PISCES 173 



(3) The blood from the head is brought into the precaval 

 sinus by two jugular veins on each side. 



The blood which is sent to the stomach, spleen, pancreas, 

 and intestine by the coeliac and mesenteric arteries is returned 

 by gastric, splenic, pancreatic, and intestinal veins. These 

 fuse to form a common trunk, the hepatic portal vein. The 

 liver, like the kidney, receives therefore a supply of arterial 

 and venous blood, and the blood is conveyed by the hepatic 

 veins into the hepatic sinus, and so to the precaval sinus. 

 The venous system is somewhat complicated by fusions and 

 expansions of the vessels, but it indicates a type of venous 

 system which in the main is the primitive one of the Craniates. 



Endocrine Organs. A large spleen is placed between the 

 two loops of the stomach, and isolated parts of the splenic 

 tissue may be seen sometimes around the outer border of the 

 stomach. It is derived from mesenchyme of the splanchnic 

 layer of the body cavity, and forms a large organ in which the 

 blood flows in ill-defined spaces. The blood is derived from 

 the anterior mesenteric artery, and is discharged into the portal 

 vein. In front of the ventral aorta a thyroid gland is to be 

 found as a rounded red body. It originates as a diverticulum 

 of the floor of the pharynx in front of the hyoid. It is the 

 homologue of the endostyle of Amphioxus. The duct is lost, and 

 it takes up the position indicated. The thymus is paired and 

 lies on each side just above the gills, and a rudimentary thymus 

 is said to be related to the spiracle. The thymus is developed 

 by bud-like outgrowths of the dorsal angle of the gill clefts. A 

 pituitary body, or hypophysis, is attached to the infundibulum, 

 and both maintain their independence. The infundibulum is 

 an outgrowth of the floor of the thalamencephalon, and the 

 hypophysis is a diverticulum of the ectoderm of the inner end 

 of the stomodeal invagination. The adrenal bodies are small 

 yellow bodies which lie dorsally between the kidneys and in 

 relation to the sympathetic ganglia. Each consists of two 

 parts, one derived from the sympathetic ganglia, and the other 

 from the peritoneum. The former is the medullary chromo- 

 phile substance, and the latter the cortical substance, of the 

 adrenals of higher animals. In the skate they are indepen- 

 dent, the medullary bodies remaining in connexion with the 



