X THE COLOURS OF FISHES 221 



often branched and contractile chromatophores, con- 

 taining different pigments, and of polygonal cells 

 of peculiarly silvery appearance called by Pouchet 

 iridocytes. The silvery iridescent appearance is 

 due to the presence of crystals of guanin. The 

 inner layer consists of the same elements. On the 

 lower surface of the skin the outer layer contains 

 iridocytes but no chromatophores, the inner consists 

 of an argenteum — that dead-white opaque layer 

 which clings to the surface of the muscles and is 

 familiar to all who have examined fishes. Though 

 no histological elements can be found in the argen- 

 teum in the adult, development shows, as might be 

 expected, that it is formed from the fusion of irido- 

 cytes. The silvery or dead -white appearance is 

 again due to crystals of guanin. The distribution 

 of the colouring elements of the skin may be sum- 

 marised in the following table :-h- 



This table is true for the flounder and, mutato 

 mutandis, for other fishes as well. The points of 

 importance are the presence, of a large amount of 

 reflecting tissue, especially on the lower surface, and 

 the absence of pigment cells on that surface. The 

 relative development of the parts varies of course 

 greatly in different fishes, but the distribution 

 appears to be fairly constant. The two layers of 

 reflecting tissue seem to have different functions in 



