272 



PRACTICAL ZOOLOGY 



of the world, so that they are very seldom 

 seen. The general needs of fishes and methods 

 of supplying them may be discussed to ad- 

 vantage with the perch as a basis (Fig. 155). 

 Habitat. Fishes are all aquatic. Some of 

 them are restricted to the salt water of the 

 sea, others to fresh water, and a very few 

 are, like the salmon, able to swim from the 

 sea into fresh water or from fresh water into 

 the sea without suffering any injury. 



Form of Body. Since the water offers 



view of a^sh (Spanish more resistance than air to movement through 



mackerel). (From it, and since fish as a rule must move rapidly 

 to catch their food and 



escape their enemies, it is not strange that 



the fish's body is long and slender, pointed 



at the ends, and compressed from side to 



side. This form offers very little resistance 



to the water '(Fig. 156). Variations in form 



depend upon the habits of the fish. For 



example, the flatfishes, or flounders (Fig. 



171), have thin bodies which adapt them for 



life on the sea bottom; the eels (Fig. 165, D) 



have a long, cylindrical body which enables 



them to enter holes and crevices, and the 



porcupine fish possesses a covering of heavy 



spines which stick straight out when it in- 

 flates itself and protect it from its enemies. 

 Locomotion. The principal locomotor 



organ is the tail, which is lashed from one 



side to the other, forcing the fish ahead, 



much as a boat is propelled by sculling or a 



steamer by its screw (Fig. 157). The tail is 



made more effective by the presence of the * ^ "(lite" 



caudal fin, which offers more resistance to Pettigrew.) 



FIG. 157. Diagram 

 showing how the tall 



