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- 

 nectidae 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 



