FISHES 181 



with both eyes on one side of the head. The very 

 young, however, have the eyes on opposite sides as 

 in ordinary fishes. As the fish grows and habitually 

 swims on one side close to the bottom, the eye on the 

 downward side, that is, the one turned away from 

 the light, begins its migration toward the light, .for 

 of what use would an eye be that is turned away 

 from the light and more or less buried in the sand.^^ 

 Nature usually is economical and makes every organ 

 serve a purpose. Therefore in the flat fishes provi- 

 sion is made to bring the otherwise worthless eye 

 around to the light, even though such a drastic 

 measure as the twisting of the skull is involved. 



The lump-fishes, the puffers, the burr-fishes, and 

 the porcupine fishes are all rather globular in form. 

 They cannot swim rapidly, and they cannot escape 

 from their enemies effectually through rapid retreats. 

 Therefore nature has provided them with other 

 means of defense. The lump-fishes are covered with 

 bony plates and the burr and porcupine fishes with 

 spines. The puffers, the burr, and porcupine fishes 

 are all capable of inflating the body, thereby making 

 themselves much larger in size and causing the spines 

 and bristle to stand erect. 



The most peculiarly shaped fishes live in the deep 

 seas. Some of these are nearly all head, others have 

 enormously large mouths, still others are provided 

 with very long tails of many shapes, each species 

 being adapted to the peculiar life it leads. 



It is an interesting fact that in many species the 

 males are smaller than the females. The greatest 

 difference in size of the sexes occurs among the top- 



