Thunnus alalunga has the spleen on the left side 

 and the stomach on the right. In the other species 

 these positions are reversed (fig. 15). 



Godsil and Byers (1944) attached considerable 

 importance to their observation that tlie straight 

 intestine in T. alalunga crosses from the right to the 

 left side, and the descending portion lies on the left 

 side. This course is obviously correlated with the 

 position of the stomach in this species. We found 

 that the intestine often does not cross over so 

 obviously and that the descending portion is com- 

 monly near the middle. Thus there is little or no 

 difference between T. alalunga and the other species 

 in position of the descending intestine, although the 

 side on which the intestine originates is different. 



The relative position of the first loop of the intes- 

 tine (where the straight intestine forms the ascending 

 portion) differs to a degree among the species (fig. 15). 

 In T. alalunga this loop is shortest and is located 

 about one-half to two-thirds the distance between 

 the posterior margin of the middle liver lobe and 

 the anus. In the other six species the loop may 

 reach from about three-fourths to nine-tenths of the 

 liver-anus distance. In Pacific forms, T. alalunga 

 is reported by Godsil and Byers (1944) to have a 

 short "fold" (27-41 percent of body cavity), T. 

 albacares a slightly longer "fold" (36-61 percent), 

 and T. thynnus orientalis and T. obesus a long "fold." 

 Their illustrations and measurements, and our ob- 

 servations, indicate such a wide range of variation 

 that only T. alalunga can be regarded as distinct in 

 this character. 



One Atlantic specimen of T. obesus among our 

 study material had two intestinal loops, a situation 

 never before reported to our knowledge. 



The length of the spleen (fig. 15) is normallj^ short, 

 seldom reaching beyond half the distance from caecal 

 mass to end of body cavity in T. alalunga, T. obesus, 

 and T. allanticus, but usually long, reaching at least 

 three-fourths of this distance in T. thynnus, T. 

 maccoyii, and T. albacares; T. tonggol is variable. 

 In all species there are exceptions. 



The gall bladder (fig. 15) in T. alalunga is normally 

 exposed along the entire right side of the straight 

 intestine. In the other species it is usually either 

 entirely hidden or a small portion may appear 

 posterior to the first intestinal loop; in the few 

 specimens in which it was largely exposed, the 

 visceral mass seemed to be distorted. 



The swimbladder (fig. 17) appears to be invariable 

 and distinctive only in T. obesus, in which it is long, 



usually slender, beginning near the transverse sep- 

 tum, and tapering to a point that reaches the poste- 

 rior end of the body cavity. 



i^\ n 



OBE 



ALB 



Figure 17. — \'entral view of swimbladder shapes of Thunnus 

 species. Fork lengths (left to right): ALA— 875, 1,030 

 mm.; ALB— 637, 700, 1,17.5, 1,420 mm.; ATL— 650, 664 

 mm.; OBE— 1,250 mm.; and THY— 954, 1,050, 1,670 mm. 



ANATOMY AND SYSTEMATICS OF TUNAS 



85 



