26 Cincinnati Society of Natural History. 
however. The arteries, like the corresponding myotomes, 
are not arranged in bilateral symmetry. The branches are 
given off alternately, especially in the anterior region. (See 
Fig. XX.) A short distance behind the junction of the 
internal carotids, the anterior dorsal aorta crosses over and 
joins the left internal carotid, just behind the origin of the 
lateral branch. 
The posterior dorsal aorta (Figs. XII, XV, XVIII, XIX, 
XX, Ao), is the posterior continuation of the same vessel 
which anteriorly forms the anterior dorsal aorta. Beginning 
behind the junction with the lateral carotids, the posterior 
dorsal aorta gives off branches as follows: 
(a) A pair of somatic branches, and usually a mesenteric 
twig to the intestinal wall. 
(6) Twigs to the right and left Avonephros (pnl, pnr). 
These may come off as separate twigs, but usually arise in 
common with somatic branches (Fig. XIX). 
(c) The coeliac artery is a relatively large vessel. It oc- 
casionally gives off a twig to the left pronephros. It then 
passes downward between the sinus venosus and the alimen- 
tary canal (to which it gives off a small branch). The coeliac 
artery then proceeds along the common bile duct to the gall- 
cyst, which it supplies. Then it divides into two terminal 
vessels, anterior and posterior, which pass along the bile 
ducts to the anterior and posterior lobes of the liver. Here 
they break up into capillaries which supply these structures. 
(d) Mesenteric arteries (Figs. XII, XV, XVIII, mes)’ are 
given off ventrally, which pass downward between the right 
and left posterior cardinal veins, in the mesentery, to the 
intestinal wall. Just above the intestinal wall they usually 
divide, one branch going to the right, the other to the left 
side of the intestine. Both branches pass to the left of the 
supra-intestinal vein, but pass one on each side of the vagus 
nerve, which lies in the median line just above the intestinal 
wall. There are about thirty mesenteric arteries, and they 
arise in a somewhat irregular manner. Often, however, they 
arise in couples, one a short distance behind the other (See 
Fig. XII). 
(ce) The somatic arteries arise regularly along the entire 
length of the dorsal aorta, anterior and posterior (Figs. XII, 
14 
