VASCULAR SYSTEM 325 



kidney and receives veins from the corresponding gonad ; but, 

 instead of traversing the liver, it passes above that organ, and 

 finally opens into the left Cuvierian duct. The course of the 

 left vein, and the relations of the vessel to the caudal vein and 

 the left Cuvierian duct, point to the conclusion that it represents 

 the left posterior cardinal of other Fishes. From its continuity 

 with the caudal vein it is also obvious that the hinder or renal 

 portion of the right trunk is a remnant of the right posterior 

 cardinal ; but the more anterior section so closely resembles the 

 postcaval vein, or inferior vena cava of the higher Vertebrates, 

 in its relations to the liver, the hepatic veins, and the sinus 

 venosus, that its identity as such seems beyond doubt, and this 

 interpretation is supported by well-known observations l on the 

 mode of origin of the inferior vena cava in Amphibia, and 

 especially the union of the independently formed inferior vena 

 cava with the posterior or inter-renal portion of the embryonic 

 right posterior cardinal vein, combined with the atrophy of the 

 anterior portion of the latter vein. 2 The singular connexions 

 and relations of these two great veins afford an additional 

 illustration of the significant transitional condition of the venous 

 system in the Dipnoi. On the other hand, the direct continuity 

 of the caudal vein with vessels which, wholly or in part, repre- 

 sent the two posterior cardinals, is a feature alike characteristic of 

 the adult Cyclostome and the embryonic Elasmobranch, Teleost, 

 and Amphibian. 



As in the Cyclostomes and Elasmobranchs, the precaudal 

 section of the embryonic subintestinal vein is represented in the 

 adult by an intra-intestinal vein which traverses the spiral valve 

 near its free edge and is a tributary of the hepatic portal 

 vein. 



The two veins from the undivided air-bladder unite to form a 

 single vessel, which, instead of joining the hepatic portal or 

 posterior cardinal veins as in other Fishes, opens into the left 

 auricle, like the pulmonary veins of the Amphibia. 



A further resemblance to the Amphibia is to be found in the 

 presence of an anterior abdominal vein. After leaving the pelvic 



1 Hochstetter, Aforphol. Jahrb. xiii. 1888, p. 153. 



- The vertebral vein, which is present only on the right side, may represent the 

 reduced anterior portion of the right posterior cardinal, as Baldwin Spencer (op. cit.) 

 has suggested. 



