PISCES 371 



to meet this risk of mortality among the young. In some cases 

 (most sharks and a few bony-fishes) the eggs are fertihzed and 

 the young hatched within the body of the mother. Only a few 

 young are produced in such forms. ^ 



The eggs of fishes are usually well supplied with yolk, seg- 

 mentation being partial (discoidal, see §53). The unsegmented 

 portion comes to be surrounded by a yolk sac and furnishes 

 nourishment for the early stages of development. 



Fig. 185. 



\ 



Pig. i8s. The Smelt {Osmerus dentex). Bull. U. S. Fish Commission. 



391. Special Adaptations. — In addition to those already 

 mentioned the group of fishes shows many adaptations to special 

 modes of life. 



Color. — Most fishes show color as the result of pigment 

 in the cells of the skin, or of delicate markings on the scales. 

 In general, the tone of color tends to accord with the environ- 

 ment. This becomes very striking in some of the less active 

 forms, as the flounders, in which the colors may change more or 

 less rapidly to accord with the bottom on which they lie. It 

 seems probable that some degree of protection from enemies 

 may thus be gained, which would be of distinct value to the 

 species. In some cases, however, the color is in sharp contrast 

 with the environment, and may be very conspicuous. This is 

 believed to be a warning coloration in some instances, accom- 

 panying some disagreeable quality. Some deep-sea forms are 

 luminescent. This is probably of considerable importance, as 

 no sunlight penetrates to that depth. 



V Electrical Organs. — In several groups of fishes (rays, eels, etc.) , 

 certain muscular tracts have become so modified that under 

 nervous stimulus instead of producing motion by contraction 



