X 
THE COLOURS OF FISHES 
219 
Pigments and Structural Colours of Fishes 
As to the detailed characters of the coloration 
of fishes, there are several points of great interest. 
We have already mentioned the occurrence of 
crystals of guanin in the skin, which are important 
factors in the production of the iridescence and the 
silvery appearance of many fishes. The exact effect 
of these crystals on the coloration has not, however, 
been sufficiently determined, and it remains un¬ 
certain how far they are instrumental in producing 
the gorgeous colours of many tropical fishes. There 
is every reason to believe that these are optical in 
nature, but exact investigations into the mechanism 
of production are still required. Owing to the 
fugitive nature of the colours, the investigation 
would need to be conducted on the spot where the 
fishes are found, so we must look to some tropical 
biological station of the future for the complete 
solution of the question. 
The most important recent work on the colours of 
fishes is that of Mr. Cunningham and Dr. M‘Munn, 
who investigated especially the colours of the Pleuro- 
nectidse or Flat-fishes. The first point is to realise 
the position of the coloured structures. We have 
already noticed that fishes possess some power of 
adapting their colours to their surroundings, and this 
proves at once that the colouring-matter is deposited 
in living cells and is not a cuticular product. In 
fact, it is the dermis which contains the elements 
giving rise to the coloration. It will be remembered 
that the scales of fishes are, for the most part, dermic 
