os VASCULAR SYSTEM 225 
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’ 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.” 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, AMforphol. Jahrb. xiii. 1888, p. 153. 
2 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. 
