Fish that swims actively in the water has a streamlined body shape which allows it

to move easily through the water with minimal frictional drag.

The body of a fish is covered with scales that overlap one another

with the free ends pointing backwardsto reduce the frictional drag in the water.

The movement of fish in water is due to theantagonistic action of the W-shaped segmental

blocks of muscles (myotomes) on both sides of thebackbones and the action of its fin.

Streamlined body shape produce the least resistance.

During swimming, the tail is swept from side to side to bend the body on either side alternately

and produce a thrust that propels that fish forward.

The sweeping of the tail is due to the contraction and relaxation

of the myotome on either side of the body that work antagonistically against the backbone.

Action of muscles and forces on the fish during swimming.

The contraction of the myotome on the right side of the body will bend to the right while

the contraction of myotome on the left side of

body will bend the tail to the left.

The continuous sweeping movement of the tail

from side to side produce a thrust that propels the fish Forward on a straight path as both the opposite Forces to the sides are

cancelled off.

The function of the fins in fish is to maintain the balance of the body during swimming.

The paired fins consists of the pectoral fins and the pelvic fins-:

I) The pectoral fins are used for steering, to changedirection and as a brake to slow down or stop the movement.

II) The pelvic fins are used for balance and to keep the fish steady by preventing diving and rolling movements.

The unpaired fins consists of one dorsal fin ,one ventral and one caudal fin or tail.

The tail is the propulsion organ.

The other unpaired fins are used for balancing by preventing

rocking and rolling movement.