II THE PIGMENTS OF ORGANISMS 37 



only occur in cases where the coloured structures are 

 not intimately connected with the blood system. 



Another substance of this group which among 

 fishes is widely utilised as a colouring agent is 

 guanin. This substance, used in the manufacture 

 of artificial or Roman pearls, is colourless or chalk- 

 like, but it occurs in the skins of fishes as small 

 crystals which frequently display a beautiful iri- 

 descence and a pearly lustre. It is to these crystals, 

 mingled with pigments, that the soles, the cod family, 

 and numerous others owe their frequently beautiful 

 colour. The crystals occur in the scales, and also 

 in the deep layers of the skin, in the peritoneum 

 and in the air-bladder, as well as elsewhere. It is 

 interesting to note that in Elasmobranchs, which are 

 ancient fishes, although guanin is present in the skin 

 it has no metallic lustre, such as it exhibits in many 

 of the modern bony fishes. Guanin is not of course 

 a pigment in the strict sense of the word, but it is 

 of much importance in producing coloration in fishes, 

 and its composition makes its occurrence in the skin 

 of great interest. It may seem that undue stress 

 is laid upon these peculiar colouring matters, but 

 they are of much interest, because while of most 

 pigments the chemical relationships are unknown, 

 the substances of the uric acid and related groups 

 have been tolerably well worked out. This is 

 partly on account of their importance in practical 

 medicine, for we all know that the production of 

 an excess of uric acid or its imperfect elimination 

 from the body is in man associated with painful 

 diseases. Now in systematic zoology we dis- 

 tinguish higher from lower animals by the fact that the 



