3l6 FISHES CHAP. 



in certain features than is the case in any other group. . The pre- 

 caudal portion of the subintestinal vein retains much of its original 

 importance and runs in the rudimentary intestinal spiral valve as 

 far as the liver, where it becomes the hepatic portal vein. From 

 the liver the blood is collected into a single hepatic vein, and by it 

 is conveyed to the sinus venosus. The caudal section of the sub- 

 intestinal vein, now known as the caudal vein, bifurcates near the 

 anus, and its two branches become directly continuous with the 

 right and left posterior cardinals, without forming a renal portal 

 system. In their forward course to the heart the posterior cardinals 

 are situated directly beneath the notochord, and after receiving 

 the blood from the kidneys and gonads, and from the numerous 

 pairs of segmental veins of the body-wall, join the corresponding 

 anterior cardinal veins, and form on each side a short transverse 

 Cuvierian duct which opens into the sinus venosus. There is 

 also a pair of inferior jugular veins which, however, unite opposite 

 the fifth pair of gill-sacs to form a single trunk ; this vessel is 

 continued backwards, externally to the medio-ventral cartilage of 

 the branchial basket, and finally opens directly into the sinus 

 venosus. 



In Elasmobranchs (e.g. Mustelus antarcticus) l the caudal vein 

 (Fig. 186) lies in the haemal canal of the caudal portion of the 

 vertebral column. On reaching the kidneys the vein divides into 

 two renal portal veins, which, however, are not directly continuous 

 with the posterior cardinal veins as in the Cyclostomata, but, 

 on the contrary, after receiving the posterior segmental and 

 oviducal veins, become continuous with the capillaries of the 

 kidneys. 



From the latter organs the blood is collected by a series of 

 renal veins, and by them conveyed to the posterior cardinals, and 

 thence to the Cuvierian ducts. In the adult, therefore, there is 

 a well-developed renal portal system, but it is worthy of note, 

 nevertheless, that this system is developed comparatively late in 

 embryonic life, and that at an earlier stage the caudal vein is 

 directly continuous with the two posterior cardinals, precisely as 

 is the case in the Cyclostomata throughout life. The posterior 

 cardinal veins are situated in the dorsal wall of the coelom (Fig. 

 187), beneath the vertebral column. For the hinder portion of 



1 T. Jeffery Parker, Phil. Trans. 177, Pt. ii. 1886, p. 702. For references to 

 Elasmobranchs in general, see Parker, op. cit. p. 725. 



