82 BRAIN AND BODY OF FISH 



mens of the razor-shell ; the crustaceans were small specimens of 

 sand-hoppers and shrimps ; the fish small sand-eels." 



The above details of the diet of this family may be tedious and 

 seem trivial, but they serve to emphasise the peculiar epicurean 

 diet of the sole, which among a large concourse of bottom-feeding 

 fish chooses to specialise in worms and despises shell-fish. It is 

 interesting to speculate on the causes of this discrimination and to 

 see if any of the facts we have noted may help to solve the problem. 



In the first place, the presence of a central acoustic lobe suggests 

 that the ancestors of the sole were surface feeders and, the absence 

 of a marked gustatory system, that they were not bottom -feeders ; 

 while the large somatic-sensory lobes remind one of the similar 

 lobes of the whiting or ling and point to an inherited predacious 

 habit of feeding. But there is other evidence pointing to their 

 ancestors having been pelagic fish, namely, the presence of a rather 

 large swim-bladder which is entirely wanting in the full-grown sole, 

 but persists until it is half an inch long and until the left eye is very 

 near the edge of the head, as is pictured in a drawing by Cunning- 

 ham. The same observer notes the eggs of the sole when fertilised 

 are buoyant and float in water. It is distinguished from the eggs 

 of other fishes in British Seas by the fact that the outer layer of 

 the yolk is divided into separate segments and that there is an 

 immense number of minute oil-globules arranged in irregular 

 patches at the surface of the yolk. These considerations suggest 

 that the sole, originally a predacious fish, has at some time found 

 itself in a shallow habitat and adapted itself to a diet of worms, and 

 that its brain pattern has remained fairly constant in accordance 

 with its sensory equipment and that its acoustic centre has become 

 of advantage in its new habitat, and its habit of tapping the sand. 



The conclusion that can be drawn from this study of the brain- 

 pattern of the sole can now be reviewed. If it is assumed that the 

 sensory activities of a hunting fish depend, as a rule, on three domi- 

 nant senses, these in the sole appear to be : I. — Olfactory, as indicated 

 by an elaborate olfactory organ and large olfactory lobes ; II. — 

 Auditory as suggested by the large central acoustic lobe ; and III. — 

 Tactile as is evidenced by the large somatic-sensory lobes ; so that 

 smell, hearing, and touch are the dominant senses ; while on the 

 other hand, sight and taste are poorly developed as indicated by 

 the small optic lobes, and the minute facial lobes, by which all 

 gustatory impressions are received. 



As regards the central acoustic lobe, it will be remembered that 

 the bleak, a surface feeding cyprinoid, the pilchard, a plankton-feed- 

 ing clupeoid, the gurnard and the mullet, all have a central acoustic 



