CHAPTER IX 

 FLAT-FISHES 



The size and abundance of flat-fishes and the flavour of their flesh 

 malces this family ver^'^ familiar and useful to man. Flat-fishes are 

 so-called from their strongly compressed, high and flat body. They 

 are unable to maintain their body in a vertical position and rest and 

 swim on one side of the body only. The side turned towards the 

 bottom is sometimes the left and sometimes the right, colourless 

 and termed the " blind " side : that turned towards the light is 

 pigmented and frec[uently has adaptive patterns. Both eyes are 

 on the coloured side, and this is the most marked sign of the asym- 

 metry of this family. In the larval stage these fish appear to be 

 symmetrical and the change in the position of the eyes has always 

 aroused great interest and also the expression of conflicting views. 



In pursuit of our study of the relation of hunting and feeding 

 liabits to the form and pattern of the brain, it would appear probable 

 that this asjanmetry would seriously interfere with the study ; 

 however, the only parts of the brain that show any distortion are 

 the olfactory lobes and, of course, the optic nerves. The optic 

 lobes, the cerebellum, and the medulla oblongata are practically 

 symmetrical. It will be of interest to enquire how this has happened, 

 and to study the history of the migrating eye. According to Norman 

 in his recently published monograph on Heterosomata, " in the early 

 stages of the development of the larva, before the bones of the skull 

 have become ossified, there is a supraorbital bar, as it is called, 

 which lies in the way of the migrating eye ; this disappears or is 

 absorbed and the eye passes through the gap, until it reaches the 

 supraorbital bar of the other side. This bar of the ocular side 

 becomes twisted over towards that side of the head by the movement 

 of the two eyes into their final position. The supraorbital of the 

 ocular side is finally absorbed, and as soon as the shifting of the 

 eyes has been completed the frontal bones appear." 



There is a genus of flat-fishes known as Psettodes which has 

 retained a more symmetrical form and in which the eyes are as 

 often found on the right side as on the left side. It is said to swim, 

 not infrequently, in a vertical position, and there are good reasons 



75 



