STRUCTURE AND MECHANISM OF FISHES 7 



spiny finned fishes (Acanthopteri^- such as the Bass and Perch, in which 

 the spiny rays are conspicuously developed. The fin rays are supported 

 by small bony rods embedded in the flesh. The fin rays can be erected 

 like the mast of a canoe by the action of muscles at the base of the fin, 

 and if the front fin ray becomes very large, several of the supporting 

 rods below the rays may fuse together to afford a firmer base. 



By quickly erecting the fins a fish can suddenly check its forward 

 motion. On the other hand in darting forward the back fins are lowered 

 and the side fins pressed against the body. The back fins also act as 

 keels to prevent the body from slipping sideways through the water. 

 The spines in the fins serve to stiffen them and are also of protective 

 value. The pectoral and ventral fins are used chiefly to steer the fish up 

 and down and in turning to right or left, and also to steady the fish 

 when resting; sometimes, especially in slow-swimming or deep-bodied 

 fishes, the pectoral fins propel the body forward, the tail acting more as a 

 rudder. 



Normal types of fish have a compressed spindle-shaped body 

 provided with a wide tail fin, but many types become excessively long, 

 with very numerous joints in the backbone; these have more or less re- 

 duced tail fins and move through the water by wriggling as do eels. 



In the sharks, which are on the whole the most primitive of existing 

 fishes, the whole body including the head is covered with a tough skin, 

 thickly studded with small points, or granules. These often take the 

 form of thorns and they are composed chiefly of a substance resembling 

 the dentine of teeth covered with a thin enamel-like layer, so that it 

 may be said that a shark has little teeth all over his skin, which are 

 identical in structure with the larger teeth inside the mouth. 



In the higher fishes the teeth are confined to the jaws, to the roof 

 and base of the mouth, and to the skin that covers the gill arches in the 

 throat. 



In the oldest ganoid fishes and in the modern gar pike the head and 

 body are encased in a hard, more or less shiny, armor. This is formed 

 in the deeper layers of the skin by bony plates, which are covered with a 

 shiny enamel-like layer called ganoine. 



The armor covering the head is not continuous but is divided up 

 into separate and distinct bony plates connected either by movable 

 joints or by contact surfaces called sutures. The armor on the body 

 in primitive fishes is composed of oblique rows of close-set diamond- 

 shape scales connected with each other by peg-and-socket joints, as in 

 the gar pike. Later these scales begin to overlap each other, the back- 



