94 ZOOLOGY sect. 



the tail may either return directly to the heait in the normal 

 manner or may go by way of the capillaries of the kidneys. 

 In the latter case there is said to be a ratal portal system, the 

 essential characteristic of which is that the kidnev has a double 

 blood-supply, one of pure blood from the renal artery, and one of 

 impure blood from the renal portal vein ; in other words, it has 

 two afferent vessels, an artery and a vein, and the latter is further 

 distinguished by the fact that it both begins and ends in 

 capillaries instead of beginning in capillaries and ending in a vein 

 of higher order. 



The blood from the gonads is returned to the cardinals by 

 veins called s-permatic {sp. x.) in the male, ovarian in the female. 

 That from the paired fins takes, in what aj)pears to be the most 

 tyi^ical case, a somewhat curious course. On each side of the 

 body there is a lateral vein (lat. v.), running in the body- wall and 

 following the course of the embryonic ridge between the pectoral 

 and pelvic fins. It receives, anteriorly, a subclavian vein (scl. v.) 

 from the pectoral fin, and posteriorly an iliac vein {il. v.) from the 

 pelvic fin, and in front pours its blood into the precavak 



The veins from the stomach, intestine, spleen, and pancreas join 

 to form a large hepatic portal vein {h. p. v.), which passes to the 

 liver and there breaks uj) into capillaries, its blood mingling with 

 that brought to the liver by the hepatic artery {h. a.), a branch of 

 the coiliac. Thus the liver has a double blood-supply, receiving- 

 oxygenated blood by the hepatic artery, and non-oxygenated but 

 food-laden blood by the hepatic portal Aein (Fig. 778, L). In 

 this way we have a hepatic portal system resembling the renal 

 portal system both in the double blood-suppl}', and in the fact 

 that the afterent vein terminates, as it originates, in capillaries. 

 After circulating through the liver the blood is poured, by hepatic 

 vtins (h. v.), into the sinus venosus. The hepatic, unlike the renal 

 portal system, is of universal occurrence in the Craniata. 



In the embryo there is a sub-intestinal vein, corresponding with 

 that of Amphioxus, and lying beneath the intestine and the post- 

 anal gut. Its posterior portion becomes the caudal vein of the 

 adult, its anterior portion one of the factors of the hepatic portal 

 vein. 



To sum up : — The circulatory organs of the branchiate Craniata 

 consist of (a) a muscular organ of propulsion, the heart, provided 

 with valves and driving the blood into {b) a set of thick-walled, 

 elastic, afferent vessels, the arteries, from which it passes into (c) a 

 network of microscopic vessels or capillaries which permeate the 

 tissues, supplying them with oxygen and nutrient matters and 

 receiving from them carbonic acid and other waste products : from 

 the capillary network the blood is carried off by {d) the veins, thin- 

 walled, non-elastic tubes by which it is returned to the heart. 

 Thus the general scheme of the circulation is simple : the arteries 



