ON SHALLOW WATER FAUNAS. 37 



coloured, one being white and the other variously tinted, 

 according to the nature of the sea bottom. The fish when 

 at rest lies on the white side, which in most flat fish is 

 the left, and so exposes the coloured side only. By flapping 

 movements of the fins it is able to bury itself more or less 

 completely in the sand of the sea bottom, and further pro- 

 tection is afi'orded by the colour of the exposed surface 

 changing until it resembles very closely that of the bottom 

 on which it is resting. This power of changing colour 

 depends on varying degrees of contraction and dilatation of 

 certain pigmented cells in the skin — the chromatophores. It 

 appears to be an entirely involuntary action, and does not 

 occur in animals that are blind. 



The same power is exercised in a still more marked 

 degree, and here apparently voluntarily, by the Octopus, a 

 shallow water animal, living in holes in rocks, and able by 

 its great strength and the powerful suckers with which its 

 arms are provided to successfully resist the tidal action of 

 the waves. 



Keturning to the flat-fish, the most curious feature in 

 their organisation is the fact that, in accordance with their 

 habit of lying on one side, both eyes are situated on one 

 side — the coloured one — of the head. When quite young a 

 sole swims vertically with its back up, and has its eyes one 

 on each side of the head. Very soon it acquires the habit of 

 swimming and Ij'ing on its left side, and in accordance with 

 this the head becomes twisted so that the left eye is brought 

 over to the right side, and both eyes can be used when the 

 animal is lying at rest at the bottom. 



There can be no doubt that flat-fish are descended from 

 more ordinarily constituted fish with their eyes one on each 

 side of the head, and in the above history we have an 

 excellent illustration of the Recapitulation Theory, which 

 explains the early developmental stages and metamorphoses 



