Childrens Illustrated Encyclopedia

23

The animal kingdom is one of the largest

groups of living things; scientists believe that

there are up to 30 million species. animals

range from tiny, simple creatures that look

like blobs of jelly to gigantic blue whales.

The huge animal kingdom is divided

into many groups. a lion, for

example, belongs to the order

Carnivora because it eats meat.

it also belongs to the class of placental

mammals. all mammals are vertebrates

(animals with backbones) and belong

to a group called chordates. an animal

is a living creature that feeds, moves, and

breeds. during its life cycle, an animal is

born, grows, matures, reproduces, and

eventually dies. it ingests (takes in) food to build

and develop its body. Food provides the animal with

the energy to move around. Some animals do not move at

all; the adult sponge, for example, spends its life anchored to a

rock. all kinds of animals from dinosaurs to dodos have become

extinct; many others, including elephants and tigers, may soon

disappear if their habitats are destroyed and if they continue

to be killed recklessly for their hides and bones.

Mouth, entrance

to the digestive

and respiratory

systems

Kidneys, part

of the excretory

system

Sex organs,

part of the

reproductive

system

SmalleST

The smallest organisms are single-celled creatures called protozoa—so

tiny, they can hardly be seen by the human eye. The tiniest mammals

are the bumblebee bat and Savi’s pygmy shrew. This pygmy

shrew measures only 2.3 in (6 cm) including its tail.

Foot bones,

part of the

musculoskeletal

system

Orbit

(hole) for

eyeball

boneS oF The SkeleTon

The skeletons of vertebrate

animals are similar in design, but

each differs in certain details

through adaptation to the way

the animal lives. a frog, for

example, has long, strong back

legs for leaping. all vertebrates

have a skull that contains the

brain and the main sense organs.

most vertebrates also have two

pairs of limbs. Some bones, such

as the skull bones, are fixed

firmly together; others are linked

by flexible joints, as in the limbs.

Vertebrae,

bones that

form the spine

Stomach,

part of the

digestive

Heart, part of the system

circulatory system

Brain, control

center of the body,

and part of the

nervous system

Back limb

bones

Cloaca, exit from the digestive

and excretory systems

Skull, made

of separate bones

joined together solidly

Front limb

bones

inTernal organS oF a verTebraTe

inside an animal such as the frog above are many different

parts called organs. organs are all shapes and sizes. each

one has a job to do. Several organs are grouped together

to form a body system, such as the digestive system, the

circulatory system, and the reproductive system. The

nervous system and the hormonal system control and

coordinate all the internal systems.

Lungs, part of the

respiratory system

inTernal SkeleTonS

The animal world can be divided into

vertebrate animals and invertebrate

animals. vertebrates have an internal

skeleton with a vertebral column or

backbone. in most cases, this is made of

bone. Some sea-dwelling vertebrates,

such as sharks, have a backbone made of

tough, rubbery gristle called cartilage.

Frog

like many animals, the common green frog is aware of

its surroundings and able to move, feed,

and reproduce. Frogs belong to

the class of animals called

amphibians. all

amphibians spend

part of their

lives in or

near water.

Eye, one of the main senses,

and part of the nervous system

TalleST

The giraffe is the

tallest animal alive

today. a large male

can measure 17 ft

(5 m) to the top

of its horns.

Animals 23-

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