfish are typical members of the phylum Cnidaria. Members ofthis phylum, called cnidarians, are characterized by two cell layers,a saclike digestive tract, and tentacles. In addition, cnidarians’ ten-tacles, which are long, flexible appendages, are arranged in a ringaround a central mouth, thereby giving these animals radial sym-metry. Cnidarians have no brain, so there is no central coordina-tion of movement. Instead, a network of nerve cells and receptorcells make up the nerve net, which is a simple nervous system;when one part of the body is stimulated, the whole animalresponds. (See Figure 7-1.)

The jellyfish is not, in fact, a fish. Jellyfish differ from true fish inmany ways; the most fundamental difference is that the jellyfish, asan invertebrate, does not have a backbone. Now you can understandthe meaning of the expression “You’re as spineless as a jellyfish.”

Jellyfish are members of the class Scyphozoa. All members ofthis class have an umbrella-shaped structure called the medusa,with tentacles hanging down from it. The medusa is composed oftwo membranes: an epidermis, or outer membrane, and a gastro-dermis (meaning “stomach skin”), or inner membrane. Lyingbetween the two membranes is a jellylike mass called the mesoglea.The shape and thinness of the membranes, and the low density of

Cnidarians 173

Medusa

Mesoglea

Epidermis

Gastrodermis

Digestive cavity

Tentacles

Mouth

Figure 7-1 Structure ofthe jellyfish, a typicalcnidarian.

the layer of mesoglea between them, enable a jellyfish to float withease. You may recall that jellyfish are considered part of the plank-ton population. However, they do have limited abilities of locomo-tion. Jellyfish use muscles to contract their medusa in a rhythmicfashion, causing them to pulsate gently through the water. Since jel-lyfish are such weak swimmers, they are often deposited on sandybeaches by waves and tides. (See Figure 7-2.)

How does the jellyfish carry out respiration? Since the mem-branes of a jellyfish are so thin, oxygen diffuses directly from thewater into the animal’s cells. The waste gas, carbon dioxide, diffusesin the opposite direction, from the cells into the water. This gasexchange occurs over the entire surface of the jellyfish.

Feeding in the Jellyfish

“Are there any jellyfish in the water?” Bathers will often ask a life-guard this question before diving into the ocean. Swimmers are con-cerned because some species of jellyfish, such as the lion’s manejellyfish (Cyanea), can inflict painful stings. The sting of some jelly-fish, such as the sea wasp, has even been known to cause death. Jel-lyfish use their stinging tentacles for defense and for getting food.The tentacles contain stinging cells, called cnidoblasts. Inside eachcnidoblast there is a coiled thread with a barb at the end, called anematocyst. (See Figure 7-3.) In most species, the barb contains aparalyzing toxin.

The nematocysts can be discharged in response to either amechanical or chemical stimulus. Just touching a tentacle (an exam-ple of a mechanical stimulus) is sufficient to cause a nematocyst to

174 Marine Invertebrates

Figure 7-2 A jellyfishstranded on the shore bywaves.

Barb

Dischargednematocyst

CnidoblastCnidoblast

Coilednematocyst

Trigger

Figure 7-3 Cnidoblastswith the nematocystscoiled and discharged.

discharge. (Note the “trigger” in the cnidoblast.) However, once anematocyst is discharged, it cannot be used again. New nematocystsare constantly produced to replace discharged ones. When thenematocysts are discharged into prey, such as a fish, they paralyze it.Contracting tentacles bring the fish up to the mouth, located in thecenter of the medusa, where it is ingested (taken in). Most of thefood is digested in the saclike digestive cavity by enzymes secretedby its lining. Digestion in the jellyfish is both extracellular (in thedigestive sac) and intracellular (in food vacuoles). Waste productsare egested, or expelled, through the mouth.

The jellyfish can also catch food with its medusa. The surface ofits medusa produces a sticky mucus. As a jellyfish moves throughthe water, plankton get stuck on this surface. Tracts of ciliated cellsmove the food from the medusa to the mouth, where it is ingested.

