II. The Subcutaneous Vessels are Homologous tfith the 



Segmental Vessels 



In all fishes the dorsal aorta and the cardinal vein give off pairs of 

 left and right segmental vessels in each myotome and these vessels and their 

 ramifications are as a rule distributed over the tissues of the sides of the 

 fish. However, as is characteristic of vascular systems in general, 

 ordinarily the relative positions of these arteries and veins and the 

 pattern of their ramifications show variations as between species, in- 

 dividuals, and the various parts of individuals. 



Now before discussing the relationship between the segmental vessels 

 and the subcutaneous vessels I will for the sake of convenience describe 

 briefly the vessels of the second myotome in the common mackerel ( Scomber 

 japonicus) and the vessels of the third myotome of the sailfish ( Istiophorus 

 orientalis ) after which I will make a comparison between these vessels and 

 the subcutaneous vessels of the Plecostei. 



The mackerel which I have examined have had eleven pairs of segmental 

 arteries in their bodies. The second of these vessels branches off from 

 the dorsal aorta directly under the fourth vertebra and immediately posterior 

 to the retractor arcus branchialis dorsalis superior (the "pharyngeal muscle'* 

 of Kishinouye). The vessel immediately proceeds outward, passes through the 

 peritoneum at the base of the rib of the fourth vertebra and then between 

 the third and fourth myotomes along the intermuscular bones, dividing into 

 anterior and posterior branches along the way. The posterior branch proceeds 

 to a point directly ventral to the nerve of the lateral line where it further 

 divides into two dorsal and ventral branches. The dorsal branch proceeds 

 posteriodorsally and toward the outer surface of the body, while the ventral 

 branch proceeds posterioventrally and toward the surface of the body. These 

 vessels run along the inner side of the dark lateral tissue belonging to 

 the third and fourth myotomes and then issue forth into the subcutaneous 

 tissues posterior to the pectoral girdle, where they become finely divided » 

 In other specimens which 1 have seen these vessels send off one or two 

 branches near their bases. 



Wow when we compare the segmental vessels of the second myotome in the 

 mackerel (sa, sav, sad in figure 5) and the subcutaneous vessels of the 

 black tuna ( Thurmus orientalis ) or the Koshinaga ( Heothunnus rarus ) (ca, eca, 

 hca in figure 1), they are almost identical in their most important points, 

 that is in the relationships of the pectoral girdle, spinal column, inter- 

 muscular bones (intercostals), myotomes, nerves of the lateral line, dark 

 red lateral tissue, and the retractor arcus branchialis dorsalis superior 

 muscle, and in the direction and branching pattern of the vessels . The 

 only point on which they differ is that in the former the dorsal and ventral 

 branches formed by the fork in the vessel ventral to the nerve of the lateral 

 line are short, while in the latter they are long. If these ventral and 

 dorsal branches in the former were only more markedly developed and if they 

 ran parallel to the axis of the body through the subcutaneous tissues, they 

 would present exactly the same condition as the latter. And if these vessels 

 in the mackerel did not split into dorsal and ventral branches and continued 

 to run longitudinally under the skin, or if the ventral branch were to 



