Childrens Illustrated Encyclopedia

206

For 518 million years, fish

have swum in the oceans. The first

fish had no scales, fins, or jaws,

unlike those we know today. The

range of fish that live in our

rivers, lakes, and seas is enormous—

from the great whale shark to the tiny

pygmy goby. in between are thousands

of other fish, such as swordfish,

sardines, parrotfish, and the comically

rounded puffer fish. some live in

freshwater; others in saltwater. some

skitter just below the surface; others swim

around in the depths. Despite their wide

variety, most fish have a streamlined shape,

ideally suited to a watery environment. a

covering of scales and slimy mucus protects fish

from parasites and helps them slip rapidly through

the water. Fish propel themselves along by their tail,

and steer and maneuver with their fins. among the

thousands of kinds of fish are some extraordinary

exceptions. lampreys have no jaws,

mudskippers can crawl across mud

flats, and catfish can crawl and

have no scales at all.

FeaTures oF a Fish

The cod has all the features of a

typical fish—a streamlined body for

speed, a powerful tail, and fins

for balance and steering.

The lateral line

along the body

is a row of sense

organs. These

organs detect

movements

made by

other

creatures

in the

water.

Space for

digestive

organs

Fish 206-

Fish were the first animals on

earth to have backbones. most fish have

a bony skeleton, like the flatfish shown

here, but there are a few exceptions. sharks

and rays, for instance, have a skeleton made

of a tough, gristly substance

called cartilage.

Mouth

First

anal fin

Second

anal fin

Caudal

fin (tail)

ScaleS

bony disks called scales

are embedded in the skin.

They protect the body and

usually overlap to allow

movement. although

there are four main

kinds, most fish have the

cycloid or ctenoid kind.

Gills

all fish can

obtain oxygen by

absorbing it from the

water through gills—blood-rich

structures on each side of the head.

Vertebral

column

(spine)

Skull

bones

Ganoid

(gar)

Ctenoid

(perch)

Placoid

(shark)

Cycloid

(salmon)

Spiny rays of

caudal fin

Skeleton of a flatfish Ribs

Fish

Lateral

line

Eye

Mouth

Operculum

(gill cover)

First

dorsal

fin

Pectoral

fin

Pelvic

fin

Vent

Heart

Gills

Pylorus

(stomach) Kidney

Internal system

of a fish

Swim bladder keeps the

fish buoyant in the water.

Intestine

insiDe a Fish

most of a fish’s internal organs lie in the lower half of

the body. The rest of the body is made of large blocks

of muscle called myotomes. Wavelike contractions of

these muscles make the tail move from side to side;

this produces swimming movements.

Brain

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