Life Cycle of the Jellyfish

How do jellyfish reproduce? Jellyfish have separate sexes, and theirlife cycle includes both sexual and asexual reproduction phases. Youcan see the stages in the life cycle of the moon jelly, Aurelia, illus-trated in Figure 7-4 on page 176. Aurelia is a saucer-shaped jellyfishthat has a weak sting; it often washes up on the beach during thesummer. In the Aurelia, as in many other jellyfish, the ovaries andtestes are organized into a four-leaf-clover pattern in the medusa.During the sexual phase, the testes produce sperm, which swim outof the male’s mouth and into the female’s mouth and digestive sac.The sperm fertilize the eggs in the ovary. Some species of jellyfishrelease eggs and sperm directly into the water, where external fertil-ization then occurs.

The fertilized egg cell, or zygote, passes through a series of devel-opmental stages before becoming an adult. As in sponges and otheranimals, organisms in the early stages of development are calledembryos. The first embryonic stage occurs when the zygote divides intwo. These cells continue to divide until the solid ball of cells calleda morula is produced. Following the morula stage, the hollow ballof cells called a blastula is formed. (Refer to Figure 6-6.)

Shortly thereafter, the surface cells of the growing blastulabecome ciliated, forming a swimming larva called the planula. Insome cases, the planula larva attaches to a hard substrate and then

Cnidarians 175

develops into a structure called a polyp. (In other cases, there is nopolyp stage.) The polyp has a mouth, tentacles, and digestive cavity,and its shape resembles an upside-down jellyfish medusa. Immaturejellyfish, in the form of small umbrella-shaped medusas, thendevelop and break off from the polyp by budding, which is the asex-ual phase in jellyfish reproduction. The life cycle is completed wheneach medusa develops into an adult jellyfish.

Comb Jellies

An animal that is often mistaken for a jellyfish, but is not a cnidar-ian, is the comb jelly. The comb jellies are classified in their ownphylum, Ctenophora. They all have eight rows of long, fused ciliathat look like combs, hence the group’s name. (See Figure 7-5.) The beating of the cilia helps move the animal through the water.

176 Marine Invertebrates

Ciliated larva (planula)

Young polyps

Mature polyp(asexual stage)

Young medusa

Mature medusa

Figure 7-4 Life cycle ofthe moon jelly Aurelia.

Figure 7-5 The comb jellyis a ctenophore (not acnidarian).

ENVIRONMENTFloating Beauties, Stinging Beasts

1. Which jellyfish is considered the most dangerous? Why?

2. How do the stinging cells of a jellyfish aid their survival?

3. Why are most victims unaware of the presence of jellyfish?

Cnidarians 177

Have you ever seen, or been stung by, a jelly-fish? Beware, jellyfish are found everywhere-from the Arctic to the Antarctic seas, and fromthe ocean surface down to the great depths.These largely transparent, often beautiful crea-tures look very delicate and vulnerable as theygently pulsate through the water. But don’t befooled. Jellyfish are among the invertebrates thatpossess deadly stinging cells for feeding and forprotection. Most marine animals have learnedto steer clear of jellyfish, with the exception ofthe loggerhead sea turtle, which regularly preyson them. Humans, too, have learned the hardway just how dangerous the “jellies” can be forpeople who enter the marine environment.There are more than 20,000 species of jellyfish,but only about 70 are potentially harmful tohumans. Which jellies are the most dangerous?Marine biologists agree that, of the dozens ofjellyfish species that can deliver a painful andtoxic sting, the following three top the “most

dangerous jellyfish” list. (Being largely transpar-ent, jellies are difficult to spot in the water.Check on jellyfish conditions before swimming.If you do get stung, wash your skin with vinegaror rubbing alcohol and immediately notify alifeguard.)

Lion’s Mane Jellyfish(Cyanea capillata)

Description: Largest of all the jellyfish,grows up to 2 meters wide (the medusa)and 40 meters long (the tentacles).

Danger: The sting causes painful reac-tions; it can be deadly.

