PISCES 



237 



surrounded anteriorly by the epididymis. The tube of the epi- 

 didymis is continued into the vas deferens which is dilated posteriorly 

 into the seminal vesicle and adjacent sperm sac. The vasa. de- 

 ferentia open into the urogenital sinus. Sperms pass along the 

 groove between the claspers of the male. 



The ovaries are anchored by peritoneum on each side of the 

 cardinal sinus. The eggs escape into the 

 body cavity, and enter the single anterior 

 aperture of the two oviducts. (Figure 1 13.) 

 The lower portions of the oviducts open 

 into the cloaca. Many of the dog fishes 

 and sharks are viviparous^ while the skate 

 is oviparous^ with a horny purse (Figure 

 114) secreted by its oviducal or "shell" 

 glands. 



Nervous System. — The brain consists 

 of the fused cerebral hemispheres with a 

 nervous roof, the optic thalamus or thala- 

 mencephalon, with dorsal pineal body, and 

 ventral pituitary body and thinly roofed 

 third ventricle within; the mid-brain with 

 paired optic lobes above, the crura-cerebri 

 below, the Aqueduct of Sylvius or iier 

 passing below; the cerebellum with its 

 anterior and posterior lobes both marked 

 by ridges and grooves; and the medulla 

 oblongata which has a thin vascular roof, 

 and lateral restiform bodies. There are 

 ten pairs of cerebral nerves, and many 

 paired spinal nerves.^ (Figure 115, Fig- 

 ure 116.) 



Sense Organs. — In the Elasmobranch Fig. 114. Egg case of a 

 eye, there is no focusing device. The shape skate. (Drawn by Norris 

 of the eye and the density of the vitreous Jones.) 

 humor aid in keeping the spherical lens close 



to the pupillary opening of the iris. The ears are sacs with three 

 pairs of semicircular canals. Within the vestibule are calcareous 



6 Norris, H. W., and Hughes, S. P. The cranial, occipital and anterior spinal 

 nerves of the dogfish, Squalus acanthias. Jour. Comp. Neurol., vol. 31, no. 5, pp. 



293-395- 



