VertebratesWhat You’ll LearnChapter 30

Fishes and Amphibians

Chapter 31 Reptiles and Birds

Chapter 32 Mammals

Chapter 33 Animal Behavior

Unit 9 ReviewBioDigest & Standardized Test Practice

Why It’s ImportantAnimals classified as vertebrates have an internal skeletonand a backbone. These features, along with the develop-ment of lungs and, in most vertebrates, limbs, haveallowed them to make the transition from life in water tolife on land with great success.

1671A torpedo fish, a type ofelectric ray, is dissected.The electric organ isexamined, but scientistsremain unaware that it produces electricity.

Understanding the PhotoVertebrate animals have structural and physiologicaladaptations that allow them to live in all of Earth’sbiomes including arctic waters, deserts, rain forests,and mountain plateaus. Guanacos—relatives ofcamels, llamas, alpacas, and vicuñas—inhabit areasof Peru, Chile, and Argentina including the Andes at altitudes up to about 4000 m.

1735Carolus Linnaeuspresents a sys-tem for classify-ing organismsbased on struc-tural similarities.

790

1680The first clocks withhands to indicateminutes are designed.

Torpedo fish

ca.bdol.glencoe.com/webquest(tl)Mark Smith/Photo Researchers, (tr)Historical Pictures Service, Chicago, (crossover)Howie Garber/Animals Animals/Earth Scenes

0790-0791 UO9 BDOL-829900 8/4/04 4:21 PM Page 790

1977A woollymammoth isfound pre-served in icein the SovietUnion.

2002Scientistsannounce the dis-covery of two newspecies of mon-keys and a newspecies of parrotin the Amazonrain forest.

1845The firstinflatable rubbertire is invented.

1969Neil Armstrongwalks on themoon.

791

1836The first livingspecies of lungfishis discovered inSouth America.

Okapi

1901Despite its zebra-like markings, theokapi is correctlyidentified as a rela-tive of the giraffe.

Ken Lucas Photo/Visuals Unlimited

0790-0791 UO9 BDOL-829900 8/4/04 4:25 PM Page 791

Fishes and AmphibiansFishes and

Amphibians

Understandingthe Photo

792

Visit to• study the entire chapter

online• access Web Links for more

information and activities onfishes and amphibians

• review content with theInteractive Tutor and self-check quizzes

Fred Bavendam/Animals Animals

Seahorses are bony fishes. Theyswim upright and slowly moveforward. An adaptation thatallows them to capture prey istheir long snout. It functions likea suction to draw in small organ-isms that pass by in the water.Seahorse species range in sizefrom about 4 cm to 20 cm.

What You’ll Learn■ You will compare and contrast

the adaptations of the differ-ent groups of fishes andamphibians.

■ You will learn about the origin of modern fishes andamphibians.

Why It’s ImportantFishes are the most diverse verte-brate group. Amphibians areadapted to live both in water andon land. The development of abony endoskeleton in fishes andlungs in amphibians were majorsteps in animal evolution.

ca.bdol.glencoe.com

0792 C30Open BDOL-829900 8/4/04 11:07 PM Page 792

Fishes30.1

What is a fish?Fishes, like all vertebrates, are classified in the phylum Chordata. This

phylum includes three subphyla: Urochordata, the tunicates; Cephalo-chordata, the lancelets; and Vertebrata, the vertebrates. Fishes belong tothe subphylum Vertebrata. In addition to fishes, subphylum Vertebrataincludes amphibians, reptiles, birds, and mammals. Recall from the pre-vious chapter that all chordates have four traits in common—a noto-chord, pharyngeal pouches, postanal tail, and a dorsal hollow nerve cord.In vertebrates, the embryo’s notochord is replaced by a backbone in adultanimals. All vertebrates are bilaterally symmetrical coelomates that haveendoskeletons, closed circulatory systems, nervous systems with complexbrains and sense organs, and efficient respiratory systems.

Classes of fishesFishes can be grouped into four classes of the subphylum Vertebrata. The

jawless fishes belong to the superclass Agnatha, which means “without jaws.”Superclass Agnatha consists of two classes: class Myxini (mik SEE nee), hag-fishes, and class Cephalaspidomorphi (se fa LAS pe do MOR fee), lampreys.

SECTION PREVIEWObjectivesRelate the structuraladaptations of fishes to their environments.Compare and contrastthe characteristics of thedifferent groups of fishes.Interpret the phylogenyof fishes.

Review Vocabularyvertebrate: an animal with

a backbone (p. 685)

New Vocabularyspawningfinlateral line systemscaleswim bladdercartilage

Answer Questions As you read Chapter 30, answer the question and fill in information about fishes behind the tabs.

Fishes Make the following Foldable to help you organize information about the diversity and originsof fishes.

Fold a sheet of paper in half lengthwise. Make the back edge about 2 cm longer than the front edge.

Turn the paper so the fold is on the bottom. Then fold it into thirds.

Unfold and cut only the top layer along both folds to make three tabs.

Label the Foldable as shown.

STEP 1

STEP 3

STEP 2

STEP 4

Fishes

What isa fish?

Originof Fishes

Diversityof Fishes

30.1 FISHES 793

0793-0802 C30S1 BDOL-829900 8/4/04 9:57 PM Page 793

Class Chondrichthyes (kahn DRIHK

theez) is comprised of cartilaginousfishes, such as sharks and rays; andclass Osteichthyes (ahs tee IHK theez)contains the bony fishes. Examplesfrom the different classes are shownin Figure 30.1.

Fishes inhabit nearly every type of aquatic environment on Earth.They are found in freshwater and salt water and adapted to living in shallow,warm water and deeper, cold andlightless water.

Fishes breathe using gillsFishes have gills made up of feathery

gill filaments that contain tiny bloodvessels. Gills are an important adapta-tion for fishes and other vertebratesthat live in water. As a fish takes waterin through its mouth, water passes overthe gills and then out through slits atthe side of the fish. Oxygen and carbondioxide are exchanged through the cap-illaries in the gill filaments. You canfind out more about the structure andfunction of gills in fishes in theMiniLab on the next page.

Fishes have two-chambered hearts

All fishes have two-chamberedhearts, as shown in Figure 30.2. Onechamber receives deoxygenated bloodfrom the body tissues, and the secondchamber pumps blood directly to thecapillaries of the gills, where oxygenis picked up and carbon dioxidereleased. Oxygenated blood is carriedfrom the gills to body tissues. Bloodflow through the body of a fish is rel-atively slow because most of theheart’s pumping action is used to pushblood through the gills.

