92 BRAIN AND BODY OF FISH 



The halibut grows to a very large size and has small smooth 

 scales. The eyes are separated by a fairly wide space. The mouth 

 is large with strong jaws, and pointed teeth equally developed on 

 both sides of the head. There is no air bladder. The upper side 

 is of an olive brown to black colour and the blind side is white. 

 The body is both longer and plumper than the majority of flat-fish. 

 On the other hand, the plaice has a shorter, deeper, and thinner 

 body, with a small mouth twisted over towards the blind side, so 

 that the jaws and teeth are more developed on that side, and the 

 teeth are adapted for crushing. 



We have already described the hunting methods of the plaice ; 

 it is only to be expected that the form of the halibut would indicate 

 a different mode of life, and we find, although it sometimes lies flat 

 on the bottom in wait for its prey, it is a powerful swimmer and 

 often goes in active pursuit of other fishes. It is a predacious fish, 

 and is found in depths up to 200 fathoms. Its diet is piincijDally 

 fish, but also feeds on crabs and shellfish, and is often found to 

 frequent grounds similar to those occupied by cod. 



The halibut is remarkable in its power to adapt its coloration 

 to its surroundings and for the rapidity with which the pigmentary 

 changes take place. Norman states that the fish does not consciously 

 imitate the suiToundings, the whole process partaking of the nature 

 of a series of reflex processes and accomplished in less than a second. 



The pattern of the brain of the hahbut can be readily surmised 

 by a consideration of its predatory habits and its deep sea habit. 

 The optic lobes are large, and the somatic sensory lobes very large ; 

 while the facial are small and no central acoustic area can be 

 demonstrated. The optic lobes are particularly enlarged in 

 a dorsoventral direction, and appear to bulge ventraUy so as to 

 give it a bi-lobed shape ; this, as in the plaice, may be associated 

 with its facility for protective coloration. 



A more difficult problem for investigation is the cerebellum ; 

 this is prominent and its lateral margins are extended ventrally 

 so as to overlap the margins of the somatic sensory lobes for some 

 distance posteriorly ; these lateral extensions are not only composed 

 of the stratum moleculare, but the stratum granulosum extends 

 downwards, so as to join the granular matter of the acustico- 

 lateralis lobes, which are not very prominent ; where these two 

 structures meet there is an accumulation of large ceUs, similar to 

 those known as the cells of Purkinje. This unusual arrangement 

 of stratum granulosum and acoustic tubercle may be explained 

 by the deep-sea habitat ; we shall have occasion to describe the 

 modifications that occur in the acoustic tubercles of such fish as 



