276 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
tory organs. Other arteries, arising from the dorsal aorta, run ventrally 
into the mesenterial structures and supply the alimentary canal and 
other viscera. Two pairs of these, the.omphalomesenteric (omphalo- 
mesaraic) and the hypogastric arteries, may be mentioned at present. 
The first of these arise in the trunk region, pass on either side of the 
intestine, and finally empty on the lower side of the body into the om- 
Fic. 282.—Diagram of the circulation in an early stage of a small yolked vertebrate 
(amphibian). , anus; ca, cv, caudal artery and vein; da, dorsal aorta; de, Cuverian duct; 
ec, external carotid; h, heart; ha, hypogastric artery; i, intestine; ic, internal carotid; 7, 
inferior jugular; j, superior jugular; J, liver; m, mouth; oma, omv, omphalomesenteric 
artery and vein; pc, postcardinal vein; si, subintestinal vein; 1-6, aortic arches. 
phalomesenteric veins, soon to be described. The hypogastric arteries 
arise from the dorsal aorta at the junction of trunk and tail and pass on 
either side of the intestine, to meet posterior continuations of the 
omphalomesenteric veins, here known as the subintestinal veins. 
Behind the origin of the hypogastric arteries the dorsal aorta is called 
the caudal artery (figs. 275, 282). 
VEINS. 
Behind the pericardium the edges of the descending lateral plates 
(p. 270) are kept from meeting by the anlage of the liver (figs. 276, 
277). The edges of the plates become grooved just as in front and each 
groove becomes rolled into a tube, lined with vascular cells, so that two 
vessels, the omphalomesenteric veins, extend backward from the heart, 
around the liver, to meet the omphalomesenteric arteries already de- 
scribed. Behind the connection of the omphalomesenteric arteries and 
veins the pair of vessels continue back, ventral to the alimentary canal 
as the subintestinal veins, until just behind the anus they fuse into a 
median tube, the caudal vein, which extends the length of the tail. 
The two subintestinal veins soon fuse to a single median vessel (fig. 
283, B) save for a loop around the anus connecting it with the caudal 
vein. The right omphalomesenteric vein disappears except for a short 
distance between the sinus venosus and the liver, leaving the left as the 