Fishes reproduce sexuallyAlthough the method may vary, all

fishes reproduce sexually. Fertiliza-tion and development is external inmost fishes. Eggs and sperm can bereleased directly into the water, ordeposited in more protected areas,such as on floating aquatic plants.Although most fishes produce largenumbers of eggs at one time, hagfishes produce small numbers ofrelatively large eggs.

794 FISHES AND AMPHIBIANS(bl)Breck P. Kent/Animals Animals, (bc)Andrew J. Martinez/Photo Researchers, (br)Jeremy Stafford-Deitsch/ENP Images

Figure 30.1Examples of fishes include jawlessfishes, cartilaginous fishes, andbony fishes.

Jawless fishes called lampreys havelong, tubular bodies without scalesand paired fins.

A

Cartilaginous fishes calledskates have a flattened bodyshape with large paired finsthat enable them to “fly”over the ocean bottom asthey search for food.

C

Most fishes you are familiarwith are bony fishes, suchas this swordfish.

B

0793-0802 C30S1 BDOL-829900 8/4/04 9:58 PM Page 794

30.1 FISHES 795

Cartilaginous fishes have internalfertilization. Skates deposit fertilizedeggs on the ocean floor. Some femalesharks and rays carry developingyoung inside their bodies. Becausethese young fishes are well developedwhen they are born, they have anincreased chance of survival.

Most bony fishes have external fertilization and development. Thistype of external reproduction in fishesand some other animals is calledspawning. During spawning, somefemale bony fishes, such as cod, pro-duce as many as 9 million eggs, ofwhich only a small percentage survive.In some bony fishes, such as guppiesand mollies, fertilization and develop-ment is internal. Most fishes that pro-duce millions of eggs provide no carefor their offspring after spawning. Inthese species, only a few of the youngsurvive to adulthood. Some fishes, suchas the mouth-brooding cichlids, staywith their young after they hatch.When their young are threatened bypredators, the parent fishes scoop theminto their mouths for protection.

Summarize the different reproductive methods ofcartilaginous and bony fishes.

Aorta

Heart

GillsCapillarynetwork

Figure 30.2Blood in a fish flows in a one-way circuitthroughout the body.

Observe and InferStructure and Function of Fishes’ Gills Fishes remove oxy-gen from the water by means of gills. Water enters a fish’smouth, moves over the gills and out through openings on thesides of the head. The gills are made up of thin filaments con-taining blood vessels. Inside the gills, blood moves in theopposite direction to the flow of water. When blood andwater flow in opposite directions, the oxygen concentrationdifference between the water and the blood is large enoughfor oxygen to diffuse from water into blood.

Procedure! Examine a prepared slide of a fish’s gill under the micro-

scope. CAUTION: Use care when working with micro-scopes and microscope slides.

@ Draw and label a sketch of the gill filaments.

Analysis1. Observe and Infer Why are the gills of fishes made up

of very thin tissue?2. Predict How might a fish adjust to water that suddenly

has less oxygen? 3. Explain Some fishes have a mutualistic relationship with

organisms, such as small shrimp, that “clean” gill filamentsby feeding on parasites that live on the gill tissue. How isthis relationship beneficial to each organism?

Gill Filaments

Gill Filaments

Water

Artery

Vein

Water

Capillary networks in filament

0793-0802 C30S1 BDOL-829900 8/4/04 9:59 PM Page 795

Most fishes have paired finsFishes in the classes Chondrich-

thyes and Osteichthyes have pairedfins. Fins are fan-shaped membranesthat are used for balance, swimming,and steering. Fins are attached to andsupported by the endoskeleton and are important in locomotion. The paired fins of fishes, illustrated inFigure 30.3, foreshadowed the devel-opment of limbs for movement onland and ultimately of wings for flying.

Fishes have developed sensory systems

All fishes have highly developed sen-sory systems. Cartilaginous and bonyfishes have an adaptation called the lat-eral line system that enables them tosense objects and changes in their envi-ronment. The lateral line system is aline of fluid-filled canals running alongthe sides of a fish that enable it todetect movement and vibrations in thewater. Find out more about the lateralline system by doing the Problem-Solving Lab on this page.

Fishes have eyes that allow them tosee objects and contrasts betweenlight and dark in the water as well.The amount of vision varies greatlyamong fishes. Some fishes that live inareas of the ocean where there is nolight may have reduced, almost non-functional eyes.

Some fishes also have an extremelysensitive sense of smell and can detectsmall amounts of chemicals in thewater. Sharks can follow a trail ofblood through the water for severalhundred meters. This ability helpsthem locate their prey.

Most fishes have scalesCartilaginous and bony fishes have

skin covered by intermittent or over-lapping rows of scales. Scales are thinbony plates formed from the skin.

Tom Brakefield/DRK Photo

Figure 30.3The paired fins of a fish include the pectoral fins and the pelvic fins. Fins found on the dorsal and ventral surfaces can include the dorsal fins and anal fin.

Dorsal fins

Pelvicfins

PectoralfinAnal

fin

Caudalfin

796 FISHES AND AMPHIBIANS

Think CriticallyWhy is having a lateral line system important? Aside fromother senses, including sight and smell, most fishes also have alateral line system.

Solve the ProblemThe lateral line system runs along either side of a fish in itsskin. Vibrations in the water cause the gelatin-like fluid in thelateral line system to move, which stimulates the receptor cellsto send messages to the fish’s brain. The fish receives informa-tion about the location of objects in the water such as coral,rocks, other fishes, and prey organisms.

Thinking Critically1. Infer If a fish’s lateral line system were damaged and

unable to function, what effect could that have on the fish?2. Explain Why might a lateral line system be important for

fishes that live in the abyss, where there is no light?3. Hypothesize Fishes in a school have the ability to change

direction almost instantly. Form a hypothesis that explainshow the lateral line system allows fishes to achieve this.

Gelatin-like fluidReceptor cells

Nerve

Lateral line

0793-0802 C30S1 BDOL-829900 8/4/04 10:00 PM Page 796

Scales, shown in Figure 30.4, can betoothlike, diamond-shaped, cone-shaped, or round. Shark scales aresimilar to teeth found in other verte-brates. The age of some species offishes can be estimated by countingannual growth rings in their scales.

Describe the differ-ent shapes scales can have.

Jaws evolved in fishesAn important event in vertebrate

evolution was the development ofjaws in ancestral fishes. The advan-tage of jaws is that they enable an ani-mal to grasp and crush its prey withgreat force. Jaws also allowed earlyfishes to prey on a greater variety oforganisms. This, among other fac-tors, explains why some early fisheswere able to grow to such great size.Figure 30.5 shows the evolution ofjaws in fishes.

