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 
