I7B 



A HISTORY OF FISHES 



It will be unnecessary to describe the brain of a fish in any 

 detail, but a brief outline of its more important features may 

 be given, commencing with those at the anterior end which 

 ha\e been derived from the original fore-brain, and proceeding 

 backwards. At the extreme front is a pair of hollow chambers, 

 the olfactory lobes, the inner cavities of which are in com- 

 munication with the parts of the brain lying immediately 

 behind. These lobes, centres of the sense of smell, are large in 

 the Cyclostomes, relatively enormous in the Sharks and Rays 

 (Fig. 7 1 a), but in the majority of Bony Fishes tend to be reduced 

 in size, and may be placed at the end of lengthy stalks (Fig. 71B). 



olfactoru loi>€ 



n ^,, cerebral- ^^ 



.''' -hemisp^iere 



~ - ootic (ohe - 



hind prcun,- 



"i^eilu i 



nervesJ[ toX 



Fig. 71. — BRAINS. 



tax ..V ^ ^ 



A. Upper surface of brain of Ray (Raia sp.),x | 



callarias), X h. 



B 



B. The same of Cod {Gadus 



In the Cyclostomes, Selachians, certain of the more primitive 

 Bony Fishes, and in the Lung-fishes, the olfactory lobes are 

 followed by another pair of outgrowths, the cerebral hemispheres, 

 which may be completely differentiated into two lobes, or may 

 coalesce to form a single cerebrum (Fig. 71 a). In the Cyclo- 

 stomes these hemispheres are very small, forming mere 

 appendages of the olfactory lobes. They are rarely developed 

 in Bony Fishes, but, instead, a bulging chamber with a non- 

 nervous roof grows forward from the fore-brain, and from its 

 sides the olfactory lobes are formed. In higher vertebrates the 

 cerebrum is the centre of the more complex mental processes, 

 such as thought and reason, but in most fishes it seems to be 



