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 Pleuronectes 
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 
