The New Childrens Encyclopedia

108

Fish

Fish are the biggest and oldest group of vertebrate.

They were the first animals to have backbones,

evolving 500 million years ago. There are around

25,000 species of fish, all of which are cold-blooded

and have bodies that are adapted to living in water.

The trunk, or back end, of the

fish is packed with swimming

muscles. It’s these muscle blocks

that make many fish good to eat.

TAKE A LOOK: GILLS

REPRODUCTION

 LARVAE Some species hatch as small,

fully formed fish, but others hatch as larvae

and will change as they grow.

 SPAWNING Many species release vast

amounts of eggs at a time, to increase the

chances of some surviving.

 MALE MOM Seahorses

are unusual in that the

female lays her eggs in the

male’s pouch and he carries

the young until they hatch.

Fish obtain oxygen using their gills. Water

is taken in through the mouth, flows over

the gills, and out under the gill covers on

the sides of the head. Most cartilaginous

fish do not have gill covers.

Gill filaments

Gill arch

Mouth

Esophagus

Direction

of water

movement Gill cover

Also called the tail fin, the

caudal fin acts as a paddle,

providing “thrust” to propel

the fish forward.

Dorsal fins give the fish stability, helping it to

make sudden changes in direction and stopping

it from rolling from side to side. This fish has

two dorsal fins, but other fish might have three

separate dorsal fins or just one.

The anal fin provides

stability as the fish swims.

There are two sets of paired fins. The pelvic

fins (shown) help “steer” the fish up and down

in the water, while the pectoral fins (not

shown) may be used for steering and propulsion

or even for “walking” along the seabed.

The gills contain a great many

blood vessels. Oxygen and

other gases are exchanged here.

Horny scales grow from

the skin, providing a

flexible protective covering.

The gas-filled swim bladder helps the

fish control its buoyancy. By inflating or

deflating its swim bladder, a fish can rise

up or sink in the water.

Fish are grouped into three orders:

O
 Jawless fish, such as lampreys,

have a suckerlike mouth and no scales.

Their bodies are supported by

notochords—a basic kind of spine that

is like a flexible rod.

O
 Cartilaginous fish include sharks,

skates, and rays. Their skeletons are

made of cartilage and their scales

resemble tiny teeth.

O
 Bony fish are the biggest class of fish.

They have skeletons made of bone.

FISH ORDERS

While some fish mate and give birth to live

young, most reproduce by releasing eggs

into the water. This is called spawning.

Often fish will gather at special spawning

sites where their young will have the best

chance of survival.

LIVING WORLD

The skeleton of a bony

fish comprises a

backbone made up of

vertebrae, fine rays to

support the fins,

and a skull.