When you think of a shark, do youimagine gaping jaws and rows ofrazor-sharp teeth? Sharks have up to20 rows of teeth that are continuallyreplaced. Their teeth point backwardsto prevent prey from escaping oncecaught. Sharks are among the moststreamlined of all fishes and are welladapted for life as predators.

Most fishes have bony skeletonsThe majority of the world’s fishes

belong to the class Osteichthyes, thebony fishes. Bony fishes, a successfuland widely distributed class, differgreatly in habitat, size, feeding behav-ior, and shape. All bony fishes haveskeletons made of bone rather thancartilage as found in other classes offishes. Bone is the hard, mineralized,living tissue that makes up theendoskeleton of most vertebrates.

30.1 FISHES 797(t)George Bernard/Animals Animals, (bl)Scott Camazine/Photo Researchers, (br)Chuck Brown/Photo Researchers

Figure 30.4Fishes can be classified by the typeof scales present. Diamond-shapedscales (A) are common to bonyfishes, such as gars. Bony fishes,such as chinook salmon, haveeither cone-shaped or round scales(B). Tooth-shaped scales (C) arecharacteristic of the sharks.

Gill arches

Gill arches

Gill slitsGill slits

Skull JawsJawless, filter-feeding fish

Beginning of jaw formation Fish with jaws

Figure 30.5Jaws evolved from thecartilaginous gill archesof early jawless fishes.Teeth evolved from skin.

AA

BB

CC

LM Magnification: 40�

0793-0802 C30S1 BDOL-829900 8/4/04 10:00 PM Page 797

In general, the development of bonewas important for the evolution offishes and vertebrates. It allowed fishesto adapt to a variety of aquatic environ-ments, as shown in Figure 30.6, andeventually to land.

Bony fishes have separate vertebrae that provide flexibility

The evolution of a backbone com-posed of separate, hard segments calledvertebrae was significant in providingthe major support structure of the ver-tebrate skeleton. Separate vertebraeprovide great flexibility. This is espe-cially important for fish locomotion,which involves continuous flexing of

the backbone. You can see how modernbony fishes propel themselves throughwater in Figure 30.7. Some fishes areeffective predators, in part because ofthe fast speeds they can attain as aresult of having a flexible skeleton.

Bony fishes evolved swim bladders

Another key to the evolutionary suc-cess of bony fishes was the evolution ofthe swim bladder. A swim bladder isa thin-walled, internal sac found justbelow the backbone in most bonyfishes. It can be filled with mostlyoxygen or nitrogenous gases that dif-fuse out of a fish’s blood. A fish with a

798 FISHES AND AMPHIBIANS(l)Kit Kittle/CORBIS, (c)Doug Perrine/DRK Photo, (r)Steinhart Aquarium/Photo Researchers

Figure 30.6Bony fishes vary in appearance, behavior, and way of life.

Figure 30.7Most bony fishes swimin one of three ways.

An eel moves its entire bodyin an S-shaped pattern.

AA mackerel flexes theposterior end of its body toaccentuate the tail-finmovement.

B A tuna keeps its body rigid,moving only its powerful tail.Fishes that use this methodmove faster than all others.

C

Predatory bony fishes, such as thispike, have sleek bodies withpowerful muscles and tail fins forfast swimming in freshwater lakesand rivers.

C

Seahorses move slowlythrough the underwaterforests of seaweed where theylive. They are unusual in thatthe males brood their youngin stomach pouches.

B

Eels have a long, snakelike bodyand can wriggle through mudand crevices in search of food.

A

l

0793-0802 C30S1 BDOL-829900 8/4/04 10:01 PM Page 798

swim bladder can control its depth byregulating the amount of gas in thebladder. The gas works like the gas ina blimp that allows the blimp tochange its height above the ground.

Some fishes remove gases from theswim bladder by expelling themthrough a special duct that attachesthe swim bladder to the esophagus. Infishes that do not have this duct theirswim bladders empty when gases dif-fuse back into the blood.

Explain the functionof a swim bladder.

Diversity of FishesFishes range in size from the tiny

dwarf goby that is less than 1 cm long,to the huge whale shark that canreach a length of about 15 m—thelength of two school buses.

Agnathans are jawless fishesLampreys and hagfishes belong to

the superclass Agnatha. The skeletonsof agnathans, as well as of sharks andtheir relatives, are made of a tough,flexible material called cartilage.Though they do not have jaws, theyare voracious feeders. A hagfish,Figure 30.8, has a toothed mouth and feeds on dead or dying fishes.

It can drill a hole into a fish and suckout the blood and insides. Parasiticlampreys use their suckerlike mouthsto attack other fishes. They use theirsharp teeth to scrape away the fleshand then suck out the prey’s blood.

Sharks and rays are cartilaginous fishes

Sharks, skates, and rays belong tothe class Chondrichthyes, shown inFigure 30.9. These fishes, like agna-thans, possess skeletons composedentirely of cartilage. Because livingsharks, skates, and rays are similar tospecies that swam the seas more than100 000 years ago, they are consideredliving fossils. Sharks are perhaps themost well-known predators of theoceans.

30.1 FISHES 799

Figure 30.8When touched, a hag-fish’s skin gives off atremendous amount ofmucus, which makes itso slimy it is nearlyimpossible to catch.

The hammerheadshark is large andfound in warmocean water. It hastwo eyes that areat opposite endsof its flattened,extended skull.

A

Most rays are ocean bottom dwellers, butthe Atlantic manta ray prefers to glide alongjust below the water’s surface.

B

Figure 30.9Cartilaginous fishes includesharks, skates, and rays.

(t)Brian Parker/Tom Stack & Associates, (bl)Bios/Peter Arnold, Inc., (br)Doug Perrine/DRK Photo

0793-0802 C30S1 BDOL-829900 8/4/04 10:01 PM Page 799

Like sharks, most rays are predatorsand feed on or near the ocean floor.Rays have flat bodies and broad pec-toral fins on their sides. By slowlyflapping their fins up and down, rayscan glide as they search for mollusksand crustaceans along the ocean floor.Some species of rays have sharp spineswith poison glands on their long tailsthat they can use for defense. Otherspecies of rays have organs that gener-ate electricity, which can stun or killboth prey and predators.

