76 THE STORY OF FISH LIFE. 



of colour which it afforded daring its death 

 struggles. 



Some colours are due to a combination of two 

 or more pigments. Thus the exquisite green 

 colour of the mackerel, so familiar to us all, is 

 due, not to a green pigment, but to a blending 

 of black and yellow chromatophores. 



But the colours of fishes are not all due to 

 pigment. Some are what is called structural. 

 For instance, the silvery iridescent appearance 

 of many fishes is due to the presence of crystals 

 of a substance known as guanin, derived as a 

 waste product of the blood. These guanin 

 crystals figure very conspicuously in the colora- 

 tion of fishes. 



We may gather then that the coloration of 

 fishes must be regarded as largely reflecting, and 

 determined by, their need for protection. It 

 may be either permanently dull or brilliant, or 

 more or less rapidly changed from one to the 

 other extreme. 



Sometimes the coloration may be brilliant at 

 one season of the year and dull at another, and 

 then is generally connected with the niceties of 

 courtship. In such cases fleeting changes from 

 dull to brilliant or vice versa, due either to excite- 

 ment or sometimes fright, are common. These 

 changes are, we have seen, due to the action of 

 contractile cells containing colouring matter 

 called chromatophores. 



But, it may be remarked, although it has been 

 shown that fishes undergo rapid and marked 

 changes of colour, of such a nature as to cause 

 them to resemble that of their immediate sur- 



