mesenteric artery arises from the dorsal aorta 

 beneath the third to fourth vertebra and forms two 

 or three main branches that go to the hver, supply 

 the vascular cones when present, and give off branches 

 to the other visceral organs. The paired cutaneous 

 arteries rise posterior to the origin of the coeliaco- 

 mesenteric artery and course laterad almost or quite 

 perpendicular to the aorta; they penetrate the lateral 

 body musculature, and on each side form two 

 branches beneath the skin that almost parallel one 

 another to the caudal peduncle (fig. 20, 21). At 

 about the level of the 30th vertebra, a posterior 

 commissure may connect the dorsal and ventral 

 branches of each side. Along their length, the 

 lateral branches give off into their interspace arteri- 

 oles, which are so dense that they seem to form a 

 solid sheet penetrating the dark muscle (chiai). 

 Cutaneous veins accompany the arteries; the two 

 parallel lateral branches join anteriorly on each side 



(fig. 22) to form a large vein that enters the duct of 

 Cuvier, which in turn enters the sinus venosus. 



If a post-cardinal vein is present it emerges from 

 the first closed haemal arch, runs toward the right 

 side in the kidney mass, and joins the right cutaneous 

 vein. There is usuall.y a cross-connection between 

 the post-cardinal and left cutaneous veins. 



Specific Characters 



The coeliaco-mesenteric artery usually has two 

 branches in T. thynnus, T. maccoyii, T. alalunga, and 

 T. atlaniicus, and three branches in T. albacares and 

 T. tonggol. In T. obesus either two or three branches 

 may be present in both Atlantic and Pacific speci- 

 mens. Exceptions were noted in T. albacares and 

 T. adanticus. 



A connecting branch near the liver between two of 

 the coeliaco-mesenteric branches is present in T. 

 maccoyii, ma}' be present or absent in T. thynnus, 



/ / / 



muilNlHlltlllH 



Figure 21. — Cutaneous system of arteries (red) and veins (blue) of Thunnus albacares. (upper) Course of cutaneous vessels 

 in superficial musculature. (A) Enlarged transverse section. (Bj Enlarged partial view of C, to show origin of venules 

 (dorsal) and arterioles (ventral). (C) Enlarged lateral view of cutaneous vessels. (D) Posterior commissure. From 

 Godsil and Byers, 1944 (fig. 31). 



ANATOMY AND SYSTEMATICS OF TUN.\S 



89 