Location: Atlantic Ocean, from the ArcticCircle to Florida and the Gulf of Mexico;Pacific Ocean, from Alaska to SouthernCalifornia.

Note: See photograph above.

Sea Nettle(Chrysara quinquecirrha)

Description: Grows up to 1 meter wideand 4 meters long; has reddish-maroonmarkings.

Danger: The sting causes painful reac-tions; it can be deadly.

Location: Atlantic Ocean, from Massachu-setts to Florida and the Gulf of Mexico;Pacific Ocean, from Alaska to SouthernCalifornia. It is particularly common inChesapeake Bay, Virginia.

Sea Wasp [Box Jelly](Chironex fleckeri)

Description: Very transparent, boxlikemedusa, can grow to size of a basketball;has approximately 60 tentacles, up to 5meters long.

Danger: The sting causes painful reac-tions; most dangerous of all jellies, it canbe deadly. Kills dozens of swimmers inAustralia every year. Its toxins attack car-diac, respiratory, and nervous systems.

Location: Pacific Ocean, near Australiaand the Philippines.

QUESTIONS

Along with short tentacles, the cilia also bring food, such as cope-pods, to the mouth. A nerve net controls movement of the cilia.Most species also have long tentacles to help capture food. Onespecies of ctenophore has nematocysts in its tentacles. Comb jelliesusually float on or near the surface of the water. They are biolumi-nescent, and light up when disturbed. Comb jellies release theirgametes into the water for external fertilization.

7.1 SECTION REVIEW

1. Explain how the jellyfish is adapted for floating.

2. How do jellyfish capture and digest their prey?

3. Describe how a typical jellyfish reproduces.

7.2 SEA ANEMONES

Sea anemones, which look like colorful underwater flowers, aremembers of the class Anthozoa (meaning “flower animal”). How arethey related to the jellyfish? Like jellyfish, sea anemones possessstinging tentacles, radial symmetry, and a nerve net. Unlike jelly-fish, the adult sea anemone lives as a polyp, attached to a substrateby means of a muscular foot. (See Figure 7-6.) As such, the seaanemone is a stationary, or sessile, animal capable of only limitedmovement. How, then, does the sea anemone respond to its envi-ronment? The sea anemone has a very simple nervous system; ifyou touch part of the sea anemone, the entire animal contracts. Forexample, if you touch a tentacle, the anemone quickly withdraws alltentacles into its fleshy polyp.

178 Marine Invertebrates

TentacleMouth

Base

Figure 7-6 The sea anemoneis a sessile cnidarian; theadult lives as a polyp.

Feeding in the Sea Anemone

How does the sea anemone capture its food? Typical of the cnidari-ans, the sea anemone uses its stinging tentacles to get food. Tinyorganisms and small fish that swim into the tentacles are paralyzedby the discharging nematocysts. You can observe discharging nema-tocysts in a sea anemone by doing the laboratory investigation atthe end of this chapter. One type of sea anemone (Metridium) cap-tures its food as jellyfish do, by trapping it in the mucus and cilia onits surface.

Interestingly, some large sea anemones have little shrimp andclownfish that live unharmed among their tentacles. The clownfishhave a protective skin coating, so they are not injured by the sting-ing tentacles. By living in the midst of the tentacles, clownfish gainprotection from predatory fish; at the same time, they protect theanemone’s tentacles from being bitten by other fish. The shrimphelp keep the anemone clean and may also help protect it frompredators. This kind of mutually beneficial relationship between dif-ferent species is known as symbiosis. The participants are referred toas symbionts. (See Chapter 21 for more on symbiosis.)

After an anemone captures its prey, it brings the food to itsmouth. Food is digested inside the digestive sac by enzymes. As inthe jellyfish, digestion in the sea anemone is mostly extracellular.Vacuoles in specialized cells in the digestive tract’s lining engulf anddigest food particles, so there is intracellular digestion as well. Undi-gested food particles and other wastes are eliminated through themouth. Because of the saclike structure of the digestive cavity, bothingestion and elimination occur through the mouth. Therefore,digestion in sea anemones and jellyfish occurs in what is called atwo-way digestive tract.

