EXCRETORY AND REPRODUCTIVE SYSTEMS. 423 



lumbar regions (the renal portal system) ; blood leaves the 

 kidneys by posterior cardinal veins, which enter the cardinal 

 sinus. 



Finally, be it noted that into the pre-caval sinus, there 

 opens the lymphatic trunk, with nutritive fluid from the 

 intestine. 



The heart lies in a pericardial cavity, which is connected 

 with the abdominal cavity by two fine canals. 



The dark red spleen lies in the curve of the stomach. 

 The red thyroid gland lies just in front of the anterior end of 

 the ventral aorta. The whitish thymus gland is a paired 

 structure lying dorsally above the gills. 



Excretory System (see pp. 398-9). — The elongated, dark 

 red kidneys lie posteriorly on each side of the vertebral 

 column. They are developed from the hind part of the 

 mesonephros. Several tubes from each kidney combine to 

 form a ureter. The two ureters of the male open into the 

 urinogenital sinus, whence the waste-products pass out by the 

 cloaca ; in the female they open into little bladders — the 

 dilated ends of the Wolffian ducts, and thence by a common 

 aperture into the cloaca. » 



The archinephric or segmental duct of each side divides 

 into a Wolffian and a Miillerian duct. The Wolffian duct 

 becomes in the male the vas deferens, in the female an 

 unimportant mesonephric duct ; the Miillerian duct becomes 

 in the female the oviduct, and in the male a mere rudiment. 



Reproductive System. — The male organs or testes lie on 

 each side of the cardinal sinus, moored by a fold of peri- 

 toneum. The spermatozoa pass from each testis by vasa 

 efferentia into the tube surrounded anteriorly by an epidi- 

 dymis. The tube of the epididymis is continued into the 

 vas deferens, which is dilated posteriorly into a seminal 

 vesicle and an adjacent sperm-sac. Finally, the two vasa 

 deferentia open into the urinogenital sinus, through which the 

 spermatozoa pass into the cloaca. In copulation, the com- 

 plex " claspers " of the male are inserted into the cloaca of 

 the female. 



The female organs or ovaries lie on each side of the 

 cardinal sinus, moored by a fold of peritoneum. In young 

 skates they are exactly like the young testes, but in the 

 adults they are covered with large Graafian follicles, each 



