X THE COLOURS OF FISHES 215 



characters ; it is unfortunately not one which is 

 likely to be settled by direct experiment, but the 

 careful observations to which the dispute has given 

 rise are worthy of all attention. 



In most Pleuronectidse the upper surface is 

 coloured with black and yellow pigment, the lower 

 is pure silvery white ; the peritoneum also is darkly 

 pigmented on its upper surface and white on the 

 lower, and this in spite of the fact that the upper 

 part does not shine through the body walls at all 

 and the lower very little. Two cases have been 

 described, however, in which the lower surface of 

 the skin is normally pigmented. In Plenronectes 

 cynoglossus the lower surface is gray and not white, 

 the colour being due to the presence of black but 

 not yellow pigment. This form " has been taken 

 at all depths up to 700 fathoms." The upper 

 surface contained both yellow and black pigment, 

 but was not so darkly pigmented as in the case of 

 shallow- water forms. It will be noted that this 

 appears like an approximation to the uniform colour- 

 ing of deep-sea forms. The other case is that of 

 Engyophrys sanctilaurentii, in which the blind side 

 has five or six dusky bands of colour occurring in 

 the anterior half of the body ; the posterior half is 

 colourless. 



Mr. Cunningham has made a series of careful 

 experiments on the artificial production of pigment 

 on the lower surface of flounders. The method 

 adopted was to keep the young flounders in tanks 

 under such conditions that the lower surface could 

 be artificially illuminated by means of mirrors. The 

 result of the experiments was to show that the 