Life Cycle of the Sea Anemone

The pattern of sexual reproduction in the sea anemone is very sim-ilar to that of the jellyfish. However, the medusa stage, typical ofthe jellyfish, is not present in the sea anemone life cycle. The dom-inant structure in the sea anemone is the polyp. Some sea anemonescan reproduce asexually by splitting in half, or they can regenerate

Cnidarians 179

from pieces of the polyp’s base that are broken off as an adultanemone moves along the substrate. They can also reproduce sexu-ally. Anemones produce embryos that develop into planula larvae,which settle and develop into adult polyps.

7.2 SECTION REVIEW

1. How does the adult body form of a sea anemone differ fromthat of a jellyfish?

2. Describe how the sea anemone obtains its food.

3. How does the sea anemone respond to stimuli?

7.3 CORALS

One of the most spectacular structures in nature is a coral reef. Someare huge, like the Great Barrier Reef off the coast of Australia, whichis about 2000 km long and 80 km wide. However, the organismresponsible for the formation of reefs is so small that it might escapenotice outside of its stony home. In fact, the coral animal is so tinythat some can be seen only through a microscope. Yet, differentspecies of coral animals are capable of building a variety of struc-tures that range in size from small and gemlike to the massive reefsthat can pose a danger to ships. Corals come in a variety of shapesand sizes, often resembling familiar objects for which they arenamed.

Coral Types and Structure

There are two types of corals: stony (hard) corals and soft corals. Thestony corals are made up of limestone (calcium carbonate), and theycan form massive stony structures. Examples of hard corals are thebrain coral, staghorn coral, and star coral. The soft corals are com-posed of a fibrous protein, which gives them flexibility. Underwa-ter, the soft corals look more like plants than animals, as they swayback and forth with the waves and currents. Examples of soft corals

180 Marine Invertebrates

are the sea fan, sea whip, and sea plume. (See Figure 7-7, and refer toFigures 3-19 and 3-20.)

The basic structure of the coral animal is the coral polyp,shown in Figure 7-8 on page 182. As you can see, the coral polypresembles a very small sea anemone. Life activities such as inges-tion, digestion, sensitivity, exchange of gases, and reproduction aresimilar in both anemones and coral polyps. Due to their similaritiesin structure and function, corals are placed in the class Anthozoaalong with the sea anemones. However, unlike sea anemones, whichlive alone or in small groups, coral polyps live in large groups ascolonial animals, attached to one another by a thin membrane.The membrane connects the polyps’ digestive systems, so there isnutritional sharing among them.

The Coral Polyp

The coral polyp is a tiny mound of tissue that contains a saclikedigestive tract and a mouth surrounded by stinging tentacles. Inhard corals, each polyp sits in a cup-shaped depression that it formson the surface of the reef. The stony depression is composed of lime-stone, which is added to—layer by layer over many years—by thepolyps that live on the surface.

Cnidarians 181

Figure 7-7 A variety ofhard corals and soft coralscan be found together ona reef.

How does a coral polyp build a reef of stone? The polyp needs agreat deal of energy to build a stony reef. It gets its energy from thefood that it eats (tiny plankton trapped by its tentacles at night) andfrom the food that is made during photosynthesis. The photosyn-thesis is actually carried out by tiny symbiotic algae called zooxan-thellae, which live inside the tissue of each polyp. The zooxanthellaeare types of dinoflagellates that are captured by the coral polyp fromthe marine environment. As in all algae, the zooxanthellae containchloroplasts, which use the energy of sunlight to make energy-richsugar (glucose) from other simple compounds. The coral animals usethe glucose to supplement the energy they get from captured foodand to build their limestone homes.

The zooxanthellae benefit by living safely within the coralpolyps’ cells and receiving nutrients from the polyps. The polypsalso benefit by receiving oxygen as a by-product of the algae’s pho-tosynthesis. This is another example of symbiosis in marine organ-isms. The life activities of the coral polyps and the zooxanthellaebenefit one another.

