Section

3 Amphibians

ObjectivesAfter completing the lesson, students will be able toB.3.3.1 Describe amphibian characteristics.B.3.3.2 Examine how adult amphibians are adapted for life on land.

Target Reading SkillSequencing Explain that organizing information from beginning to end helps students understand a step-by-step process.

AnswersAdult frog; Fertilized eggs; Tadpole hatches; Hind legs develop; Front legs develop; Tail is absorbed.

Teaching Resources

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Preteach

Build Background KnowledgeIdentifying Amphibian CharacteristicsHave students describe any animals they know of that live part of their life on land and part in the water (Frogs, salamanders). Then ask: What characteristics allow these animals to live on land and in water? (Ability to change/undergo metamorphosis; body structures that allow movement, feeding, and breathing on land or in water).

L2

Skills Focus Inferring

Materials dried yellow and green peas; paper cup; green construction paper, approximately 1 m × 1 m; clock or watch with second hand

Time 10 minutes

Tips To intensify the camouflage effect, make sure that the green background closely matches the color of the green peas.

L1 Expected Outcome Students should pick up more yellow peas than green peas from the green background.

Think It Over Being a color that blends in with the environment makes frogs harder for predators to see and thus more likely to survive and reproduce. learning modality: visual

3 Amphibians

What’s the Advantage of Being Green?1. Count out 20 dried yellow peas and 20 green ones. Mix them

up in a paper cup.2. Cover your eyes. Have your partner gently scatter the peas

onto a large sheet of green paper.3. Uncover your eyes. Have your partner keep time while you pick

up as many peas, one at a time, as you can find in 15 seconds.4. When 15 seconds are up, count how many peas of each color

you picked up.5. Repeat Steps 2 through 4, but this time you scatter the peas

and keep time while your partner picks up the peas.6. Compare your results with those of your partner and your

classmates.

Think It OverInferring Many frogs are green, as are their environments. What advantage does a frog have in being green?

Reading PreviewKey Concepts• What are the main characteristics

of amphibians?

• What are some adaptations of adult amphibians for living on land?

Key Terms• amphibian • tadpole • lung • atrium • ventricle • habitat

Target Reading SkillSequencing As you read, make a cycle diagram like the one below that shows the different stages of a frog’s metamorphosis during its life cycle. Write each step of the process in a separate circle.

What’s that sound coming from the pond? Even 1 kilometeraway you can hear the shrill calls of frogs called spring peeperson this damp spring night. By the time you reach the pond, thecalls are ear-splitting. You might think that the frogs must behuge to make such a loud sound. But each frog is smaller thanthe first joint of your thumb! In the beam of your flashlight,you see the puffed-up throats of the males, vibrating with eachcall. Female peepers bound across roads and swim acrossstreams to mate with the noisy males.

What Is an Amphibian?A frog is one kind of amphibian; toads and salamandersare other kinds. An amphibian is a vertebrate that isectothermic and spends its early life in water. Indeed, theword amphibian means “double life,” and amphibianshave exactly that. After beginning their lives in water,most amphibians spend their adulthood on land,returning to water to reproduce.

Spring peeper �

Fertilizedeggs

Frog Metamorphosis

Adult frog

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Instruct

What Is an Amphibian?

Teach Key ConceptsExploring Amphibian DevelopmentFocus Tell students that, during their life cycles, amphibians live both on land and in water.

Teach Direct students to Figure 14. Have students trace the stages of development, stating at least one difference between each stage (Front legs develop, tail disappears, and so on).

Apply Ask: Why is water essential for amphibian life? (Amphibian eggs are laid and hatched in water; larvae live in water; adults must stay moist to absorb oxygen through their skin.) learning modality: visual

Teaching Resources

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Independent PracticeTeaching Resources

• Guided Reading and Study Worksheet: Amphibians

Student Edition on Audio CD

For: More on the frog life cycleVisit: PHSchool.comWeb Code: ced-2033

Students can review the frog life cycle in an online interactivity.

