Britannica Student Encyclopedia - 2010

mouths. The pseudopods reach out to

surround the food and flow over it.

To reproduce, the nucleus of the amoeba

simply splits into two pieces. The two

halves pull apart, and each half takes

part of the cytoplasm. In this way, a

single amoeba becomes two.

Amphibian

Most of the animals known as amphibians

can live on land or in water.

Amphibians are vertebrates, or animals

with backbones. Amphibians were the

first vertebrates to appear on land. The

early amphibians were the ancestors of

all reptiles, birds, and mammals.

There are three basic groups of amphibians.

The first group consists of frogs

and toads. The second group consists of

salamanders, including newts and mud

puppies. The third group is made up of

wormlike creatures called caecilians.

Altogether there are more than 4,000

species, or types, of amphibian.

Where Amphibians Live

Amphibians are found nearly all over the

world. They are most common in hot,

humid areas such as the tropics. Some

adult amphibians live only on land.

Other types live only in water. However,

most amphibians spend part of the time

on land and part of the time in water.

Physical Features

Most amphibians are relatively small.

However, they vary greatly in length,

from less than 0.4 inch (1 centimeter) to

more than 60 inches (150 centimeters).

Amphibians do not have hair or feathers.

Their skin takes in oxygen. In other

words, amphibians breathe through

their skin. Most kinds also breathe

through lungs or organs called gills.

Amphibian skin takes in water, too.

Most amphibians have moist skin. In

most species, if the skin dries up, the

animal soon dies. The skin often contains

glands that make a slimy fluid

called mucus. Mucus helps protect the

skin from drying out.

Many amphibians also have glands in

the skin that make poison. This helps

protect an amphibian from other animals

that want to eat it. For example,

poison may make the amphibian taste

bad or hurt the predator’s mouth.

Some

amphibians

change their

skin color to

blend in with

their surroundings.

This

makes them

harder for

enemies to

see.

BRITANNICA STUDENT ENCYCLOPEDIA Amphibian 109

 

Frogs and Toads

Adult frogs and toads are tailless. Their

eyes are large and well developed. They

have short, thick bodies and four legs.

Frogs use their long, powerful back legs

for leaping and swimming. Toads have

shorter back legs than frogs do. They

walk or hop about. Unlike other

amphibians, toads have skin that is often

dry, with bumps that look like warts.

Salamanders

Salamanders have long tails. Most

salamanders have a small head, a short

body, and four legs. If a salamander

loses its tail or a leg, it can usually grow

a new one.

Caecilians

Caecilians look like worms or snakes.

They have long, slender bodies with no

arms or legs. Skin or bone covers their

eyes, so the animals are blind or nearly

blind. Most kinds live underground,

where they tunnel about.

Behavior

Amphibians are cold-blooded. This

means that their body temperature is

always about the same as the temperature

of their surroundings. If their environment

gets too hot or cold,

amphibians move to another place to

become more comfortable. For instance,

a frog may move from the shade into the

sunshine to warm its body.

During cold winters some amphibians

become very inactive for months. They

enter a resting state called hibernation.

They become active again when it gets

warmer. Some amphibians also become

inactive during hot, dry summers.

Adult amphibians eat a wide variety of

foods. Frogs, toads, and salamanders eat

mainly small boneless animals such as

Tree frogs, such as the green tree frog, have

special sticky feet that help them climb trees.

Salamanders (top) have smooth, wet skin,

like frogs’ skin. Caecilians (bottom) are

amphibians that look like worms.

110 Amphibian BRITANNICA STUDENT ENCYCLOPEDIA

 

insects, worms, and spiders. Some large

salamanders and frogs eat small birds

and mammals. Most caecilians eat

worms.

Life Cycle

Most amphibians lay eggs, but some

species give birth to live young. In most

egg-laying species, the female deposits a

large number of eggs in the water.

The life cycle of most amphibians is

divided into two phases. In the first

phase the amphibian lives in the water

in an immature form called the larva. In

frogs and toads the larva is also called a

tadpole. The larva begins life with gills,

which allow it to breathe underwater

like a fish. The larva stage may last from

several weeks to five years, depending on

the species.

In the second phase the larva develops

into the adult form. In most species, the

animal loses its gills and develops lungs.

It is then capable of living on land at

least part of the time. The animal’s body

structure may change in many other

ways, too. For example, a tadpole grows

legs and loses its tail. The change from

larva to adult is called metamorphosis.

However, some species do not have separate

larva and adult forms. They are

born in the same form as adults, only

smaller.

..More to explore

Animal • Frog • Hibernation

• Metamorphosis • Poison • Salamander

• Toad

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