PISCES. 367 



mentation being partial (discoidal). The unsegmented por- 

 tion comes to be surrounded by a yolk sac and furnishes nour- 

 ishment for the early stages of development. 



384. 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 

 buried in the cells of the skin, or of delicate markings on the 

 scales. In general, the tone of color accords 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. Some deep-sea forms are phosphorescent. This is 

 probably of considerable importance, as no sunlight penetrates 

 to that depth. 



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 con- 

 traction they form and accumulate electrical energy which may 

 be discharged at the will of the animal. This power certainly 



FIG. 183. 



FIG. 183. Young Sea-bass (Centropristis striatus). Photo from life by Dr. R. W. 



Shufeldt. 



Question on the figure. Locate the pelvic fin and compare with other 

 fish as to position. 



