KIDNEY 



URETER 



Figure 22. — Post-cardinal vein in relation to cutaneous veins 

 and kidney in Thunnus atlanticiis. Also typical of T. obesus, 

 T. albacarex, and T. tonggnl. 



T. obesus, and T. albacares; and appears to be absent 

 in T. alalunga, T. allanticus, and T. ionggol. Godsil 

 and Byers (1944) implied that this connection is 

 always, or nearly always, present in Pacific T. 

 albacares. Godsil and Holmberg (1950) used its 

 supposed absence in Atlantic T. t. thynnus as one 

 character that differentiates Atlantic from Pacific 

 specimens {T. t. orienlalis), a conchision which our 

 observations do not support. 



The cutaneous artery usually originates at the 

 level of the tliird or fourth vertebra in T. thynnus, 

 T. maccoyii, and T. alalunga, and at the sixth to 

 eighth vertebra in T. albacares, T. obesus, T. allanti- 

 cus, and T. tonggol. 



The cutaneous arteries pass laterally between the 

 third and fourth ribs in all T. thynnus, T. maccoyii, 

 and T. alalunga examined by us (also between the 

 second and third according to Godsil and Holmberg, 



19o0) ; in T. albacares, T. obesus, T. tonggol, and 

 T. atlanticus they pass between the fifth and sixth 

 ribs, or occasionally between the fourth and fifth. 

 Branching occurs between the fourth and fifth inter- 

 muscular bones in T. thynnus, T. maccoyii, and 

 T. alalunga, and l)ctwcen the sixth and seventh in 

 T. albacares, T. obesus, T. tonggol, and T. atlanticus. 

 Godsil and Holmberg (1950) reported more T. mac- 

 coyii with branching between the fifth and sixtli, and 

 we observed this in one specimen of T. t. thynnus. 

 In a significant number of our T. albacares, as well 

 as two T. obesus, branching occurred between the 

 seventh and eighth intermuscular bones. 



A posterior commissure is present in T. thynnus, 

 T. maccoyii, T. albacares (fig. 21), T. tonggol, and 

 T. allanticus, but absent in T. alalunga. In T. 

 obesus it is present or absent (fig. 20). In all species 

 except T. albacares and T. maccoyii, we encountered 

 specimens in which we could not ascertain the pres- 

 ence of a commissiue. 



We noted the number and position of the rows of 

 arterioles and venules arising from the lateral cut- 

 aneous vessels in relatively few si)ecimens of each 

 species (see fig. 23). T. alalunga, T. albacares (fig. 

 21), T. atlanticus, and T. tonggol, have one row from 

 each vessel; in T. alalunga it originates on the mesial 

 side, and in the other species on the lateral side. In 

 T. thynnus, T. maccoyii, and T. obesus (fig. 20) two 

 rows, one mesial and one lateral, arise from each 

 vessel. Kishinouye (1923) reported two rows of 

 venules and a single row of arterioles in Japanese 

 T. thynnus orientalis. California specimens have 

 two rows of arterioles (Godsil and Byers, 1944, and 

 our observations). We feel certain that Kishinouye 

 either was mistaken or relied on an unusual specimen. 



In T. albacares large parallel trunks connect the 

 posterior epibranchial and the cutaneous arteries on 



OBE 



ALB 



ALA 



Figure 23. — Rclation.'ihips of arterioles and venules to cuta- 

 neous artery (light) and vein (dark). Three patterns 

 represented by T. obesus (OBE), T. albacares (ALU), and 

 T. alalunga (ALA). Lateral view. After Kishinouye 

 (1923). 



90 



U.S. FISH AND WILDLIFE SERVICE 