Subclasses of bony fishesScientists recognize two subclasses

of bony fishes—the lobe-finnedfishes, including lungfishes, and theray-finned fishes. Figure 30.10 showsexamples of these subclasses. Thelobe-finned fishes are represented byseven living species: six species oflungfishes, which have both gills and

lungs, and the coelacanth, shown inFigure 30.10B. In the ray-finnedfishes, such as catfish, perch, salmon,and cod, fins are fan-shaped mem-branes supported by stiff spines calledrays. You can compare the interrela-tionships between the structures of aray-finned fish in Figure 30.11.

Origins of FishesScientists have identified fossils

of fishes that existed during the lateCambrian Period, 500 million yearsago. At this time, ostracoderms (OHS trah koh durmz), early jawlessfishes, were the dominant verte-brates on Earth. Most ostracodermsbecame extinct at the end of theDevonian Period, about 354 millionyears ago. Present-day agnathansappear to be the direct descendantsof ostracoderms.

800 FISHES AND AMPHIBIANS(tr)Tom McHugh/Photo Researchers, (l)Steinhart Aquarium/Photo Researchers, (br)Ron & Valerie Taylor/Bruce Coleman, Inc.

Figure 30.10Lobe-finned fishes and ray-finned fishes are the subclasses of the bony fishes.

Lungfishes represent an ancient group oflobe-finned fishes.Lungfishes, such as thisAfrican lungfish, haveboth gills and lungs.

A

The coelacanth, anothertype of lobe-finned fish,was thought to be extinctuntil living coelacanthswere caught off the coastof Africa in 1938.

B

You can easily see the raysthat support the pectoral finsof this flying fish, an exampleof a ray-finned fish.

C

PhysicalScience

Connection

Buoyancy anddensity of fluidsA fluid—a liquid ora gas—exerts anupward, buoyantforce on an objectimmersed in it.This buoyant forceis equal to theweight of the fluiddisplaced by theobject. The weightof fluid dependson the object’svolume. When afish’s swim bladderis inflated, thevolume of the fishincreases and thebuoyant force onthe fish increases.How does thisaffect the depth ofthe fish in water?

0793-0802 C30S1 BDOL-829900 8/4/04 10:02 PM Page 800

30.1 FISHES 801

Lateral line system When fishes swim past obstacles, pressure changes occur in the water. Fishescan detect these changes with their lateral linesystems, which enable them to swim in the dark and in complex coral reefs.

AA

Gills Gills are thin, blood-vessel-richtissues where gases are exchanged.

DD

Scales Scales arecovered with slipperymucus, allowing a fish tomove through water withminimal friction.

CC

Fins The structureand arrangement of fins are related to aparticular type oflocomotion. Tropicalfishes that live amongcoral reefs tend to havesmall fins that allowmaneuvering betweenrocks and in crevices. Atuna has larger, broadfins for moving quicklythrough open water.

EE

Kidney

Urinary bladder

Reproductive organ

StomachIntestine

LiverHeart

Rainbow trout,Oncorhynchus mykiss

A Bony FishFigure 30.11The bony fishes, class Osteichthyes, include some of theworld’s most familiar fishes, such as the bluegill, trout, min-now, bass, swordfish, and tuna. Though diverse in generalappearance and behavior, bony fishes share some commonadaptations with other fish classes. Critical Thinking Explainhow the different organ systems in bony fishes worktogether, allowing a fish to function in the water.

Swim bladder A swim bladder is a gas-filled sac thatenables a fish to control the depth at which it swims. Tobecome more buoyant and rise in the water, the fish inflatesits swim bladder with gases extracted from the water. Thefish reabsorbs these gases to become less buoyant and sink.

BB

Pat & Tom Lesson/Photo Researchers

0793-0802 C30S1 BDOL-829900 8/4/04 10:02 PM Page 801

Understanding Main Ideas1. Identify three characteristics of fishes.

2. Compare how modern jawless fishes and cartilagi-nous fishes feed.

3. Identify the function of the circulatory system infishes. Explain how it works.

4. Explain how a flexible skeleton is related to locomotion in fishes.

Thinking Critically5. Why was the development of jaws an important

step in the evolution of fishes?

6. Summarize Construct a diagram that shows the phylogeny of fishes and the development of important characteristics in their evolution. For more help, refer to Summarize in the SkillHandbook.

SKILL REVIEWSKILL REVIEW

802 FISHES AND AMPHIBIANS

Figure 30.12Ostracoderms, the earli-est vertebrate fossilsknown, were character-ized by bony, externalplates covering the bodyand a jawless mouth.Infer How did thesejawless ostracodermsobtain food?

Weighed down by heavy, bonyexternal armor, ostracoderms, shownin Figure 30.12, were fearsome-looking animals that swam sluggishlyover the murky seafloor. Although allostracoderms had cartilaginous skele-tons, they also had shields of bonecovering their heads and necks. Thedevelopment of bone in early verte-brates was an important evolutionarystep because bone provides a place formuscle attachment, which improveslocomotion. In ancestral fishes, bonethat formed into plates provided pro-tection as well.

Scientists hypothesize that the jaw-less ostracoderms were the common

ancestors of all fishes. Modern carti-laginous and bony fishes evolvedduring the mid-Devonian Period.Lobe-finned fishes, such as coela-canths (SEE luh kanths), are anotherancient group, appearing in the fossilrecord about 395 million years ago.They are characterized by lobelike,fleshy fins and live at great depthswhere they are difficult to find. Thelimblike skeletal structure of fleshyfins is thought to be an ancestral con-dition of all tetrapods (animals withfour limbs). The earliest tetrapodsdiscovered also had gills and thereforewere still aquatic.

Anaspid

Heterostracan

Cephalaspid

ca.bdol.glencoe.com/self_check_quiz

0793-0802 C30S1 BDOL-829900 8/4/04 10:03 PM Page 802

30.2SECTION PREVIEWObjectivesRelate the demands of a terrestrial environ-ment to the adaptationsof amphibians.Relate the evolution of the three-chamberedheart to the amphibianlifestyle.

Review Vocabularymetamorphosis: a series of

changes in an organismcontrolled by chemicalsubstances (p. 751)

New Vocabularyectothermvocal cord

30.2 AMPHIBIANS 803

What is an amphibian?The striking transition from a completely aquatic larva to an air-

breathing, semiterrestrial adult gives the class Amphibia (am FIHB ee uh) itsname, which means “double life.” The class Amphibia includes three orders:Caudata (kaw DAH tuh)—salamanders and newts, Anura (uh NUHR uh)—frogs and toads, and Apoda (uh POH duh)—legless caecilians, shown in

Figure 30.13. Amphibians havethin, moist skin and most havefour legs. Although most adultamphibians are capable of a terres-trial existence, nearly all of themrely on water for reproduction.Fertilization in most amphibians isexternal, and water is needed as amedium for transporting sperm.Amphibian eggs lack protectivemembranes and shells and must belaid in water or other moist areas.Review some adaptations thatallow frogs to live a “double life”in Figure 30.14 on the next page.

