XIV NERVOUS SYSTEM Ey AC 
cerebellum is entirely absent. The ventricles are subject to 
some individual variation. Third and fourth ventricles are 
generally recognisable, either as isolated cavities or connected 
by a remnant of the mesocoele. In the feeble development of 
the prosencephalon, in the striking preponderance of the mid-brain 
b.0, 
MC, 
Fig, 213.—The brain of a Dog-Fish (Scylliwm canicula). A, dorsal view ; B, ventral 
view. The choroid plexuses covering the roof of the third and fourth ventricles 
have been removed. 0.0, Olfactory lobe ; ep, origin of the stalk of the pineal body ; 
J.6 (in A), prosencephalon ; 7.6 (in B), cerebral hemispheres ; fr, fourth ventricle ; 
h.b, cerebellum ; h.p, pituitary body ; 7.f, lobi inferiores ; m.b, optic lobes ; m.d, 
medulla oblongata ; sc, saccus vasculosus ; th, thalamencephalon ; ¢.0 (i) olfactory 
peduncle ; i.-x. cranial nerves. (From Wiedersheim. ) 
over the rest of the brain, and in the absence of a cerebellum, 
Myxine is unique amongst Craniates. . 
In Elasmobranchs among Fishes the brain attains a much 
higher grade of structure. In Seyllium (Fig. 213) there is 
a large prosencephalon, and directly in front of it a pair of 
imperfectly differentiated cerebral hemispheres, while from its 
antero-lateral regions the large olfactory lobes arise. The proso- 
coele divides in front into four diverticula, of which the two 
