THE STRUCTURE AND ACTIVITIES OF FISHES 273 



the water. The other fins aid the fish in maintaining an upright 

 position and help it to steer the body up or down, straight 

 ahead, or from one side to the other. 



The shapes and positions of the fins differ in different species; 

 for example, the caudal fin of the perch (Fig. 155) is bilaterally 

 symmetrical and is adapted to swimming straight ahead, whereas 

 that of the sturgeon (Fig. 163, A) is longer above than below 

 and tends to force the body downward to the bottom, where this 

 species obtains its food. 



Fishes may often be seen suspended in the water and almost 

 motionless. This they are able to do because of the presence 



V 



\ 



FIG. 158. Fish scales. 



A, placoid ; B, ganoid ; C, ctenoid ; D, cycloid. (From Parker and 



Haswell.) 



of an air bladder within the body which decreases their weight 

 until they are exactly as heavy as the amount of water they dis- 

 place. 



Protection. - - The enemies of the adult fish are principally 

 other fish, birds such as kingfishers, herons, gulls, and terns, and 

 flesh-eating mammals like the otter. From most of these the 

 fish escape by swimming away rapidly, but from ordinary physi- 

 cal injury they are protected by an exoskeleton of scales or 

 bony plates. 



SCALES. - -The scales are of several kinds; those most com- 

 monly seen are thin, oval structures (Fig. 158, C and D) which 



