292 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
portion of the postcardinals which have joined the anterior ends of the 
subcardinals (fig. 294, C). The changes in the postcardinals and the 
renal portal system of mammals will be described below. 
In Ceratodus (dipnoi, fig. 296, A) there are some differences from the above 
account. Thus the anterior portion of the right postcardinal (not shown in the 
figure) loses its connexion with the vessels behind and acts as a vertebral vein, 
taking the blood from the intercostal veins of that side back to the heart. The 
Fic. 296.—A, venous system of Ceratodus, dorsal view, after Spencer; B, of a urodele, 
ventral view. ab, abdominal vein; av, ven advehentes; b, brachial; c, caudal; cd, Cuvierian 
duct; ej, external jugular; #, heart; hp, hepatic portal; 27, inferior jugular; 7, jugular; i, 
iliac; J, liver; /c, lateral cutaneus; m, mesonephros; p, postcava; pc, postcardinal; r, vene 
revehentes; s, subclavian; /, testes. 
caudal and the subcardinals form a continuous trunk, the revehent vessels forming 
side branches. The posterior portions of the postcardinals grow back into’ the 
tail as paired vessels, forming no connexion with the caudal vein. In Protopterus 
the vertebral vein is lacking, the subcardinals are not fused behind while the 
advehent veins are connected with the caudal. 
The development of lungs brings about the appearance of one or 
more pairs of pulmonary veins which bring the (arterial) blood from 
these organs to the heart. These arise as an outgrowth from the 
