A further resemblance to the Amphibia is to be found in the presence of an anterior abdominal vein. After leaving the pelvic limb each femoral vein divides into two branches; one of these forms a renal portal vein as previously described; the other, which may rightly be termed a pelvic vein (pv.v), unites with its fellow to form a median anterior abdominal vein (a.ab). Pursuing its course forwards in the ventral abdominal wall, the vein eventually reaches the heart and opens into the sinus venosus. The direct connexion of the anterior abdominal vein with the heart is yet another example of the retention in the adult Neoceratodus of a transitory embryonic feature in the developing Amphibian.[[370]]
Fig. 192.—Venous system of Protopterus. a, Auricle; a.c, anterior cardinal; an.v, anastomotic veins; c, intestine; f.v, femoral or iliac vein; g.b, gall-bladder; h.p.v, hepatic portal vein; i.j.v, inferior jugular; ov.v, ovarian veins; p, pericardium; p.c.v, left posterior cardinal; p.v′, parietal or segmental veins; s, stomach; sb.v, subclavian. Other reference letters as in Fig. 191. (From Newton Parker.)
As in other Fishes, the blood from the head is conveyed to the Cuvierian ducts by an anterior cardinal and an inferior jugular on each side. There are no lateral veins, the blood from the pelvic fins flowing into the renal portal system or into the anterior abdominal vein, and that from the pectoral fin through subscapular and brachial veins into the Cuvierian ducts. Lateral cutaneous veins are, however, present; and, as in Elasmobranchs (e.g. Mustelus antarcticus), anastomose anteriorly with the subscapular vein and behind with the caudal vein.
Less is known of the venous system of Protopterus,[[371]] 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 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 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.
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.)