50 BRAIN AND BODY OF FISH 



surface feeding habits, as this area is very highly developed ; this 

 is also the case in the bleak, which has a similar habitat and method 

 of feeding. In fact, in the bleak figure, the central acoustic area 

 becomes a definite lobe which projects backwards from the base 

 of the cerebellum. The central acoustic area is also seen in the roach, 

 but is less marked ; but in the bottom-feeding Cyprinoids it is only 

 feebly represented. These facts suggest that surface feeding fish 

 find it advantageous to have an increased power of hearing. It is of 

 great interest to find that these considerations receive strong support 

 from observations made by Bhimachar on the Cyprinoids of the 

 Madras tanks. 



In a paper which was published in the Proceedings of the Royal 

 Society, he states, " The acoustic area or lobe is the terminal centre 

 in the brain of the auditory function. This area is very prominently 

 developed in all the sight feeders, and fairly well developed in the 

 ground feeding fish which come to the surface to take in air. But in 

 the purely ground feeding fish as Nemachilus, which is not exposed 

 to the influence of external sound waves this area is almost com- 

 pletely absent. Compared with the central acoustic area of the 

 British planliton feeding fishes, such as the bleak, the Indian forms 

 like rasbora, nauria, etc., have not only a larger central acoustic 

 area but also an extension of this area behind the cerebellum in 

 the form of a distinct central acoustic lobe. Tliis is evidently due 

 to a more perfect sight-feeding habit of the trojjical fishes and their 

 consequent exposure to the effects of external sounds. In the 

 accessory air-breathing Cyprinoids and Siluroids the extent to 

 which the central acoustic area or lobe is developed gives a strong 

 indication of the air-breathing habit." 



This confirmation of the observation of a British observer by 

 an independent Indian naturalist makes the conclusion, that the 

 auditory function of the central acoustic area or lobe in siu-face 

 feeding Cyprinoids is justified, will be supported by further observa- 

 tions on several other families of fish as w^e shall have occasion to 

 describe in the following chapter. 



We have now reviewed the relations of all the special senses to 

 the various lobes of the carp family and we arrive at a certain 

 conclusion which may be expressed as a law, "The pattern of the 

 brain of a bony fish is determined by the proportional representa- 

 tion of the special senses in its feeding or hunting equipment." 



