Figure 18. — Ventral view of kidney and ureter of Thunnus 

 species. Forward extent shown only in ALA and ATL. 

 Fork lengths (left, right); ALA— 875 mm.; ALB— 700, 

 1,175 mm.; ATL— 533 mm.; OBE— 1,310, 1,350 mm.; 

 THY— 457, 872 mm.; and TON— 910 mm. 



tance before they join at the posterior end of the 

 kidney. In those specimens in which the junction 

 occurs well forward, the angle between the branches 

 is less than in T. thynnus or T. maccoyii. In T. 

 albacares and T. longgol the branches usually con- 

 verge gradually, at a very slight angle, and join at 

 some distance craniad from the end of the kidney tail. 

 When the junction is almost at the posterior end of 

 the tail, the branches may be almost parallel for a 

 short distance. The junction of the branches in T. 

 atlanticus usually occurs far anteriorly, and the 

 angle between the branches is usually large. Some 

 specimens, however, resemble T. albacares. 



Our observations on the urinary bladder are few, 

 but they suggest more variation than was implied by 

 Godsil and Byers (1944). The extent to which the 

 bladder is embedded in the dorsal body wall or 

 projects freely into the body cavity seems to be 

 partly a matter of interpretation. In all species, 

 most of the bladder was contained in the membrane 

 between the left and right gonads. Some T. thynnus 

 and T. alalunga had much of the posterior part 

 closely attached to the body wall, but the anterior 

 part was separated from the body wall and contained 

 in the membrane of the gonads. The anterior tip 

 of the bladder actually projected free of the mem- 

 brane in some specimens of T. obesus. In all three 

 of these species, at least some specimens had the 

 bladder, except for its posteriormost end, entirely 

 within the membrane, not attached to the body wall, 

 and without a freely projecting anterior tip; this was 

 the only condition observed in the other four species. 



Dorsal Connective Tissue 



Covering most of the dorsal body wall, dorsal to 

 the peritoneimi but ventral to the kidney, is a region 

 of tough, white fibrous connective tissue. In T. 

 alalunga the sheet becomes extremely thick poste- 

 riorly. In T. albacares a thick raised median cord 

 forms in the anterior half. The other species have 

 a rather uniform thin sheet of tissue, perhaps slightly 

 thickened posteriorly. 



VASCULAR SYSTEM 



Important papers on the circulatory system of 

 Thunnus include those of Kishinouye (1923), Godsil 

 and Byers (1944), and Godsil and Holmberg (1950). 



General Description 



The pericardial cavity is separated from the pleuro- 

 peritoneal cavity by a transverse septum, the walls 



.\XATOMY AND SYSTEMATICS OF TUNAS 



87 



