422 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



The vence 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 (PI. VIII., Fig. 1, 

 L, II., III., 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. ex. and c. in.), which I have called the 

 external and internal cai*otids. 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 cceliaco-mesenterica (Fig. 1, cm), 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 oesophagus. The first . branch supplies the air- 

 bladder, the second the oesophagus 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. 1 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 ereliaeo-mesenteriea, 

 xcepting the splenic and genital arteries, see Mr. Maeallum's paper. 



