1 64 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



is distinct and of the usual type, but in front of this are four pairs of lobes 

 which are interpreted by Retzius as, first, a pair of olfactory lobes, connected 

 with the enormously developed nasal capsule (p. 83); second, the cerebral 

 hemispheres; third, the mid-brain (the 'twixt-brain being covered) ; and fourth, 

 the cerebellum, which covers the fossa rhomboidalis. Lying in the median 

 groove is a prominence which is regarded as the habenula. In both genera 



^ 



WJT^r^ 



r-xt 



Fig. 170. Fig. 171. 



Fig. 1 70. — Dorsal and lateral views of the brain of Petromyzon planeri after Ahlborn. 

 The telae chorioideae removed, and the epiphysial structures not shown in the side view. 

 c, cerebrum; e, epiphysial structures; is, saccular part of infundibulum; la, acustic lobe; 

 Ih, left habenular ganglion; li, lobular part of infundibulum; Iv, lobe of vagus; m, mid- 

 brain; of, olfactory lobe; pc, posterior commissure; rh, right habenular ganglion; s, 

 first spinal nerve; th, thalamus; 1-12, cranial nerves. 



Fig. 171. — Brain of Bdellcstoma (Princeton, 2204). 0, skeleton of olfactory organ, 

 the brain behind this; V-X, nerves. 



(Myxine and Bdellostoma) the ventricular cavities are greatly reduced, the aque- 

 duct ending blindly in the mid-brain, in front of which is only the third ventricle, 

 completely cut off from the rest. 



ELASMOBRANCHS (figs. 172, 193) usually have the brain somewhat com- 

 pact, but in a few it is long and slender. The more striking features are the 

 slight development of the intercerebral fissure, the large hemispheres being 

 lateral expansions just in front of the diencephalon. The optic lobes are large 

 and the large cerebellum overlaps both lobes and the fossa rhomboidea. The 

 olfactory lobes arise from the antero-lateral angle of each hemisphere; their 