L2

L2

Monitor Progress L2

AnswersFigure 14 Developing legs and losing the tail allows frogs to move on land.

A tadpole

Adult frogs reproduce sexually.

Eggs are fertilized out-side the female’s body.

Swimming tadpoles hatch from the eggs.

4 Hind legs develop.

Front legs develop.

The tail is absorbed, and development is completed.

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5

1

2

3

Groups of Amphibians The two major groups of amphib-ians are salamanders and frogs and toads. You can distinguishbetween the groups by the presence of a tail in the adults. Sala-manders keep their tails in adulthood, while almost all frogsand toads do not.

Reproduction and Development Amphibians have a lifecycle that suits the “double lives” they lead. Eggs are fertilizedinternally in most salamanders and externally in most frogsand toads. Fertilized eggs develop in water. After a few days,larvae wriggle out of a jelly that coats the eggs and begin a free-swimming, fishlike life.

The larvae of most amphibians grow and eventually undergometamorphosis. You can trace the process of frog metamorphosisin Figure 14. The larva of a frog or a toad is called a tadpole.

Unlike tadpoles, the larvae of salamanders look like adults.Most salamander larvae undergo a metamorphosis in whichthey lose their gills. However, the changes are not as dramaticas those that happen during a frog or toad’s metamorphosis.

What is a frog larva called?

FIGURE 14

Life Cycle of a FrogDuring its metamorphosis from tadpole to adult, a frog’s body undergoes a series of dramatic changes. Applying Concepts How do these changes prepare a frog for living on land?

For: More on the frog life cycleVisit: PHSchool.comWeb Code: ced-2033

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Living on Land

Teach Key ConceptsComparing Amphibian AdaptationsFocus Ask students to describe some differences between adult frogs and tadpoles.

Teach Write two headings on the board, Larvae and Adults. Then read characteristics of each aloud. Have students discuss which heading is appropriate for each characteristic. (Example: Larvae—herbivores, swim, obtain oxygen through gills, have one-loop circulatory system; adults—carnivores, crawl/leap, breathe through lungs, have two-loop circulatory system)

Apply Ask: How does acquiring lungs change the life of amphibians? (They are able to leave the water, breathe air, and live on land.) learning modality: logical/mathematical

Teaching Resources

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Address Misconceptions

Toads Do Not Cause WartsFocus Explain that the belief that someone who touches a toad will get warts is merely a superstition.

Teach Ask: Can you think of any reason why this superstition came about? (Toads’ skin is bumpy and looks “warty.” People assumed that toads’ “warts” were contagious.)

Apply Explain further that although toad skin will not cause warts, it does sometimes secrete substances that can severely irritate skin. Ask: What adaptation would these secretions offer? (Irritation would discourage predators.) learning modality: logical/mathematical

For: Respiration and Circulation activityVisit: PHSchool.comWeb Code: cep-2032

Students compare and contrast the circulatory systems found in fishes, typical adult amphibians, and birds.

L2

L1

Blood vessels in lungs

Blood vessels in body

Right atrium

Left atrium

Oxygen-rich bloodOxygen-poor blood

Key

Three-chamberedheart

Ventricle

Living on LandOnce an amphibian becomes an adult and moves onto land, itssurvival needs change. It must now get its oxygen from the air,not the water. Fins no longer help it move. The respiratoryand circulatory systems of adult amphibians are adapted forlife on land. In addition, adult amphibians have adaptationsfor obtaining food and moving.

Obtaining Oxygen Amphibian larvae use gills to obtainoxygen from the water they live in. During metamorphosis,most amphibians lose their gills and develop lungs. Lungs areorgans of air-breathing vertebrates in which oxygen gas andcarbon dioxide gas are exchanged between the air and theblood. Oxygen and carbon dioxide are also exchanged throughthe thin, moist skins of adult amphibians.

