CH. XX] EXCRETORY SYSTEM 465 



FIG. 226. ScylUum canicula ? . Ventral view of viscera. 



1. Left naris. 2. Mouth. 3. Pectoral fin. 4. Pelvic fin. 



5. Aperture of cloaca. 6. Pericardial cavity. 7. Ventricle. 7'. Conus 

 arteriosus. 8. Auricle. 9. Sinus venosus. 10. Coelomic opening 

 of oviducts. 10'. Falciform ligament. 11. Shell-gland. 12. Ovi- 

 duct. 13. Ovary reflected over to the right so as to show 12, which lies 

 external to the attachment of the ovary. 14. Liver. 15. Proximal 

 limb of stomach. 16. Distal limb of stomach. 17. Intestine. 



18. Rectum. 19. Spleen. 20. Pancreas. 21. Pancreatic duct. 

 22. Bile-duct. 23. Dorsal fin. 24. Spinal cord. 25. Noto- 



chord in centrum of vertebra. 26. Caudal artery. 27. Caudal vein. 

 28. Lateral line. 29. Myotomes. 30. Abdominal pores. a. He- 

 patic artery. b. Intestinal branch of anterior mesenteric artery. 

 c. Lienogastric artery. d. Gastric branch of lienogastric artery 

 (posterior gastric artery). e. Splenic branch of lienogastric artery. 

 /. Portal vein. g. Intestinal vein. h. Splenic vein. 



kidney is the metanephros, and its tubules unite into about six 

 main ducts, which are termed ureters. These unite with one another 

 a very short distance from the cloaca to form a metanephric duct 

 or common ureter. There is also a blind sperm sac into whose 

 posterior end the vesicula seminalis opens and which immediately 

 after receives the ureter. The compound duct thus formed meets 

 its fellow in the middle line and so there is a single urino-genital 

 sinus which opens into the cloaca behind the anus. In the female 

 the mesonephros is more vestigial than in the male and its duct 

 (the archinephric duct) is in front a very fine tube which lower down 

 dilates and meets its fellow to form a median urinary sin us. This 

 receives the five or six ureters on each side from the metanephros, 

 and opens into the cloaca behind the oviduct. There is thus no 

 common ureter in the female, and the so-called common ureter of 

 the male is formed in quite a different way from that in which the 

 ureter of the higher Craniata is formed (see Fig. 216, G, H). 



Actual sexual congress or copulation takes place in the Elasmo- 

 branchs ; the most posterior rays of the pelvic fins called the 

 claspers are enlarged, and used to distend the cloaca of the female 

 to allow of the entrance of spermatozoa (Fig. 227). This is cor- 

 related with the large size and small number of the eggs and their 

 long retention in the oviduct. In the male the spermatozoa are 

 stored in a swollen portion of the vas deferens, the vesicula\ 

 seminalis, or in special pouches termed the sperm-sacs. It is 

 probable that the claspers, the large eggs and the division of the 

 kidney into two parts are specialisations peculiar to modern 

 Elasmobranchs. 



The Elasmobranchs are the Sharks, Dog-fish, Skates and Rays 

 of our seas. They are almost exclusively marine and are a group 



s. & M. 30 



