422 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
The vene branchiales leave the gills at the dorsal end of the arch 
much as the arteries entered at the ventral end. Both the artery 
and vein of the first arch are straight vessels entering and leaving 
near the termination of the filaments, and so not requiring a branch. 
The fourth vein leaves the gill below the bend in the arch. 
Each branchial vein sends a branch backwards to the hyoid and 
mandibular regions while yet within the gill. 
THE ARTERIAL SYSTEM. 
The course and relation of the branchial veins (Pl. VIII., Fig. 1, 
I., IL, ILI., IV.) are as follows. The first branchial vein runs at 
right angles to the longitudinal axis of *the skull, and near its base 
gives off two branches (c. ew. and ¢. im.), which I have called the 
external and internal carotids. It then turns backwards along the 
ventral surface of the anterior cardinal, and is joined by the second 
branchial vein. The vessel thus formed unites with its fellow from 
the opposite side to form the aorta descendens. An artery to the 
pharynx, &c., springs from it at varying points. The vessels formed 
by the union of the third and fourth branchial veins enter from each 
side immediately below. 
The first branch from the descending aorta, after the junction of 
all the branchial veins, is a small impair artery from its median 
ventral surface to the ‘head-kidney.’ (Fig. 1, hk.) Immediately 
behind it, arises the arteria ccliaco-mesenterica (Fig. 1, em), a large 
single stem which supplies all the viscera, except the kidney. It 
passes downward between the air-bladder and the head-kidney, and 
to the right of the esophagus. The first branch supplies the air- 
bladder, the second the cesophagus and stomach, the third is the 
hepatic artery, the next branches pass to the anterior end of the 
intestinal tract, and then the splenic artery is given off. Here the 
mesenteric artery divides into two stems which follow respectively 
the right and left walls of the mesenteric fold and supply by many 
nearly parallel branches each its own half of the intestine.’ The 
left vis. that branch situated upon the attached portion of the 
mesentery is the larger, and from it springs the genital artery near 
the anterior end of these organs. 
One other impair artery is given off into the body cavity at its 
1 For the distribution in the various organs of the branches of the arteria cceliaco-mesenterica, 
xcepting the splenic and genital arteries, see Mr. Macallum’s paper. 