Circulatory System A tadpole’s circulatory system has a sin-gle loop and a heart with two chambers, like that of a fish. Incontrast, the circulatory system of many adult amphibians hastwo loops and a heart with three chambers. You can trace thepath of blood through an amphibian in Figure 15. The twoupper chambers of the heart, called atria (singular atrium),receive blood. One atrium receives oxygen-rich blood from thelungs, and the other receives oxygen-poor blood from the rest ofthe body. From the atria, blood moves into the lower chamber,the ventricle, which pumps blood out to the lungs and body.Oxygen-rich and oxygen-poor blood mix in the ventricle.

For: Respiration and Circulation activityVisit: PHSchool.comWeb Code: cep-2032

FIGURE 15Respiration and Circulation This adult salamander has lungs and a double-loop circulatory system. Interpreting Diagrams What kind of blood is in the ventricle?

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Modeling Amphibian Circulation

Materials 25 red balloons

Time 15 minutes

Focus Have students trace the path of blood in Figure 15.

Teach Position students at five stations, representing parts of the circulatory system—lungs, body, right atrium, left atrium, ventricle. Place 25 balloons, representing oxygen, at the lungs station. Slowly clap your hands to indicate heartbeats. At each heartbeat, students change stations in the direction of blood flow. For example, students at the body station will move to the right atrium. Students at the ventricle move to the lungs or back to the body. As students pass the lungs station, they pick up a balloon. At the body station they drop a balloon. A student holding a balloon represents oxygen-rich blood. A student without a balloon represents oxygen-poor blood. Continue for about 4 minutes.

Apply Ask: Where in an adult amphibian’s circulatory system is oxygen acquired? (In the lungs and skin)Where does the mixing of oxygen-rich and oxygen-poor blood occur? (In the ventricle) learning modality: kinesthetic

L2

Skills Focus Making models

Materials plastic bags, heavy rubber bands, pail of water or sink

Time 15 minutes

Tips Make sure that students insert only their fingers into the water. No part of their palm should be under water as it will provide too much resistance. Encourage

L1 students to experiment with moving their bagged hands through the water with their fingers spread as wide as possible and with their hands balled into a fist.

Extend Ask: How do some kinds of birds benefit from webbed feet? (Because these birds spend some part of their lives in water) learning modality: kinesthetic

Monitor Progress L2

Writing Have students write a brief paragraph comparing and contrasting the characteristics of fishes and adult amphibians. (Compare: Both lay eggs in water and have internal skeletons, a backbone, and a closed circulatory system. Contrast: Fishes have gills, most adult amphibians have lungs; amphibians have a two-loop circulatory system and fish have one loop.) Students can save their paragraphs in their portfolios.

AnswerFigure 15 A mixture of oxygen-rich blood and oxygen-poor blood

The webbed hind feet of this African clawed frog help it swim through water.

Sticky pads on the toes of this tree-dwelling frog give it a secure foothold as it climbs.

Obtaining Food Although most tadpoles are herbivores,most adult salamanders, frogs, and toads are carnivores thatfeed on small animals. Frogs and toads usually wait for theirprey to come close. But salamanders, unlike frogs and toads,actively stalk and ambush their prey.

Frogs and toads have camouflage that helps them obtainfood. Most frogs and toads are brownish-green, making themhard to see in their environment. In the Discover Activity, youlearned that it is hard to see something green against a greenbackground.

Movement A vertebrate that lives on land needs a strong skel-eton to support its body against the pull of gravity. In addition, aland animal needs some way of moving. Fins work in water, butthey don’t work on land. Most adult amphibians have strongskeletons and muscular limbs adapted for moving on land.

Salamanders usually crawl in their environments, but frogsand toads have adaptations for other kinds of movements. Per-haps you’ve tried to catch a frog or a toad only to have it leapaway from you. The legs of frogs and toads have adaptations forleaping. Leaping requires powerful hind-leg muscles and a skele-ton that can absorb the shock of landing. The feet of frogs andtoads have adaptations, too, as you can see in Figure 16.

Webbing Along 1. Fill a sink or pail with

water.2. Spread your fingers and

put your hand into the water just far enough so that only your fingers are under water. Drag your fingers back and forth through the water.

