pnd of the stomach, and a very large caecal mass is 

 attached to its origin by several ducts that are not 

 externally apparent. The intestine proceeds caudad 

 for half or more the length of the body cavity 

 (straight intestine), forms a loop, runs craniad 

 (ascending portion) almost to the pylorus, then forms 

 another loop and continues in a nearly straight line 

 (descending portion) to the anus. The spleen is 

 located between the straight and ascending portions. 

 The gall bladder is a long, tubular sac rising from the 

 riglit lobe of the liver, attached to the dorsal wall or 

 the left side of the straight intestine. A swim- 

 bladder, when present, is situated dorsad to the 

 main visceral mass, and may be rudimentary or well 

 developed. 



Specific Characters. — The ventral surface of the 

 liver (fig. 16) is striated in T. alalunga, T. maccoyii, 

 T. ihynnus, and T. obesus. These striations give 

 the impression of being denser and extending farther 

 toward the center of the middle lobe in T. alalunga, 

 T. maccoyii, and T. thyiuui^ than in T. obcsus, but 

 this difference is not easy to detect. We have seen 

 only one instance of striations being limited to the 

 peripheral margins, at least of the middle lobe, as 

 described by Kishinouye (1923) and Godsil and 

 Byers (1944) for Pacific, and by Morice (1953) for 

 Atlantic T. obesus. The peripheral nature of these 

 striations has been used as a major diagnostic char- 

 acter of the nominal genus Paralhunnm^. In T. 

 albacares, T. atlanticus, and T. tonggol the liver lacks 

 striations. 



In T. alalunga, T. maccoyii, T. Ihynnus, and 

 T. obesus the three liver lobes are subequal in length, 

 the lateral lobes most often slightly shorter than the 

 middle lobe. In T. albacares, T. atlanticus, and 

 T. tonggol the right lobe is much longer and narrower 

 than the middle or left lobe. 



Correlated with the ventral striations on the livers 

 of T. alalunga, T. maccoyii, T. thynnus, and T. obesus, 

 vascular cones are associated dorsally with each lobe. 

 The middle lobe always has a single large cone; the 

 other lobes usually have two or more, and these may 

 be somewhat difficult to distinguish from ordinary 

 l)lood vessels. In the left lobe, we rccortled two to 

 five cones in T. alalunga; two to six, usually two or 

 three, in T. thynnus; two to four in T. maccoyii; one 

 to six, usually one or two, in T. obesus. In the right 

 lobe we found: two to eight, usually two, in T. 

 alalunga; one or two, usually two, in T. thynnus; two 

 in T. maccoyii; one to four, usually one or two, in 

 T. obesus. 



Figure 16. — Ventral view of livers of Thunnus species, 

 showing shape and presence or absence of striations. 

 Fork lengths (left, right): AL.\— 875, 1,030 mm.; .\LB— 

 700, 1,175 mm.; .\TL— 568, 650 mm.; OBE— 1,450, 1,.540 

 mm.; THY— 457, 757 mm.; .ami TON— '.)10, <»2I mm. 



Morice (1953) described the liver of T. albacares as 

 having two efferent vessels at the anterior end, but 

 noted only a single vessel in T. alalunga, T. thynnus, 

 and T. obesus. He probably overlooked the vessel 

 in the right lobe near the junction with the middle 

 lobe, because this vessel is smaller than the one in 

 the middle lobe. All specimens that we examined 

 had two such efferent vessels. 



84 



U.S. FISH AND WILDLIFE SERVICE 