Amphibians

(tl)O.S.F./Animals Animals, (tr)Joe McDonald/DRK Photo, (b)Zig Leszczynski/Animals Animals

Figure 30.13Caecilians, orderApoda, are long, limb-less amphibians.

A Double LifeUsing Prior KnowledgeWhen you think of an amphibian, do you picture a jumping frog? Maybe a salamander sitting on a log? Perhaps you think of tadpoles in pond water. Amphibians are a diverse group of animals that can be found both on land and in water. In this section you willexamine the characteristics ofamphibians that allow them to spend at least part of their lives on land.Infer Make a list of charac-teristics that are necessary foran animal to live successfully on land.

Pickerel frog (above)and tadpoles (inset)

0803-0815 C30S2 BDOL-829900 8/4/04 9:36 PM Page 803

A FrogFigure 30.14Many species of frogs look similar. As adults, they have short, bulbous bodies with no tails. This adaptation allowsthem to jump more easily. Critical Thinking Compare and contrast the lateral line system of a fish to thetympanic membrane of a frog.

Rod Planck/Photo Researchers

Green frog, Rana clamitans

Eyes Some frogs’ eyesprotrude from the tops oftheir heads—an adaptationthat enables them to staysubmerged in the waterwith only their eyes abovethe surface.

AA

Tongue A frog’s tongueis long, sticky, and fastenedto the front of the mouth.These adaptations allowfrogs to snare their prey,such as flies, with amazingaccuracy.

CC

Calls Male frogs usesound to attract females.Females have distinctcalls to indicate whetheror not they are willingto mate.

EE

804 FISHES AND AMPHIBIANS

Vocalcords

Heart

Liver

Intestine

Fat bodies

Backbone

Tympanic membrane Vibrationsfrom water or air are picked up by thetympanic membrane and transmitted tothe inner ear and then to the brain. Thetympanic membrane also amplifies thesounds frogs make.

BB

Lungs Lungs enable adultamphibians to breathe air.

DD

Legs The hind legs of afrog are muscular. If you have ever tried to catch afrog, you can appreciate these powerful leg muscles.

FF

0803-0815 C30S2 BDOL-829900 8/4/04 9:37 PM Page 804

Adult frog

Young frogs have structuresneeded for life on land.

Tadpoles with legs feedon plants in the water.

Young, legless tadpoles live off yolk storedin their bodies.

BB

Fertilized eggsAA

CC

DDEE

Amphibians are ectothermsAmphibians are more common in

regions that have warm temperaturesall year because they are ectotherms.An ectotherm (EK tuh thurm) is ananimal that has a variable body temperature and gets its heat fromexternal sources. Because many bio-logical processes require particulartemperature ranges in order to occur,amphibians become dormant inregions that are too hot or cold forpart of the year. During such times,many amphibians burrow into themud and stay there until suitable con-ditions return.

Amphibians undergo metamorphosis

Unlike fishes, most amphibians gothrough the process of metamorpho-sis. Fertilized eggs hatch into tadpoles,the aquatic stage of most amphibians.You can compare tadpoles with adult

frogs in the MiniLab on the next page.Tadpoles possess fins, gills, and a two-chambered heart as seen in fishes. Astadpoles grow into adult frogs andtoads, they develop legs, lungs, and athree-chambered heart. Figure 30.15shows this life cycle.

Young salamanders resemble adults,but, as aquatic larvae, they have gillsand usually have a tail fin. Most adultsalamanders lack gills and fins. Theybreathe through their moist skin orwith lungs. Salamanders in the familyPlethodontidae have no lungs andbreathe only through their skin.Completely terrestrial salamanderspecies do not have a larval stage; theyoung hatch as smaller versions ofadults. Most salamanders have fourlegs for moving about, but a few haveonly two front legs.

Explain the processof metamorphosis in amphibians.

30.2 AMPHIBIANS 805

Adult frog

Young frogs have structuresneeded for life on land.

Tadpoles with legs feedon plants in the water.

Young, legless tadpoles live off yolk storedin their bodies.

BB

Fertilized eggsAA

CC

DDEE

Figure 30.15The amphibian life cycle caninclude an aquatic tadpole stageand a terrestrial adult stage.

parthenogenesisfrom the Greekword parthenos,meaning “virgin”and the Latin wordgenesis, meaning“birth”; Someamphibians, reptiles,and insects repro-duce asexuallythrough partheno-genesis. Insects usu-ally produce allmales, while reptilesand amphibians usu-ally produce allfemales.

0803-0815 C30S2 BDOL-829900 8/4/04 9:38 PM Page 805

Walking requires more energyThe laborious walking of early

amphibians required a great deal ofenergy from food and large amountsof oxygen for aerobic respiration. Theevolution of the three-chamberedheart in amphibians ensured that cellsreceived the proper amount of oxy-gen. This heart was an important evo-lutionary transition from the simplecirculatory system of fishes.

In the three-chambered heart ofamphibians, one chamber receivesoxygen-rich blood from the lungsand skin, and another chamberreceives oxygen-poor blood from thebody tissues. Blood from both cham-bers then moves to the third cham-ber, which pumps oxygen-rich bloodto body tissues and oxygen-poorblood back to the lungs and skin so it can pick up more oxygen. Thisresults in some mixing of oxygen-rich and oxygen-poor blood in theamphibian heart and in blood vesselsleading away from the heart. Thus,in amphibians, the skin is much moreimportant than the lungs as an organfor gas exchange.

Because the skin of an amphibianmust stay moist to exchange gases,most amphibians are limited to life onthe water’s edge or other moist areas.Some newts and salamanders remaintotally aquatic. Amphibians, such astoads, have thicker skin, and althoughthey live primarily on land, they stillmust return to water to reproduce.

Amphibian DiversityBecause most amphibians still

complete part of their life cycle inwater, they are limited to the edges ofponds, lakes, streams, and rivers or toareas that remain damp during part ofthe year. Although they may not beeasily seen, amphibian species arenumerous worldwide.

George Bernard/Animals Animals

Compare and ContrastFrog and Tadpole Adaptations An adult frog and its larval stage—a tadpole—areadapted to different habitats. How are the structures of a frog and a tadpole adapted to their environments?

