H4 PISCES 



The post-cardiac portion of the embryonic subintestinal vein 

 contributes to the formation of the hepatic portal and renal portal 

 systems. It runs below the intestine, bifurcates to surround the 

 rectum, and joins again to a median caudal vein. This posterior 

 portion comes into connection with the hinder ends of the posterior 

 cardinals, and separates oft* from the intestinal portion (Fig. 73). 

 In front the subintestinal vein joins the paired omphalo-mesenteric 

 veins, forming a complete loop, which subsequently becomes broken 

 up into capillaries in the liver. The anterior ends of the loop 

 form the hepatic veins ; the posterior ends form the portal veins, 

 receiving blood from the remains of the subintestinal and from 

 other newly formed veins of the alimentary canal. 



The cardinal veins receive the segmental parietal veins and 

 enter the ductus Cuvieri. The anterior cardinal of the embryo, 

 running ventrally below the spinal nerves, is partially replaced by 

 a large jugular vein (anterior cardinal sinus) passing forwards 

 outside the nerves to the head. This sinus sends a branch down 

 the hyoid arch, which joins the ventral inferior jugular vein (Fig. 

 70). The posterior cardinals, having joined the caudal vein, break 

 up in the adult kidney, or mesonephros, into the renal portal system 

 of capillaries. Two ventral longitudinal epigastric or lateral veins 

 unite just above the pelvic girdle, receive the iliac veins, then run 

 forward to open into the ductus Cuvieri, near which point they 

 receive the brachial veins (Figs. 70, 74). 



A special yolk-sac circulation is established in the embryos of 

 those fish whose eggs are supplied with a very large quantity of 

 yolk. It may be entirely venous and derived from the sub- 

 intestinal vein (Teleostei, Fig. 75), or partly arterial and partly 

 venous (Elasmobranchs, Fig. 76). In the latter case are found a 

 vitelline artery — a branch of the aorta passing out to the yolk-sac 

 on one side— and a vitelline vein returning behind to the hepatic- 

 portal region of the subintestinal vein (Balfour [29], Ziegler [512], 

 Wenckebach [485]). In the Elasmobranch the stalk of the yolk- 

 sac comes off from the gut in front of the hepatic diverticulum ; in 

 the Teleostomes the yolk is situated farther back, on or behind the 

 diverticulum. 



In the intestine of all living Pisces, excepting the Teleostei 

 (p. 362), is found a 'spiral valve'; it is a spiral infolding of the 

 Avail provided with blood-vessels, and serving to increase the 

 absorptive surface (Figs. 77, 78 [Parker, 313]). Another primitive 

 structure is the cloaca — formed partly by the expansion of the 

 posterior end of the enteron, partly by an invagination from the 

 exterior. Into the cloaca open the rectum by the anus in front, 

 and the urinary and genital ducts farther back (Fig. 90). 



Although it is not possible to give a satisfactory definition of 

 a large group like the Tisces, exhibiting such a wide range of 



