of whic-li are formed posteriorly by peritoneum, 

 anteriorly by pericardium and the walls of the sinus 

 \enosus and the ducts of Cuvier, which enter the 

 sinus. No specific differences have been observed 

 in these structures or in the heart itself. 



After leaving the heart, blood is carried in the 

 ventral aorta, which sends an afferent branchial 

 artery into each of the gill arches. After circulating 

 in the capillaries of the gill lamellae, the blood from 

 each gill arch enters an efferent branchial (epihranch- 

 ial) artery. The anterior two epibranchials of each 

 side unite to form a common trunk, and these trunks 

 join as the "Y" of the aorta, usually beneath the 

 second vertebra, to form the dorsal aorta (fig. 19). 

 The posterior two epibranchials of each side also 

 unite, and their short common trunks join the dorsal 

 aorta, usually beneath the third vertebra. The 

 dorsal aorta continues posteriorly along the dorsal 

 body wall to the first haemal canal. The coeliaco- 



Khu'RE 19. — Representative piiUiTii.-i of anterior hranclie.s 

 of dorsal aorta in 7'/ihh«».s species. (Left) T. alalunga; 

 (nii(lille) T. (tttanticiit;; (riglit) T. tonggol. Y: Y of aorta. 

 ANT KPI; anterior epil>ranehials. I'OST EIT: posterior 

 epibranchials. C-M: coeliaco-mesenteric. SEG: segmen- 

 tal. L. CUT and R. CUT: Left and right cutaneous. 

 VI'^RT: vertebrae separated by dashed lines. 



CSOf^'" 



,fnirp'',fi'p 



uuiiunintiimm 



iiy;jjjjjju//jjjjjj;jjjjj; 

 c 



KiCiURE 20.— Cutaneous system of arteries (red) and veins (bhiel of Tbunnus ohcsus. (upper) Coursse of cutaneous vessels in 

 superficial musculature. (A) Enlarged transverse section. (H) Enlarged partial view of C, with portions of the arterial 

 walls cut away to show origin of arterioles and venules. (C) Enlarged lateral view of cutaneous vessels. (D) Posterior 

 course of cutaneous vessels; no posterior commissure. From Godsil and Byers, 1944 (fig. 66). 



88 



U.S. FISH .\N'D WILDLIFE SERVICE 



