Body Covering 33 



to working out a really gopd waste-disposal system. Those 

 guanin crystals are waste products of the blood, carried out 

 to the skin and deposited there where they can do no harm; 

 and the same thing may be true of scales. As soon as a good 

 excretory system came on the market, guanin crystals went 

 out of use. 



Against this theoretically interesting but visually unexcit- 

 ing background, the color pattern of the fish is traced by 



Pigment Partly Fully 



contracted expanded expanded 



Figure 6. CHROMATOPHORE, GREATLY MAGNIFIED 



pigments in the skin. Some fish never vary, but the majority 

 have the ability to change color to a greater or less extent. 

 To accomplish this they make use principally of a simple 

 but ingenious set of mechanisms known as chromatophores. 

 Chromatophores are little sac-like cells, shaped like many- 

 armed stars, which are scattered through the skin in great 

 numbers. They contain pigment. The pigment can become 

 practically invisible by withdrawing into the center of the 

 cell, or it can expose its color in varying degrees by the extent 

 to which it spreads out into the arms of the star. 



Each chromatophore contains only one color — red, orange, 

 yellow, or black — and it is the amount of this color exposed 

 to view, in combination with the amount of each other color 

 exposed to view by the other chromatophores, and also in 

 combination with a certain amount of permanent, invariable 