182 Marine Invertebrates

Tentacles Coral polyps

Limestone

Digestivecavity

Mouth

Zooxanthellae

Figure 7-8 Structure ofthe coral polyp, shown incross section.

Coral Reefs

As mentioned in Chapter 3, a reef forms when coral larvae settleon a substrate in the sand and develop into polyps. What kindof building materials do the coral polyps need to construct thereef? A coral reef is a massive limestone structure composed ofthe compound calcium carbonate (CaCO3), the same material thatmakes up mollusk shells. To build the reef, the coral polyp needsa source of calcium and carbon. Cells within the polyp absorb cal-cium (Ca) from the seawater; and the zooxanthellae take up car-bon dioxide (CO2) that is produced by the polyp duringrespiration. When these two substances combine inside the tissueof the polyp, the CaCO3 needed to build the limestone reef isproduced.

Interestingly, the process of reef building is similar in principleto the production of bones and teeth in your own body. Teeth andbones are composed of the mineral calcium. Special cells in yourskeletal system remove calcium from the blood to build your teethand bones.

Coral reefs are found in tropical areas, where the waters arewarm and clear. Since the zooxanthellae require energy from sun-light, the waters that corals live in must be clear and free from sed-iments. And coral polyps need warm temperatures in order tosecrete their calcium carbonate skeletons. Each new generation ofpolyps lays down another layer of stone as the reef grows upward ata rate of about 2.5 cm per year. The reefs provide homes and habi-tats for many other animal species. Large reefs also protect nearbycoastlines from the harsh effects of ocean waves.

At low tide, the top of the reef may lie just beneath the water’ssurface. Ships that come too close may hit a reef and damage theirhulls. As a result, coasts with coral reefs are graveyards for manyships. Likewise, many reefs are damaged when ships run agroundon them. A large ship, or its anchor, can break off chunks of livingcoral and damage the protective mucus coating on the coral polyps.Coral reefs are also damaged by water pollutants, severe storms,freshwater runoff, sediments, and unusually low or high water tem-peratures (which can disturb the algae living within the polyps).When this happens, a condition known as “coral bleaching”occurs, in which the living polyps die off, and only the white coral

Cnidarians 183

skeletons remain. This problem has been noticed in several placesaround the world. (See Figure 7-9.)

7.3 SECTION REVIEW

1. Why are coral reefs found only in clear tropical waters?

2. Compare the coral polyp with a sea anemone. What are thesimilarities and differences?

3. Why is the coral polyp considered a colonial animal?

7.4 HYDROIDS

Another type of cnidarian that may resemble a plant is the hydroid.Hydroids, which are members of the class Hydrozoa, live in theintertidal and subtidal zones. Hydroids are actually colonial animals,made up of many individual polyps that function together as a sin-gle organism. The snail fur (Hydractinia) is a hydroid colony whosepink and reddish growth coats the shells of many kinds of oceananimals, including hermit crabs, There are some exceptions to therule; for example, the hydroid Tubularia is not colonial.

184 Marine Invertebrates

Figure 7-9 An example ofcoral bleaching: the polypshave died, leaving behindthe stony white skeleton.

One common hydroid, Obelia, forms bushy colonies attachedto rocks, seaweeds, and other substrates in the intertidal zone. Obeliais made up of two types of polyps: feeding polyps (part of the sessileasexual phase) that have nematocysts, and reproductive polyps thatform the brief medusa (free-swimming sexual) phase. (See Figure7-10.) The life cycle of Obelia is similar to that of both jellyfish andsea anemones. Like the jellyfish, hydroids have a medusa and aplanula larva. However, like the sea anemone (which has no medusaphase), the hydroid’s dominant phase is the asexual polyp.

