Thunnus obesus (7 Atlantic specimens, 697-1,360 

 mm.; 3 Indian Ocean, 630-1,270 mm.; 1 eastern 

 Pacific, 1,600 mm.) liave type A grooves and could 

 not be distinguished from T. thy?mus. 



Thunnus ahdunga (20 specimens, 520-1250 mm.), 

 like T. thi/nnus and T. obesus, have type A grooves, 

 but, although the grooves vary greatly in width, 

 they tend to be much narrower than in the latter 

 two species. Usually the grooves are widely sepa- 

 rated, but in a few specimens, the partitions are 

 narrow. 



Thunnus tonggol (4 specimens, 373-924 mm.) have 

 type B grooves, two grooves per centrum, quite 

 variable in size, with a shallow canal connecting 

 them. The grooves tend to be larger than those of 

 T. atlnnticus and smaller than those of T. thynnus 

 or T. obesus. In the two larger specimens, the anter- 

 ior grooves strongly resemble those of T. thynnus or 

 T. obesus in being larger and close together. 



Thunnus atlanticus (20 specimens, 322-665 mm.) 

 have type B grooves, with two very small pits 

 on each centrum connected by a very narrow canal. 

 The canal is not evident in some specimens, giving 

 the impression that the two grooves are separated 

 by a sharp ridge. In this respect T. atlanticus is 

 distinctive. The largest specimens, however, closely 

 resemble small T. tonggol, with small grooves con- 

 nected by an obvious canal, the grooves becoming 

 elongate posteriorly. 



The proportion of length to depth of the 36th 

 vertebra is relatively greatest in T. albacares (.¥=60) 

 1.2-1.9, x=1.4 and in T. alalunga (jV=40) 1.1-1.7, 

 f=1.5. In T. alalunga the vertebrae are often 

 considerably smaller anteriorly than posteriorly (fig. 

 14), accounting for the high ratio. In the other 

 species, this proportion is: T. Ihyyinus (N = 58) 

 0.8-1.3, x=l.l;T. maccoyii {N= 16) 0.8-1.3, x= l.O; 

 T. obesus (A^=30) 0.9-1.4, x=1.2; T. atlanticus 

 {N=23) 1.0-1.4, f=1.3; T. tonggol {N=8) 1.1-1.5, 

 5=1.3. 



VISCERA 



The relative position, shape, and size of the various 

 internal organs provide excellent diagnostic char- 

 acters (fig. 15). These organs are treated here 

 by systems. Important works on the viscera of 

 Thunnus include those of Eschricht and Miiller 

 (1837), Kishinouye (1923), Frade (1925), Serventy 

 (1942), Godsil and Byers (1944), and Godsil and 

 Holmberg (1950). 



Figure 14. — Left lateral view of vertebrae 35-39, showing 

 differences in proportions of vertebra 36. (top) Thunnus 

 albacares, (middle) T. alalunga, (bottom) T. obesus, typical 

 of other species not illustrated. 



Digestive System and Associated Organs 



General Description. — At the anterior end of the 

 body cavity the liver abuts against the transverse 

 septum and caps the other organs. It is usually 

 composed of three lobes, only the middle of which 

 is plainly visible in ventral view; the other two 

 lobes lie along the lateral body wall, hidden by the 

 other organs. The ventral surface of the liver of 

 some species appears striated owing to the parallel 

 arrangement of blood vessels (both arterial and 

 venous) near its surface. The species with striated 

 livers also possess, on the dorsal surface of the liver, 

 several large "vascular cones," each comprising 

 numerous vessels bound in a common sheath. These 

 are absent in species with unstriated livers. In all 

 species (Morice, 1953, to the contrary) there are two 

 efferent (venous) vessels leading directly from the 

 anterior surface of the liver into the sinus venosus. 

 The esophagus merges indistinguishably (in external 

 view) into the stomach, which forms a blind sac 

 posteriorly. The intestine rises from the anterior 



AN.VTOMY .\ND SYSTEMATICS OF TUN.\S 



81 



