TELEOSTEAN FISHES 



ol.- 



o.n 



op. I. 



pc 



As regards the nervous system, the brain (Fig. 151) shows two striking 

 differences from that of the elasmobranch. There is a large well developed 

 cerebellum (c) which projects back as a somewhat tongue-shaped or- an 

 over the roof of the fourth ventricle. If, however, the brain is cut through 

 with a sharp knife in the sagittal plane it is seen that a forward con- 

 tinuation of the cerebellum dips inwards underneath the optic lobes, 

 encroaching upon the cavity of the mid-brain. This is known as the 

 valvula cerebelli. Whereas in the elasmo- 

 branch the cerebellum as it grows in length 

 bulges outwards, in the teleost on the 

 other hand only its posterior or tailward 

 portion bulges outwards, its anterior por- 

 tion bulging inwards into the ventricular 

 cavity so as to form the valvula cerebelli. 



There is reason to suspect that a some- 

 what similar though more extensive modi- 

 fication has taken place in the region of 

 the hemispheres. The portion of brain-wall 

 which in the typical vertebrate bulges out 

 to form the hemisphere appears to be re- 

 presented in the teleost by a solid thicken- 

 ing which projects into the cavity of the 

 anterior portion of the brain from its floor 

 (Fig. 151, c.s.) and is covered over by 

 a thin membranous roof (pr.). 



The last feature which need be men- 

 tioned is that the optic lobes are of rela- 

 tively enormous size in most teleosts (Fig. 

 151, op. 1.) in correlation with the very 

 high development of the eyes. 



The Teleost possesses the same outfit 

 of sense organs as the Elasmobranch. 

 The eyes, apart from their large size, show an interesting pecul- 

 iarity in regard to their focusing arrangements. A projection from 

 the choroid known as the falciform process penetrates the vitreous body 

 in the neighbourhood of the optic nerve and passes in a meridional direc- 

 tion towards the lens, to the equator of which it is attached by a curious 

 bulb (the " campanula Halleri "). The falciform process is provided 

 with longitudinal muscle fibres which by their contraction pull the lens 

 nearer the retina so as to focus more distant objects the eye when in 

 a state of rest being focused for objects close at hand. Here we have 



sp.c. 



FIG. 151. 



The brain of a teleost (Salmon). 

 (From Wiedersheim, The Cambridge 

 Natural History, vol. vii.) c, Cere- 

 bellum ;. c.s, solid mass projecting 

 into cavity of fore-brain ; ol, olfactory 

 lobe ; op.l, optic lobe ; pn.o, pineal 

 organ ; pr, thin roof of fore-brain, 

 partially removed ; sp.c, spinal cord. 

 Roman figures indicate cranial nerves. 



