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 Neoceratodiis, 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 caiidal 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 Neoceratoclus, 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- 

 cardium 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. Bohm. Oesellsch. 1845, p. 643. 