ProcedureCAUTION: Wear disposable gloves, and use a forceps whenhandling preserved specimens. Always wash your hands after working with live or preserved animals.! Copy the data table.@ Examine a living or preserved adult frog and larval

(tadpole) stage. # Observe the first seven traits listed. Complete your data

table for these observations.$ Use references to fill in the information for the last three

traits listed.

Analysis1. Explain How do hind leg muscles aid adult frog survival?2. Analyze Correlate the type of respiratory organ in an

adult and a tadpole with their differing habitats. 3. Analyze Correlate the type of appendages (arm, leg, tail)

in an adult and a tadpole with their differing habitats.4. Analyze Correlate mouth size in an adult and a tadpole

with their differing diet.5. Explain How do eyes aid in the survival of both stages?6. Think Critically How can skin color and texture aid in

adult frog survival?

Rana temporaria

Data Table

Trait or Information Tadpole Adult

Limbs present?

Eyes present?

Tympanic membrane present?

Tail present?

Mouth present?

Nature of skin (color and texture)

General size

Respiratory organ type

Diet

Habitat

0803-0815 C30S2 BDOL-829900 8/4/04 9:39 PM Page 806

Frogs and toads belong to the order Anura

Frogs and toads are amphibians withno tails. Frogs have long hind legs andsmooth, moist skin. Toads have shortlegs and bumpy, dry skin. Like fishes,frogs and toads have jaws and teeth.Adult frogs and toads are predators thateat invertebrates, such as insects andworms. Many species of frogs andtoads secrete chemicals through theirskin as a defense against predators. Youcan find out more about poisonousfrogs in the Connection to Chemistry atthe end of this chapter.

Frogs and toads also have vocalcords that are capable of producing awide range of sounds. Vocal cordsare sound-producing bands of tissuein the throat. As air moves over thevocal cords, they vibrate and causemolecules in the air to vibrate. Inmany male frogs, air passes over thevocal cords, then passes into a pair ofvocal sacs lying underneath thethroat, as shown in Figure 30.16.

Most frogs and toads spend part oftheir life cycle in water and part onland. They breathe through lungs orthrough their thin skins. As a result,frogs and toads often are among thefirst organisms to be exposed to pol-lutants in the air, on land, or in thewater. Declining numbers of frogspecies, or deformities in local frogs,sometimes indicate the presence ofpollutants in the environment.

Analyze the impor-tance of environmental conditionsto a frog’s health.

Salamanders belong to the order Caudata

Unlike a frog or toad, a salamanderhas a long, slender body with a neckand tail. Salamanders resemblelizards, but have smooth, moist skinand lack claws. Some salamanders are

totally aquatic, and others live indamp places on land. They range insize from a few centimeters in lengthup to 1.5 m. The young hatch fromeggs, look like small salamanderadults, and are carnivorous.

Caecilians are limbless amphibians

Caecilians are burrowing amphib-ians, have no limbs, and have a short, orno, tail. Caecilians are primarily tropi-cal animals with small eyes that oftenare blind. They eat earthworms andother invertebrates found in the soil.All caecilians have internal fertilization.

Origins of AmphibiansImagine a time 360 million years

ago when the inland, freshwater seaswere filled with carnivorous fishes.One type of tetrapod had evolved thatretained gills for breathing and afinned tail for swimming. Earlytetrapods may have used their limbsto move on the bottom of marshlandsfilled with plants. In later fossils, thefour limbs are found further belowthe body to lift it off the ground.

30.2 AMPHIBIANS 807William Leonard/DRK Photo

Figure 30.16Most male frogs havethroat pouches that,along with the tym-panic membrane,increase the loudness oftheir calls. Infer Howmight this adaptationbenefit frogs?

0803-0815 C30S2 BDOL-829900 8/4/04 9:40 PM Page 807

Most likely, amphibians arose as theirability to breathe air through well-developed lungs evolved. The successof inhabiting the land depended onadaptations that would provide support,protect membranes involved in respira-tion, and provide efficient circulation.

Challenges of life on landLife on land held many advantages

for early amphibians. There was alarge food supply, shelter, and nopredators. In addition, there wasmuch more oxygen in air than in

water. However, land life also heldmany dangers. Unlike the tempera-ture of water, which remains fairlyconstant, air temperatures can varygreatly. In addition, without the sup-port of water, the body was clumsyand heavy. Some of the efforts byearly amphibians to move on landprobably were like movements ofpresent-day salamanders. The legs ofsalamanders are set at right angles tothe body. You can see in Figure 30.17why the bellies of these animals mayhave dragged on the ground.

Amphibians first appeared about360 million years ago. Amphibiansprobably evolved from an aquatictetrapod, as shown in Figure 30.18,around the middle of the PaleozoicEra. At that time, the climate onEarth is known to have become warmand wet, ideally suited for an adaptiveradiation of amphibians. Able tobreathe through their lungs, gills, orskin, amphibians became, for a time,the dominant vertebrates on land.

(t)M.C. Chamberlain/DRK Photo, (c)M.P. Kahl/DRK Photo, (b)William Leonard/DRK Photo

Figure 30.17Adaptation to life on landinvolves the positioning oflimbs. The evolution of ani-mal with four limbs led tothe diversification of landvertebrates.

Reptiles, such as this crocodile, have legs onthe sides of their bodies, like amphibians, butthe limbs have joints that enable them to bendand hold the body up off the ground.

C

Mammal bodies are raised above the groundwith limbs that are positioned underneath the body. This position allows greater speed of locomotion, making mammals, such as this cheetah, the fastest-moving land animals.

B

The salamander, anamphibian, has legsthat extend at rightangles to its body.

A

808 FISHES AND AMPHIBIANS

Figure 30.18Transitional fossils of aquatictetrapods from the DevonianPeriod show that they had amphib-ian characteristics, but they alsoretained some fishlike features.

0803-0815 C30S2 BDOL-829900 8/4/04 9:40 PM Page 808

Understanding Main Ideas1. Describe the events that may have led early ani-

mals to move to land.

2. Identify and describe three characteristics ofamphibians.

3. List several reasons why amphibians are depen-dent on water.

4. Relate the demands of a terrestrial environmentto the adaptations of amphibians.

Thinking Critically5. How does a three-chambered heart enable

amphibians to obtain the energy needed formovement on land?

6. Get the Big Picture Make a diagram that tracesthe evolutionary development of amphibians fromlungfishes. For more help, refer to Get the BigPicture in the Skill Handbook.

