88 ZOOLOGY SECT. 



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

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

 veins (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 



0,0 



i III 



br.a 



a.brc. 



ft U 



FIG. 728.- -Diagram illustrating the course of the circulation in a Fish. Vessels containing aerated 

 blood red, those containing non-aerated blood blue, lymphatics black. B. capillaries of the body 

 generally ; E. of the enteric canal ; G. of the gills ; K. of the kidneys ; L. of the liver ; T. of 

 the tail. a. br. a. afferent branchial arteries ; ac. auricle ; c. a. conus arteriosus ; (/. ao. dorsal 

 aorta ; e. br. a. efferent branchial arteries ; /;. p. r. hepatic portal vein ; k. <. hepatic vein ; 

 Ic. lacteals ; ly. lymphatics ; _/>/. tv. r. pre-caval veins ; /. p. r. renal portal veins ; s. r. sinus 

 venosus ; c. ventricle ; c. ao, ventral aorta. The arrows show the direction of the current. 



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 

 spring from the heart, or from arteries of a higher order, and end in 

 capillaries ; the veins begin in capillaries and end in vessels of a 

 higher order or in the heart. Actually, however, the system is 

 complicated (a) by the interposition of the gills in the course of 

 the outgoing current, as a result of which we have arteries serving 

 as both afferent and efferent vessels of the respiratory capillaries, 

 the efferent arteries taking their origin in those capillaries after 

 the manner of veins ; and (&) by the interposition of two important 

 blood-purifying organs, the liver and the kidney, in the course of 

 the returning current, as a result of which we have veins acting 



