CIRCULATORY SYSTEM 801 



The blood is collected from these capillaries into a median longitudinal 

 sub-intestinal vein, through which the blood flows forward to pass into 

 the hepatic portal vein at the origin of the liver. This portal vein breaks 

 up into capillaries within the liver and is then collected in the hepatic 

 vein which extends along the dorsal portion of the digestive gland, where 

 it turns downward and forward to join the caudal end of the ventral 

 pulsating vessel. 



The vascular system of Amphioxus, therefore, consists primarily of 



(I) a dorsal vessel represented by the paired and unpaired dorsal aortae, 



(II) a ventral vessel represented by the subintestinal vein and the ven- 

 tral aorta, and (III) commissural vessels represented by the afferent and 

 efferent branchial arteries and the intestinal capillaries. This is quite 

 similar to the circulation in the earthworm except for two important 

 differences. The blood in the ventral vessel of Amphioxus travels for- 

 ward, that in the dorsal vessel backward just the reverse of what 

 occurs in the earthworm, while the ventral vessel is broken up into two 

 parts, by the interposition in its course of the capillaries of the liver, so 

 that all the blood from the intestine has to pass through the liver before 

 reaching the ventral aorta. This passage of the intestinal blood through 

 the vessels of the liver constitutes what is called the hepatic portal sys- 

 tem, which is characteristic of all vertebrates. 



FISHES 



The circulation in fishes corresponds quite closely in the main to 

 that of the chick's embryonic circulation. It is built about the gill sys- 

 tem. The blood is pumped forward from the ventral heart through the 

 gills, and is then, as arterial blood, carried backward in the dorsal aorta. 

 This scheme of circulation wherever found is interpreted as primarily 

 aquatic. 



The heart consists of four chambers : (a) sinus venosus, (b) auricle, 

 (c) ventricle, and (d) conus arteriosus, through which blood passes in 

 the order given. The sinus and auricle lie dorsal to the ventricle. 



In the lampreys there is no portal system. 



In the dogfish (Figs. 446, 449, 454), the circulation is laid out in 

 accord with the branchial system. The blood brought to the heart by 

 the venae cavae is pumped forward through a common ventral aorta 

 which divides into five pairs of afferent branchial arteries, each of which 

 carries blood to one set of branchiae. A corresponding efferent branchial 

 vessel picks up the aerated blood from the branchiae and carries it to a 

 dorsal aorta, through which it is distributed to all parts of the body, both 

 anteriorly and posteriorly. The general systemic, hepatic-portal, and 

 renal-portal systems return the blood to the heart along dorsal vessels, 

 called anterior and posterior cardinal veins. 



The fish-type of circulation is built primarily along lines laid down 

 by the branchial respiration and the heart pumps blood forward and 



