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Show 1874.] PROF. T. H. HUXLEY ON MENOBRANCHUS. 201 to be the case in Proteus anguinus. On the right side, a vein from the stomach opens into the pulmonary vein." Van der Hoeven adds to this description a more precise account of the truncus arteriosus (which he terms arterial trunk, "slagader-lijken stam"), and its terminal dilatation into an elongated oval bulbus arteriosus (I. c. p. 30) : - " At the origin of the truncus arteriosus lie three semilunar valves, and higher up beneath the bulbus three more. In the bulbus itself is a freely projecting solid plate, by which its cavity is almost divided into two semicanals." He further states that the auricle " is divided by an imperfect septum;" but he gives no account of the form or structure of this septum. In the specimen dissected by me, the heart (Plate XXXII. figs. 1 to 6) was lodged in an oval pericardial cavity, situated between the styliform second basibranchial and the curious chondrification of the linea alba and of the adjacent parts of the intermuscular septa, which Mayer (/. c. p. 85) justly interpreted as the sternum. Thick masses of longitudinal muscular fibres lie on each side of the pericardium, and represent the sterno-hyoid muscles. The heart consists of a dorsal division, composed of the sinus venosus (S) and of the auricles (A), and of a ventral division, consisting of the ventricle ( F) and the truncus arteriosus (T.a). The sinus venosus lies immediately over the posterior half of the ventricle, and is formed by the junction of the two inferior caval trunks described by Mayer. The right and left superior cavte (R.s.v.c, L.s.v.c.) open into these at their passage into the sinus venosus, and might fairly be said to communicate directly with the latter. The sinu-auricular aperture is situated in the right half of the posterior wall of the spacious auricular cavity. Though its lips project somewhat into the auricular cavity, they can hardly be regarded as truly valvular. The auricular chamber is very spacious, and extends forwards nearly as far as the anterior end of the pericardium. It lies above the truncus arteriosus in front, and the anterior half of the ventricle behind. On each side, it is produced into a saccular dilatation, which extends over the truncus and ventricle to the ventral wall of the pericardium on the left side, but is much less developed on the right side, where it leaves the ventricle uncovered. The wall of the right dilatation presents three or four longitudinal folds. On the dorsal face of the sinus venosus, between the two superior cavse and the diverging inferior caval trunks, lies the pulmonary vein (P. v. fig. 4). It is very narrow posteriorly, but dilates in front, and, turning to the left, opens into the posterior part of the auricular cavity, to the left of the middle line. The existence of the anomalous arrangement of the pulmonary veins in the specimen described by Mayer is therefore rendered doubtful, though I am unwilling to suggest that so accurate an observer was altogether mistaken. The right wall of the dilatation of the pulmonary vein is continued downwards, forwards, and to the left side, nearly as far as the dorsal lip of the auriculo-ventricular aperture, as a delicate plate formed PROC. ZOOL. Soc-1874, No. XIV. 14 |
