The Fish

Essay by MohammedHHigh School, 12th gradeB+, June 2004

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Osteichthyes-Bony fish

(eg.Tilapia sp.)

Most bony fish belong to the group of ray-finned fish called Teleostei. Most food, sport and aquarium fish belong to this group. About 30000 species and sub-species are known.


Bony fish occur in seawater, fresh and estuaries.

External characteristics

The body is divided into head,trunk,and tail.On the head are a mouth, two large eyes, two external nostrils for smelling and an operculum(a bony plate which covers the gill-slits)one each side of the body.

The trunk is bilaterally flattened and covered with dermal scales. The paired fins are the pectoral and pelvic fins. The dorsal fin can be paired or unpaired. Ventral, just before the anal fin opens the cloaca. On either side of the body is the lateral line (a group of sensory organs) which detects streaming movements and vibrations in the water with which the fish determines its depth.

The tail is muscular.


The fins control locomotion, which is caused by the muscular caudal (tail) fin of the fish. Fig.1:The dorsal and pelvic fins serve to keep the body upright when the fish makes a sudden change of direction. The pectoral and pelvic fins balance and steer the fish as it moves. The caudal fin curves slowly outwards and then rapidly back to the midline, the tail thus presses backwards against the water and in this way the fish is driven forward. This action is repeated on the other side causing a further forward movement of the fish. The swim bladder, is situated in the trunk, below the vertebral column. Some fish fill the swim bladder by gulping in air at the water's surface, others slowly fill the swim bladder with oxygen diffusing into it from the surrounding blood-vessels. Likewise, the volume of air can be reduced by...