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 :— 
Upper Surface. 
Lower Surface. 
Outer layer 
Inner layer 
Iridocytes and Chromatophores 
Iridocytes and Chromatophores 
Iridocytes only 
Argenteum 
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 
