XII VASCULAR SYSTEM 327 
Less is known of the venous system of Protopterus,' but it is 
certain, nevertheless, that it presents a more advanced grade of 
evolution than in Neoceratodus, and, except for the doubt as to the 
existence of an anterior abdominal vein, it is essentially similar 
to that of a Urodele Amphibian in which the right posterior 
cardinal vein has aborted. 
The caudal vein (Fig. 192) divides into right and left renal 
portal branches, neither of which, however, is directly continuous 
with the inferior vena cava or the left posterior cardinal; on the 
contrary, each renal portal vein is joined by the corresponding 
iliac or femoral vein, and also by numerous segmental veins, and 
then distributes the whole of its venous blood to the kidney. 
The radicles of the inferior vena cava and the left posterior 
cardinal are formed by the renal veins from the two kidneys, and 
in their forward course to the heart both veins receive in addition 
genital and segmental veins. In its course through the liver the 
inferior vena cava receives several hepatic veins, and finally 
opens into the sinus venosus, while the left posterior cardinal 
vein joins the corresponding Cuvierian duct, which also receives 
anterior cardinal, inferior jugular, and subclavian veins. There 
is an intra-intestinal vein as in eoceratodus, but an anterior 
abdominal vein has yet to be discovered. The two pulmonary 
veins from the double air-bladder form a single trunk before 
communicating with the left auricle. 
With the exception of certain doubtful details which need 
further investigation, the venous system of Lepidosiren” seems to 
resemble that of Protopterus. 
The Heart.—The heart is more anteriorly placed than in 
other Vertebrates, being situated directly behind and beneath the 
last pair of branchial clefts and internal to the ventral portion of 
the pectoral girdle. The organ is enclosed in a pericardial cavity, 
which, in the adult, is separated from the abdominal portion of 
the coelom by a transverse pericardio-peritoneal septum, and in 
the Lamprey (Petromyzon) is partially enclosed within a cartila- 
ginous, cup-like modification of the hinder part of the branchial 
basket. In the Ammocoetes-stage of the Lamprey the peri- 
eardium is in communication behind with the general coelom, 
but the connexion is lost in the adult. In Elasmobranchs the 
1 Newton Parker, Trans. Roy. Irish Acad. xxx. 1892, p. 179. 
2 Hyrtl, Abhand. d. Bihm. Gesellsch. 1845, p. 643. 