Portuguese Man-of-War

An interesting and much-feared hydroid colony is the Portugueseman-of-war (Physalia), so named by British sailors in the eighteenthcentury as an insult to the ships of the Portuguese navy. Physalia isclassified with the hydrozoans, not with the true jellyfish which itresembles, because it is a colony made up of different types of

Cnidarians 185

Tentacles

Feeding polyp

Medusa bud

Reproductivepolyp

Figure 7-10 Structure ofObelia, a colonial hydroid;its dominant phase is thesessile asexual polyp.

polyps. Like other cnidarians, it has batteries of nematocysts on itstentacles. The sting of the Portuguese man-of-war is very painful.People become ill from the sting; some even die.

The different polyps that make up Physalia are adapted for avariety of functions. One kind of polyp makes up the gas-filled bagthat keeps the colony afloat. This gas-filled float, composed of manyindividual buoyant polyps, resembles a sail. Winds push against thesail, thus moving the colony along in the water. (Physalia does notactively chase its food.) Another kind of polyp makes up the sting-ing tentacles, which are several meters long and hang like a deadlycurtain beneath the float. Other polyps digest food that is caughtand killed in the tentacles. And, as in other hydroids, some polypsserve a reproductive function. All in all, the Portuguese man-of-waris quite a remarkable organism—yet one that you would not want tomeet up close and personal! (See Figure 7-11.)

7.4 SECTION REVIEW

1. Why are hydroids referred to as colonial animals?

2. List the different functions of hydroid polyps.

3. Why is the Portuguese man-of-war classified as a hydroid?

186 Marine Invertebrates

Figure 7-11 The Por-tuguese man-of-war,Physalia, a colonialhydroid.

Laboratory Investigation 7

PROBLEM: How can you observe the discharging nematocysts of a stingingtentacle?

SKILL: Using the compound microscope to observe tiny cell structures inaction.

MATERIALS: Tiny sea anemone or jellyfish tentacles, forceps, slides, coverslip,compound microscope, absorbent paper, medicine dropper, dilute acetic acid(or clear vinegar).

PROCEDURE

1. Cut a small piece of a fresh jellyfish tentacle or, using your forceps, remove atentacle from a sea anemone. (CAUTION: Do not touch the tentacles withyour hands!)

2. Place the tentacle on a glass slide. Put 1 or 2 drops of seawater on the ten-tacle. Carefully apply the coverslip.

3. Place the wet mount slide on the microscope stage under the low-powerobjective.

4. Look for the tentacle under low power. Focus along the edge of the tentacle.In your notebook, draw a section of the tentacle.

5. Now place a piece of absorbent paper on the glass slide to the left of thecoverslip. Fill the medicine dropper with clear vinegar or dilute acetic acid.

6. While looking through the microscope at the edge of the tentacle, squeeze 2or 3 drops of acid on the slide so that the acid makes contact with the rightside of the coverslip. Then place the absorbent paper in contact with the left

Observing Stinging Tentacles

Cnidarians 187

188 Marine Invertebrates

side of the coverslip. (See Figure 7-12.) The paper will absorb the seawaterand draw the acid into contact with the tentacle.

7. Look for many fine threads shooting out from the tentacle. These threadsare the discharging nematocysts. Sketch your observation. (Refer to Figure7-3 on page 174.)

8. Examine the nematocyst under high power. Draw and label the structure.

OBSERVATIONS AND ANALYSES

1. What is the adaptive value of stinging tentacles?

2. Why do you think the nematocysts reacted to the acetic acid?

3. Why do some fish avoid swimming near sea anemones?

Dropper

Coverslip

Slide

Absorbentpaper

Dilute acetic acid(or vinegar)

Tentaclespecimen

Figure 7-12 Acetic acid willmake the nematocysts in atentacle discharge.

Answer the following questions on a separate sheet of paper.

Vocabulary

The following list contains all the boldface terms in this chapter.

cnidarians, cnidoblasts, colonial animals, coral polyp, hydroid,medusa, mesoglea, nematocyst, nerve net, planula, polyp, radialsymmetry, sea anemones, symbionts, symbiosis, zooxanthellae

Fill In

Use one of the vocabulary terms listed above to complete each sentence.

