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 

 FIG 156 -- Front resistance than air to movement through 



view of a fish (Spanish 



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

 Dean - ) 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 

 caudal fin, which offers more resistance to 



FIG. 157. Diagram 

 showing how the tail 

 of a fish is used in 

 swimming. (After 

 Pettigrew.) 



