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 _Neoceratodus_, 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_[372] 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 cartilaginous, cup-like modification of the hinder part of the branchial basket. In the Ammocoetes-stage of the Lamprey the pericardium is in communication behind with the general coelom, but the connexion is lost in the adult. In Elasmobranchs the {328}two cavities are connected by a single pericardio-peritoneal canal, or by two such canals; and in _Chimaera_, and in the Sturgeon (_Acipenser_) and _Polyodon_, by a single canal. [Illustration: FIG. 193.—Diagram of the structure of the heart in different Fishes. A, In an Elasmobranch; B, in _Amia_; and C, in a Teleost. _a_, Auricle; _b.a_, bulbus aortae; _c.a_, conus arteriosus; _s.v_, sinus venosus; _v_, semi-lunar valves; _v′_, auriculo-ventricular valve; _v.a_, ventral aorta; _vt_, ventricle. (From Boas.)]