SKILL REVIEWSKILL REVIEW

30.2 AMPHIBIANS 809

Frogs and Toads3700 species

Salamanders369 species

Caecilians168 species

Placoderms

Lobe-finnedfishes

7 species Ray-finned Fishes18 000 species

Sharksand Rays850 species

Ostracoderms

Earlytetrapod

Reptiles

Lampreys17 species

43 speciesHagfishes

Osteichthyes

Agnatha

Amphibians

Chondrichthyes

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

Figure 30.19The radiation of classes of fishesand orders of amphibians showstheir relationships.

ANIMALS

Recall that early vertebrates evolvedfrom swimming fishes to aquatictetrapods. Scientists have found fossilevidence that supports the hypothesisthat limbs first evolved in aquatic ani-mals. Some of these aquatic verte-brates had lungs and evolved intoanimals that crawled on the bottom of

marshlands, and finally evolved tofully developed amphibians that couldlive on land. Although the fossilrecord for fishes and amphibians is incomplete, most scientists agree that the relationships shown in Figure 30.19 represent the best fit forthe available evidence.

ca.bdol.glencoe.com/self_check_quiz

0803-0815 C30S2 BDOL-829900 8/4/04 9:40 PM Page 809

810 FISHES AND AMPHIBIANS

Before You BeginTo help identify organisms,taxonomists have devel-oped classification keys. Adichotomous key can con-sist of a set of questionsfor identifying organisms.When you use this type ofa dichotomous key, youanswer yes or no to aquestion based on thecharacteristics of theorganism you are trying to identify. Depending onyour answer, you aredirected to another ques-tion that further narrowsdown the identification.When you have answeredall the questions, you willarrive at the name of theorganism or the group towhich it belongs. In thislab, you will create a sim-ple dichotomous key tohelp you identify differentamphibians.

Making a DichotomousKey for Amphibians

ProblemHow is a dichotomous key made?

ObjectivesIn this BioLab, you will:■ Design and construct a dichotomous key for amphibians.■ Classify organisms on the basis of structural characteristics.

Materialssample keys from guidebooksfield guides for amphibian identification

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

1. Study the field guides for amphibians. Note characteris-tics, such as the number of legs present, length of hindlegs, length of tail (if present), texture of skin, and thestructure of gills (if present).

2. Make a list of yes or no questions based on the differentcharacteristics of amphibians.

3. Begin with a broad question, then based on their answer,direct the user to another question that will further nar-row down the identification of an organism.

4. Continue to construct your key using the question-and-answer system until the user can classify an organism intoa particular group, such as family or genus.

5. Exchange dichotomous keys with another team. Usetheir key to classify some amphibians.

PROCEDUREPROCEDURE

PREPARATIONPREPARATION

0803-0815 C30S2 BDOL-829900 8/4/04 9:43 PM Page 810

30.2 AMPHIBIANS 811

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

1. Compare and Contrast Was the dichoto-mous key you constructed exactly like thoseof other students? Explain.

2. Evaluate What characteristics were mostuseful for making a classification key foramphibians? What characteristics were notuseful for making your key?

3. Think Critically Why do keys typically offertwo answer choices rather than a larger num-ber of choices?

4. Give reasons why anotherstudent might not be able to identify the groupto which an amphibian belongs using your key.

ERROR ANALYSIS

Field Investigation Find out which amphi-bians live near you, and make a dichotomouskey specific for the amphibians in your area.Test the accuracy of your key by using it inthe field to classify amphibians.

Web Links To find out more about amphibians, visitca.bdol.glencoe.com/amphibians

00 Second Proof

0803-0815 C30S2 BDOL-829900 8/4/04 9:46 PM Page 811

Painkiller Frogs

The most colorful frogs in the world are found in South and Central America. These

poisonous frogs, including 130 species of theDendrobatidae family, range in size from 1 to5 cm. Although all frogs have glands that producesecretions, these frogs secrete toxic chemicalsthrough their skin. A predator will usually dropthe foul-tasting frog when it feels the numbing orburning effects of the poison in its mouth.

The frogs advertise their poisonous personali-ties by bright coloration; they may be red orblue, solid colored, marked with stripes or spots,or have a mottled appearance. The poisonsecreted by these frogs is used by some nativepeople to coat the tips of the darts that they usein their blow guns for hunting. Thus, these frogsare known as poison-arrow frogs.

The secretions of the poison-arrow frogs ofthe frog family Dendrobatidae are alkaloid tox-ins. The chemical structure of an alkaloid toxinincludes a ring consisting of five carbon atomsand one nitrogen atom. The toxins secreted bypoisonous frogs act on an ion channel betweennerve and muscle cells. Normally, the channel isopen to allow movement of sodium, potassium,and calcium ions. The toxins can block the flowof potassium and stop or prolong nerve impulsetransmission and muscle contraction. One groupof alkaloids affects the transport of calcium ions,which are responsible for muscle contraction.Current research indicates that these alkaloidsmay have clinical applications for muscle dis-eases and as painkillers.

Frog poison eases pain Recent researchshows that a drug derived from the extract fromsome poison-arrow frogs, including Epipedobatestricolor, works as a powerful painkiller. The drugABT-594 may have the same benefits as mor-phine, but not the side effects. Morphine is the primary drug used to treat the severe and

unrelenting pain caused by cancer and seriousinjuries. Side effects of morphine include sup-pressed breathing and addiction. The “frogdrug” does not interfere with breathing and doesnot appear to be addictive in initial testing.Another benefit of ABT-594 is that as it blockspain, it does not block other sensations, such astouch or mild heat. One day, pain that you expe-rience might be eased by a frog!

812 FISHES AND AMPHIBIANSJohn Netherton/Animals Animals

Poison-arrow frog, Epipedobates tricolor

Alkaloid toxinCl

O

N

N

H

Evaluate the Impact of Research Research onnewly discovered organisms, such as poisonousfrogs, may result in drugs to treat specific disor-ders in humans. Evaluate the impact of thisresearch on scientific thought and society. Identifydiseases that could be treated using toxins frompoisonous frogs.

To find out more about poisonous frogs,visit ca.bdol.glencoe.com/chemistry

0803-0815 C30S2 BDOL-829900 8/4/04 9:48 PM Page 812

Section 30.1Key Concepts■ Fishes are vertebrates with backbones and

nerve cords that have expanded into brains.■ Fishes belong to four classes: two classes of

jawless fishes: lampreys and hagfishes, thecartilaginous sharks and rays, and the bonyfishes. Bony fishes are made up of twogroups: the lobe-finned fishes, includinglungfishes, and the ray-finned fishes.