3. Now dry your hand and cover it with a small plastic bag. Secure the bag around your wrist with a rubber band.

4. Repeat Step 2. Note any difference in the way in which your fingers push the water.

Making Models Use your model to explain how a frog’s webbed feet help it move through water.

FIGURE 16Adaptations for Movement Some frogs have sticky pads on their toes for climbing. Others have webbed feet for swimming.

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Monitor Progress L2

AnswersFigure 17 Destruction of habitat and chemicals in the environment

The specific environment in which the animal lives

Assess

Reviewing Key Concepts1. a. An amphibian is an ectothermic vertebrate that spends its early life in water . b. Vertebrate, ectotherm, has double life (in water and on land). c. Salamander larvae resemble salamander adults. Larvae lose their gills during metamorphosis, but they do not undergo the dramatic physical changes seen in frog and toad metamorphosis.2. a. Lungs, two-loop circulatory system and three-chambered heart, camouflage for stalking prey on land, strong skeletons and muscular limbs for movement on land b. Crawling, leaping, climbing; each adaptation helps amphibian get around on land c. Blood leaves the ventricle and travels to the lungs, where it picks up oxygen. Then blood returns to the ventricle via the left atrium. Then it goes to the body, drops off oxygen to the body’s cells. Finally, it returns to the ventricle via the right atrium and begins the cycle again.

ReteachCall on students to give one characteristic of amphibians and tell how the characteristic helps the species survive in its environment.

Performance AssessmentOrganizing Information Have students create a cycle diagram of the life cycle of a frog.

Teaching Resources

• Section Summary: Amphibians• Review and Reinforce: Amphibians• Enrich: Amphibians

L1

L2

Keep Students on Track Review students’ amphibian choices for appropriateness. Before students begin work on their amphibian models, review their fish models. Discuss with students ways in which amphibians differ from fish and how these differences will be reflected in their amphibian model.

Design a Web Site Before students create their Web sites, choose good examples of professional Web sites for them to view. Have them discuss what makes the Web sites interesting to look at and easy to use.

Section 3 Assessment

Target Reading Skill Sequencing Review your cycle diagram about frog metamorphosis with a partner. Add any necessary information.

Reviewing Key Concepts1. a. Defining What is an amphibian?

b. Summarizing What are three main characteristics of amphibians?

c. Comparing and Contrasting How is the metamorphosis of a salamander different from the metamorphosis of a frog?

2. a. Reviewing What are four adaptations of adult amphibians for living on land?

b. Describing What are three adaptations frogs and toads have for moving? How does each adaptation help the amphibian survive in its environment?

c. Sequencing How does blood move in the circulatory system of an amphibian? (Hint: Start with blood leaving the ventricle of the heart.)

3

Web Site Design the home page of a Web site that introduces people to amphibians. First, come up with a catchy title for your Web site. Then, design your home page, the first page people will see. Consider these questions as you come up with your design: What information will you include? What will the illustrations or photos show? What links to specific topics relating to amphibians will you have?

Amphibians in Danger Worldwide, amphibianpopulations are decreasing. One reason for thedecrease is the destruction of amphibian habitats. Ananimal’s habitat is the specific environment in whichit lives. When a swamp is filled in or a forest is cut, anarea that was moist becomes drier. Few amphibianscan survive for long in dry, sunny areas. But habitatdestruction does not account for the whole problemof population decrease. Amphibians are decliningeven in areas where their habitats have not beendamaged. Because their skins are delicate and theireggs lack shells, amphibians are especially sensitive tochanges in their environment. Poisons in the envi-ronment, such as pesticides and other chemicals, canpollute the waters that amphibians need to live andreproduce. Even small amounts of these chemicalscan weaken adult amphibians, kill amphibian eggs,or cause tadpoles to become deformed.

What is a habitat?

FIGURE 17Golden Frog Golden frogs, like the one shown here, are rarely seen anymore in their native habitat—the rain forests of Panama. Relating Cause and Effect What are two possible causes for the decrease in the number of golden frogs?

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