the bodies of mammals and birds give
off heat.
Movement
Reptiles use various methods to move
from place to place. Most lizards walk
on four limbs. Some use only their hind
limbs when running. Certain snakes and
legless lizards move by pushing their
belly scales against rough surfaces and
pulling the body forward. Some desert
snakes use a type of movement called
sidewinding. They repeatedly bend and
lift the body to move across the sand.
Flippers or webbed feet help sea turtles
swim. Sea snakes use their flattened tails
like paddles in the water. Alligators and
crocodiles swim by whipping their tails
from side to side.
No living reptile can fly. However, some
tropical snakes and lizards can glide
from tall trees by flattening their bodies.
Life Cycle
Most reptiles lay eggs. Some lay just one
egg. Others can lay up to 200. It usually
takes about 60 to 105 days for the eggs
to hatch. Certain snakes and lizards give
birth to live young.
Most reptiles take at least a year to
mature. Unlike mammals, reptiles may
grow throughout their lives. Some reptiles
live longer than any other vertebrates.
Many species of reptile can live
Flying lizards cannot really fly. But they can
glide down easily from high places.
Some snakes
and the stinkpot
turtle let
out foul odors
to chase away
enemies.
52 Reptile BRITANNICA STUDENT ENCYCLOPEDIA
for more than 20 years in zoos. Certain
turtles have lived for more than 100
years.
#More to explore
Alligator • Animal • Bird • Crocodile
• Dinosaur • Lizard • Snake • Turtle
Republic
A republic is a form of government in
which the people elect, or choose, their
leaders. In most countries with a republican
government, the people elect the
head of the government, such as a president.
They also elect other people, called
representatives, to make laws.
The leaders of a republic are responsible
to the people who elected them. If the
people do not like the way their leaders
govern, they can vote them out of office.
This means that the people are the
source of power. This sets republics apart
from countries led by a king or another
all-powerful ruler.
Republics Today
Today most countries are true republics,
meaning that the people choose their
leaders. These republics practice a form
of democracy. A democracy is any government
in which the people have a say.
The United States, Mexico, India,
France, Kenya, South Korea, Peru, and
Indonesia are only a few of the world’s
many true republics.
Some countries call themselves republics
simply because a monarch (a king or a
queen) is not the leader. They are not
always democracies. For example, China
calls itself a people’s republic. But the
Chinese people have no choice in who
their leaders are.
Other countries practice democracy but
are not really republics, either. In the
United Kingdom the people vote for
representatives in Parliament (the country’s
lawmaking body). But they also
have a monarch. However, the monarch
has little real power.
An eastern box turtle gets its first look at the
world after hatching from a leathery egg. It
is on its own and will get no care from its
parents.
The president of Finland, Tarja Halonen,
ran for reelection in 2006. In a republic,
people elect their leaders.
BRITANNICA STUDENT ENCYCLOPEDIA Republic 53
History
The first republic was in ancient Rome.
In 509 BC the Romans set up a republic
that lasted for almost 500 years. A group
of elected leaders called the Senate was
the highest power in the government.
The next important democratic republics
formed in the late 1700s. In 1788
the United States set up the first modern
republic. An elected president leads the
U.S. government. Groups of elected
leaders called senators and representatives
make the laws.
#More to explore
Democracy • Government • Monarchy
• Rome, Ancient • United States
Government
Respiratory
System
All animals need a steady supply of oxygen
in order to live. Oxygen is a gas
found in air. It helps to turn food into
energy. This process creates another
gas—carbon dioxide. Carbon dioxide
must be removed from the body. The
respiratory system is the body’s way of
breathing in oxygen and breathing out
carbon dioxide.
Insects breathe through holes in the
sides of their bodies. Fish and young
amphibians have organs called gills to
take in oxygen from water. Mammals,
birds, reptiles, and adult amphibians
breathe through lungs.
The Human Respiratory
System
The human respiratory system includes
several parts: nasal cavity, throat, voice
box, windpipe, bronchi, and lungs. Air
enters this system through the nose. It
then moves into the nasal cavity, a hollow
space behind the nose. Air also
enters through the mouth. These two
airways meet at the upper part of the
throat, called the pharynx.
Insects have no lungs. They use holes called
spiracles and air sacs to breathe.
Fish have gills instead of lungs. A fish takes
in oxygen as water flows past its gills.
54 Respiratory System BRITANNICA STUDENT ENCYCLOPEDIA
From the pharynx, two passageways
open downward. One is for food, and
the other is for air. The passage for air
contains the voice box, or larynx. Below
the voice box is the windpipe, or
trachea.
