win their independence
were the 13
colonies that
became the
United States.
An illustration from 1698 depicts the last
expedition of the French explorer Sieur de
La Salle
BRITANNICA STUDENT ENCYCLOPEDIA Americas, Exploration and Settlement of the 107
Amman
Population
(2004 estimate)
1,036,330
Amman is the capital of the Middle
Eastern country of Jordan. The city
spreads over a series of rolling hills.
Amman is Jordans largest city. It is also
Jordans main center of business,
finance, and international trade.
People have lived in the area for thousands
of years. In ancient times the city
was the capital of the Ammonites, a
people often mentioned in the Bible.
Egypt conquered the city more than
2,000 years ago. Many other powers
controlled the city over the years. But
for reasons that are still unknown,
Amman disappeared by AD 1300.
In 1878 some people fleeing Russia created
a small village in the area. Ammans
revival began in 1921. That year it
became the capital of a territory called
Transjordan. Transjordan (now Jordan)
became an independent country in
1946. After that, Amman grew rapidly.
..More to explore
Jordan
Amoeba
An amoeba is a one-celled creature that
is among the simplest of all living organisms.
Most amoebas are so small that
they can only be seen though a microscope.
There they look like tiny blobs of
colorless jelly.
Amoebas are found in fresh and salt
water. They also live in the moist body
parts of other animals and in moist soil.
At least six types of amoeba are found in
human beings.
An amoeba has few parts. It has a thin
covering called a membrane. Inside is a
nucleus, food vacuoles, and a contractile
vacuole. The food vacuoles digest food.
The contractile vacuole gets rid of extra
water.
Amoebas move by changing their shape.
First, an amoeba extends a part of its
jellylike body outward, like a stubby
finger projecting out of a closed fist. This
projection is called a pseudopod, or false
foot. The amoeba then pours the main
Amman has many remains of
buildings from ancient times. A
theater built by the Romans
almost 2,000 years ago is just
one example.
108 Amman BRITANNICA STUDENT ENCYCLOPEDIA
part of its body into the pseudopod,
which makes the pseudopod grow
bigger. Eventually the pseudopod gets so
big that it becomes the whole body. New
pseudopods form as old ones disappear.
For food, amoebas depend mainly on
plants and other one-celled animals.
Amoebas use their pseudopods as
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 predators 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.