118 



THE ANATOMY OF VERTEBRATED ANIMALS. 



The kidneys of the Elasmohranchii do not extend so far 

 forward as those of most other fishes. The ureters generally 

 become dilated near their terminations, and open by a common 

 urinary canal into the cloaca behind the rectum. 



The brain is well developed. It usually presents a large 

 cerebellum, overlying the fourth ventricle, the side-walls of 

 which {corpora restlformia) are singularly folded (Fig. 37, A.^ 

 a) ; and moderate-sized optic lobes, which are quite distinct 

 from the conspicuous thalamencephalon, or vesicle of the third 



f\Q. 37. — The brain of the Skate {Raia batis). A. From above ; JB. A portion of the van- 

 tral asyject enlar<,'ed : s, the olfactory bulbs ; a, the cerebral hemispheres which are niiited 

 Id the middle line; Z/, the thalamencephalon ; c, the mesencephalon; (?, the cerebellum : 

 a a, the plaited bands formed by the corpora restiformia; I., II., IV., V., the cerebral 

 nerves of the corresponding' pairs ; / the medulla oblongata ; w, a blood-vessel. In B. : 

 ch, the chiasma of the optic nerves; A, the pituitary body; wand v vessels connectwl 

 with it; k, the saecvs vasculoeus ; /3, the pyramids of the medulla oblongata. 



