40 SUPPLEMENT TO THE BOOK OF THE BLACK BASS. 



takes on the characteristic colors and spots of the brook- 

 trout. 



The external coloration of fishes depends on the pres- 

 ence of variously-colored pigment-cells in one or both lay- 

 ers of the skin. These pigment-cells are under the influ- 

 ence or control of the nervous system, and are able to cause 

 changes of color which may be rapid or temporary, or more 

 or less permanent. 



In some fishes, as the Black Bass, the change in colora- 

 tion is involuntary, and is, without much doubt, occasioned 

 by an increase or decrease in the number of the different 

 pigment-cells, owing to the influences of light, depth of 

 water, temperature, surroundings, etc. 



In some other fishes the change of coloration is much 

 more rapid, and seems to depend on a contraction or ex- 

 pansion of the pigment- cells already developed, and which 

 are very sensitive to surrounding conditions, especially to 

 light. Owing to this rapid change in the colors of certain 

 fishes, when exposed to the light, they are thought by some 

 to be endowed with the power of changing their colors *at 

 will, or voluntarily. This, however, is not very likely. 



