COMPARATIVE STUDY OF SCOMBROID FISHES. 
375 
the segment of the fifth vertebra, in Paraihunnus and Neothunnus in the segment 
of the eighth or ninth vertebra. Eschricht observed that the dorsal aorta becomes 
abruptly slender after the ramification of these cutaneous arteries, in the follow¬ 
ing words :— “ Nach dem Abgang der uiieriae axillares wird die Aorta plötzlich 
mehr als um die Hälfte dünner im Durchmesser 
In the Thunuidae the cutaneous artery runs obliquely backward and 
dorsalward, passing behind the third ( Thunnus) or fifth rib ( Paraihunnus and 
Neothunnus), and reaches the surface of the body before the intermuscular bone, 
attached above the root of the respective rib. Before reaching the surface of 
the body each artery is split into two equal branches, running dorsal and 
ventral to the lateral median line, nearly parallel to each other (fig. 3). They 
are united again in the caudal portion by a transverse commissure, and the 
commissure is again united to the dorsal aorta by a pair of horizontal 
segmental arteries (fig. 4). Each branch of the cutaneous arteries gives off, at 
the surface of the body, segmental arteries obliquely backwards along the borders 
of myotomes for some distance and then bends forwards. The dorsal branch 
sends dorsal segmental arteries only, and the ventral branch ventral segmental 
arteries only. These cutaneous segmental arteries send arterioles axially, 
along myocommata or straightly inward (fig. 3). Both dorsal and ventral 
cutaneous arteries, moreover, send one or two rows of very numerous parallel 
arterioles, quite close to each other. These arterioles run obliquely inward, 
along the boundary between the red and dark red portions of the lateral 
muscle. They are soon divided into several smaller canals and always run in 
association with similar venules making a membranous sheet investing and 
nourishing the dark red portion of the lateral muscle. The arterioles in the 
vascular sheet gradually unite again reduced in numljer at about midway 
between the surface and the axis of the body, and vanish in capillaries, so 
that the dark red portion of the lateral Hiuscle is not entirely covered with 
fine bloodvessels and capillaries near the axial part. The vascular sheet is 
thick near the surface of the bod}', becoming gradually thin as it approaches 
the axis of the body. In the Plecostei, except Euthynnus, the cutaneous 
arteries always run on the axial and inner side of the accompanying veins. 
Generally the axial margin of the cutaneous vein partly covers or i3 at least 
apposed to the abaxial margin of the accompanying cutaneous artery ; but in 