1. A jellyfish’s umbrella-shaped structure is the ____________________.

2. The stinging cells inside tentacles are called ____________________.

3. An adult sea anemone lives as an attached ____________________.

4. The name for the algae that live inside coral polyps is____________________.

5. The Portuguese man-of-war is a dangerous colonial ____________________.

Think and Write

Use the information in this chapter to respond to these items.

6. Diagram the life cycle and reproduction of a jellyfish.

7. Describe the main difference between a coral colony and ahydroid colony.

8. Name two similarities and two differences between corals andsea anemones.

Chapter 7 Review

Cnidarians 189

Inquiry

Base your answers to questions 9 through 11 on the following diagram,which illustrates the life cycle of the moon jelly Aurelia, and on yourknowledge of marine biology.

9. What kind of reproduction is illustrated in this life cycle?a. sexual only b. asexual only c. both asexual and sexuald. parthenogenesis

10. Which statement is not correct regarding the stages of this lifecycle? a. The polyp stage lives attached to a substrate.b. The medusa drifts among other plankton. c. All stages ofdevelopment are free-swimming. d. The free-swimming andsessile stages alternate.

11. During which two stages does embryonic development takeplace? a. young medusa to mature medusa b. ciliatedlarva to young polyp c. young polyp to mature polypd. mature polyp to young medusa

Multiple Choice

Choose the response that best completes the sentence or answers thequestion.

12. Inside each stinging cell of an anemone is a coiled thread with a barb at the end, called a a. cnidoblast b. ciliac. nematocyst d. zooxanthellae.

190 Marine Invertebrates

Ciliated larva(planula)

Young polypsMature polyp

(asexual stage)

Young medusa

Mature medusa

13. A jellyfish responds to external stimuli through use of itsa. nematocysts b. mesoglea c. nerve netd. zooxanthellae.

14. Corals and jellyfish are classified together mainly because they a. share the same habitat b. have stinging tentaclesc. are tropical d. are planktonic.

15. The large reef-building corals grow only in tropical watersbecause of the warm temperatures and a. plentiful sunlightb. room for more growth c. greater water pressured. fewer predators.

16. Which of the following organisms with tentacles is leastrelated to the others? a. sea anemone b. brain coralc. octopus d. Portuguese man-of-war

17. All of the following are functions of the jellyfish medusaexcept a. reproduction b. locomotion c. protectiond. ingestion.

18. The cnidarians are characterized by the presence of stingingtentacles and a a. streamlined shape b. saclike digestivetract c. tube-shaped digestive tract d. ventral nerve cord.

19. The basic structure of the coral animal is the a. polypb. medusa c. nematocyst d. cnidoblast.

20. The beneficial living arrangement between the coral polypsand the tiny algae within them is known as a. radialsymmetry b. symbiosis c. zooxanthellae d. cnidarian.

21. Which of the following is planktonic in its adult stage?a. sea anemone b. elkhorn coral c. hydroid d. jellyfish

22. In the cnidarians, oxygen and carbon dioxide gas exchangeoccurs through the a. gills b. lungs c. mouth d. cellmembranes.

23. What is the main functionof the structures labeled Ain the animal shown here?a. reproductionb. food-gettingc. locomotiond. respiration

Cnidarians 191

A

24. The cnidarians have tentacles that grow in a ring around acentral mouth, an arrangement known as a. symbioticb. planktonic c. colonial d. radial symmetry.

Research/Activity

� Do a report on a species of jellyfish that lives in your area. Ifpossible, photograph jellyfish that have washed up on yourbeach or that can be seen floating near a pier. If you live inland,try to report on a freshwater cnidarian, such as the Hydra. Youcan do additional research in the library or on the Internet.

� The Portuguese man-of-war is really many different organismsthat function together as one. Report on how Physalia accom-plishes this task. Include a labeled drawing.

� Participate in a NOAA-sponsored jellyfish sighting survey bygoing on line with Sea Nettle Homepage.

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