234 



THE ELASMOBRANCH FISHES 



anterior (a.l.), intermediate (i.l.), and inferior lobes (i.l.h.). To these parts 

 are to be added the paired suj^erior lobes (s.l.) which lie at the sides of and 

 above the intermediate lobes. 



The mesencephalon (fig. 213, op.l.) is a conservative segment and yet it 

 varies considerably in different forms. In many it is relatively inconspicuous 

 because of the extreme development of the cerebellum, while in others it comes 



Fig. 213. Brain of Scymnus. (From Burckhardt.) A. Side view. B. Median sagittal view. 

 ch., cerebellum; c.r., restiform body; (q)., pineal stalk; ffin., median longitudinal bundles; 

 in., infundilnilum ; i.L, anterior lolie ; Lv., lobe of the vagus ; m-.n., median olfactory nucleus ; 

 op.l., optie lobe ; p.e., pallial eminence ; p.L, posterior or inferior lobe of hypophysis ; v.s., 

 vascular sacs ; //, optic nerve. 



to be unusually large. In all forms the roof of the mesencephalon is composed 

 of a right and a left optic lobe {op.l., figs. 210 and 213) which are hollow out- 

 pockets from the dorsal side of the mesencephalic segment. It is largely among 

 the cells of these lobes that the fibers of the optic nerve terminate. The ventro- 

 lateral part of this segment of the ])rain (see fig. 213b) is enlarged by longi- 

 tudinal swellings, the lateral fiber tracts of the mesencephalon. Through the 

 ventral walls the third cranial or oculomotor nerve ])asses, and from the roof 

 the fourth nerve leaves the brain stem (IT, figs. 210 and 212). 



The metencephalon as a segment is usually well developed in the Elasmo- 

 branchs. Dorsally it consists of the cerebellum (ch.) and ventrally it is swollen 

 by large fiber tracts {Scy)iinus, fig. 213b). The cerebellum is usually rhomboid 

 in shape and divided dorsally by a median longitudinal furrow into right and 

 left halves (fig. 210a). These may be further separated into anterior and 

 posterior parts l)y a second furrow at right angles to the first. In some forms, 

 as has been said, the cerebellum comes to be very complex and of inniiense size. 



