THE ELASMOBRANCHII. 119 



v^entricle. The third ventricle itself is a relatively wide and 

 short cavity, which sends a prolongation forward, on each side, 

 into a large, single, transversely-elongated mass (Fig. 37, a), 

 which is usually regarded as the result of the coalescence of 

 the cerebral hemis23heres, but is perhaps, more properly, to be 

 considered as the thickened termination of the primitive en- 

 cephalon, in which the lamina terminalis and the hemispheres 

 are hardly dijfferentiated. The large olfactory lobes are usually 

 prolonged into pedicles, which dilate into great ganglionic 

 masses where they come into contact with the olfactory sacs 

 (Fig. 37, A.^ s). The latter ahvays open upon the under-sur- 

 face of the head. A cleft, which extends from each nasal aper- 

 ture to the margin of the gape, is the remains of the embryonic 

 separation between the naso-frontal process and the maxillo- 

 palatine process, and represents the naso-palatine passage of 

 the higher Vertehrata. The optic nerves fuse into a complete 

 chiasma (Fig. 37, JB.^ c/i), as in the higher Vertehrata. In 

 some Sharks, the eye is provided with a third eyelid or nictitat- 

 ing membrane, moved by a single muscle, or by two muscles, 

 arranged in a manner somewhat similar to that observed in 

 birds. In both Sharks and Rays, the posterior surface of the 

 sclerotic presents an eminence which articulates with the ex- 

 tremity of a cartilaginous stem proceeding from the bottom of 

 the orbit. 



Except in Chimcera, the lab}rrinth is completely enclosed 

 in cartilage. In the Ra3's, the anterior and posterior " semi- 

 circular " canals are circular, and open by distinct narrow ducts 

 into the vestibular sac. In the other JE lasmohranchii they are 

 arranged in the ordinary way. A passage, leading from the 

 vestibular sac to the top of the skull, and opening there by a 

 valvular aperture, represents the canal by which, in the verte- 

 brate embryo, the auditory involution of the integument is at 

 first connected with the exterior. 



The testes are oval, and are provided with an epididymis 

 and vas deferens, as in the higher Vertehrata, The vas def- 

 erens of each side opens into the dilated part of the ureter. 

 Attached to the ventral fins of the male are peculiar append- 

 ages, termed claspers. 



The ovaria are rounded, solid organs. There are usually 

 two, but in some cases, as in the Dogfishes and nictitating 

 Sharks, the ovary is single and symmetrical. The oviducts 

 are true Fallopian tubes, which communicate freely with the 

 abdominal cavity at their proximal ends. Distally, they dilate 

 into uterine chambers, which unite and open into the cloaca. 



