Figure 32. — Dorsal wall of botly cavity of Thuimus thynnus. Ventral view with viscera removed and head end to the left. 

 Left: 7'. /. orientalis, 1,450 mm. fork length, from California, showing the comparatively narrow anterior bulge with lateral 



concavity and wide lateral trough. Right: T. t. thynnus, 1,850 mm. fork length, from the western \orth .Atlantic, showing 

 the wide anterior bulge without a lateral concavity. 



coarse compared with T. maccoyii. We could detect 

 no differences. 



The caecal mass is so variable in size that its di- 

 mensions cannot be used to differentiate populations. 



The relative length of the lateral liver lobes of 

 western Atlantic specimens encompasses the differ- 

 ences in lobe lengths suggested by Codsil and 

 Holmberg. 



The stomach length cannot logically be u.sed as a 

 specific character, since this is a highly distensible 

 organ, the dimensions of which will vary under 

 different physiological states. 



Swimbladder dimensions vary with size, becoming 

 larger with growth, as shown by Serventj' (1956a) 

 for T. maccoyii from Australia and bj' us for western 

 Atlantic T. t. thynnus and eastern Pacific T. t. orien- 

 talis. Abe (1955) reported that the swimbladder 

 was well developed in a 1,470-mm. specimen thought 



to be T. maccoyii from the eastern Indian Ocean. 

 Kishinouye (1923) and Fradc (1925) illustrated 

 swimbladders for western Pacific and eastern At- 

 lantic specimens, respectively, that are ver.y similar 

 to those of larger specimens from other regions, and 

 Kishinouye noted that the swimbladder is short and 

 very narrow in immature specimens of .Japanese T. 

 thynnus orientalis, but short and wide in adults. 



Godsil and Holmberg described the posterior end 

 of the kidney of T. maccoyii specimens as truncate. 

 This condition was observed in several specimens of 

 western Atlantic T. t. thynnus, which displayed all 

 variations that have been described, but our speci- 

 mens of 7'. maccoyii did not show this condition. 



The branching of the ureter of T. maccoyii was 

 said to differ in that the branching occurred well 

 anterior in the kidney mass. We observed this con- 

 dition in both T. t. thynnus and T. t. orientalis; in 



114 



U.S. FISH .\XI) WILDLIFE SKHVICE 