■ Jawless, cartilaginous, and bony fishes mayhave evolved from ancient ostracoderms.

Vocabularycartilage (p. 799)fin (p. 796)lateral line system

(p. 796)scale (p. 796)spawning (p. 795)swim bladder (p. 798)

Fishes

Key Concepts■ The class Amphibia includes three orders:

Caudata—salamanders and newts, Anura—frogs and toads, and Apoda—legless caecilians.

■ Adult amphibians have three-chamberedhearts that provide oxygen to body tissues,but most gas exchange takes place throughthe skin.

■ Land animals face problems of dehydra-tion, gas exchange in the air, and supportfor heavy bodies. Amphibians possessadaptations suited for life on land.

■ Amphibians probably evolved from ancient aquatic tetrapods.

Vocabularyectotherm (p. 805)vocal cord (p. 807)Amphibians

STUDY GUIDESTUDY GUIDE

CHAPTER 30 ASSESSMENT 813(t)Tom Brakefield/DRK Photo, (b)O.S.F./Animals Animals

Section 30.2

To help review informa-tion on fishes, use the OrganizationalStudy Fold on page 793.

Fishes

Class Organisms Characteristics

Myxini Hagfishes Jawless, cartilaginous skeleton, gills

Cephalaspidomorphi Lampreys Jawless, cartilaginous skeleton, gills

Chondrichthyes Sharks, skates, rays Jaws, cartilaginous skeleton, paired fins, gills, scales, internal fertilization

Osteichthyes Lobe-finned fishes, Jaws, bony skeleton, paired fins, ray-finned fishes gills, scales, swim bladder

ca.bdol.glencoe.com/vocabulary_puzzlemaker

0803-0815 C30S2 BDOL-829900 8/4/04 9:49 PM Page 813

Review the Chapter 30 vocabulary words listed inthe Study Guide on page 813. Match the wordswith the definitions below.

1. thin bony plates formed from the skin of fish 2. thin-walled, internal sac found just below

the backbone in bony fishes3. fan-shaped membranes that are used for bal-

ance, swimming, and steering by fish4. a line of fluid-filled canals running along the

sides of a fish

5. Which of the following characteristics do allvertebrates have at some point in theirdevelopment? A. bilateral symmetry, exoskeletons, lungsB. lungs, postanal tail, backboneC. pharyngeal pouches, notochord, backboneD. closed circulatory systems, segmented

bodies, external fertilization6. In general, the respiratory system and circu-

latory system of fishes, as illustrated below,are composed of ________.A. gills and a two-chambered heartB. gills and a three-chambered heartC. gills and a four-chambered heartD. none of the above

7. How is the word amphibia related to theorganisms classified in this group?A. Most amphibians spend part of their lives

in water and part on land.B. Amphibians live their entire lives on land.C. Amphibians use swim bladders for

breathing in water but use lungs on land.D. Many amphibians have a two-chambered

heart that develops into a three-chamberedheart when the animal matures.

8. Advantages to life on land for early amphib-ians included ________.A. more food and no predatorsB. greater temperature variationsC. more food and more predatorsD. less oxygen in the air than water

9. Complete the following concept map byusing the following vocabulary terms: lat-eral line system, scales, swim bladder.

10. Open Ended Relate the importance of theevolution of bone in fishes to the evolutionof vertebrates.

11. Open Ended Relate the different types ofbody shapes of fishes to the habitat andbehavior of fishes.

12. Open Ended Hypothesize how a fish with-out a swim bladder is able to maintain orvary its position in the water column.

13. Writing in Biology Compare and contrastthe advantages and disadvantages of the dif-ferent reproductive methods of fishes.

14. There are sev-eral interpretations of how fishes can beclassified. Visit toinvestigate the different ways fish groups arecurrently classified. Present the results ofyour research to your class in the form of aposter or multimedia presentation.

REAL WORLD BIOCHALLENGE

Fishes

are vertebrates that

two-chamberedheart

1. 2.

3.

have a

sense vibrations

with a

have a

protect skin with

814 CHAPTER 30 ASSESSMENT

ca.bdol.glencoe.com

ca.bdol.glencoe.com/chapter_test

0803-0815 C30S2 BDOL-829900 8/4/04 9:50 PM Page 814

CHAPTER 30 ASSESSMENT 815

15. Design an Experiment Monkey frogs livein extremely dry regions of South America.They secrete wax from skin glands andspread the wax over their bodies. Form ahypothesis about the function of this wax,and design a controlled experiment thatwould test your hypothesis.

16. Writing in Biology Scientists do not knowthe ultimate effect of stocking freshwaterecosystems with fishes for food and sport.Decline of native species has been observedin areas where stocking occurs. Give possi-ble reasons that explain why native speciesdecline when new species are introduced.

Constructed Response/Grid InRecord your answers on your answer document.

21. Open Ended You are trying to find the optimum pH for hatching frog eggs. Design a controlledexperiment that would yield quantitative data.

22. Open Ended What might frogs be communicating when they make vocalizations? Explain whyscientists use frog calls to identify different species of frogs.

ca.bdol.glencoe.com/standardized_test

Multiple ChoiceUse this diagram to answer question 17.

17. The sequence that best describes amphibiandecline is ________.A. fungus infection increases frogs’ risk for

damage by UV light caused by intensedryness

B. intense dryness reduces pond depth,causing more exposure to UV light thatincreases risk for fungus infection

C. intense dryness causes an increase in UVlight that kills frogs

D. increase in the depth of ponds causesmore fungus infections in frogs

Use the following graph to answer questions 18–20.

18. How many eggs hatched at 20°C?A. 150 C. 95B. 120 D. 65

19. At which temperature did the fewest eggshatch?A. 15°C C. 25°CB. 20°C D. 30°C

20. Based on the information presented in thegraph, which temperature would you use foroptimal hatching of eggs?A. 15°C C. 25°CB. 20°C D. 30°C

Nu

mb

er o

f fr

og

eg

gs

hat

chin

g

Temperature15˚C 20˚C 25˚C 30˚C

60

30

90

120

150

180

Number of Frog Eggs Hatching at VariousTemperatures in 5 Days.

Global climate change

Intense dry events

Reduced water depth in ponds

Frog embryos exposed to more ultraviolet (UV) light

Increased risk of infection

Increased rates of infection by fungus

0803-0815 C30S2 BDOL-829900 8/4/04 9:51 PM Page 815