CIRCULATING SYSTEM. This system is not so simple as it is described by Cuvier, Poli, &c. Bojanus in the Anadonta first described its correct anatomy. By means of mercurial injections it may be readily made out in the common scallop or Pecten maximus The venous blood from the liver, ovaries and intestine, does not go to the branchiæ immediately, but a portion of it enters, on each side, a sinus or system of veins situated upon the adductor muscle, the remainder is poured into a large vessel, which is distributed to a dark coloured excretory organ, situated at the base of the branchia on each side. These organs have other veins which open either into the sinus or into the branchial arteries. The sinus receives likewise the veins of the muscle, and also in part the blood of the mantle. It opens into the branchial arteries by two valvular openings. The blood from the viscera must in great part pass through the tissue of the excretory organs to enter the branchial artery, and the tissue itself appears to be entirely formed by these veins. Bojanus from this considers these bodies as the organs of respiration. The auricles, besides the branchial veins, receive the extremities of the large veins of the mantle, small hepatic branches, and other veins from the neighbouring parts. Poli has figured this circumstance in the Arca, Spondylus, &c. The branchial artery then has its principal origin from the large veins or venous sinus, situated upon the adductor muscle; it then receives veins from the organs above mentioned, also others from the root of the branchia and from the mantle it then at regular distances gives branches to the processes of the branchia, which run into the corresponding ramuscules of the branchial vein. The branchial vein, formed by these ramuscules, lies nearer to the processes of the two vessels, and is crossed by the divisions of the artery; it is joined by the extremity of the great vein of the mantle, and by small veins from the liver, &c., as described above, and forms the auricle. No valve exists between the veins and auricles. The latter have projecting processes upon them, secreting perhaps the fluid of the pericardium. They are connected by a transverse vein, receiving some small vessels from the pericardium, &c. A valve formed by two semilunar membranes exists at the entry of each auricle into the ventricle. The pericardium, always situated in the : 1 A translation of his paper may be found, 'Journ. de Physique,' t. 89, with observations upon it by M. Blainville. back of the animal, except in the oyster, contains a thickish, transparent fluid. The ventricle is muscular, and is pierced by the rectum, which, in some species, though not in the animal whose circulation is now described, likewise traverses more or less of the aorte. The ventricle gives off an anterior and a posterior aorta, and a valve exists at the commencement of each. The former runs over the liver, giving hepatic arteries, and surrounding the mouth with a ring, furnishes the labial, ovarian, intestinal, pedal, and other arteries. The latter goes backwards, and furnishes, principally, the muscle and mantle with arteries. The coats of the veins are thin, but they are readily injected; those of the arteries thicker, but these vessels are not easily filled with mercury. Blood taken from the auricles is almost colourless,' separates on standing into a liquid and solid part, and, microscopically examined, its globules, which are about a thousandth part of an inch in diameter, show some appearance of movement, even out of the vessels. This fact appears to have been observed by Mayer. The heart is slow in its pulsations; they are generally from twenty to thirty in the minute. In the Unio and Anadonta the sinus, corresponding to that of the Pecten, lies under the pericardium, receiving anteriorly large veins from the mantle, viscera, &c., and posteriorly other vessels from the posterior part of the body. Part of its blood goes to the branchial artery on each side, a few twigs enter the auricles and the remainder goes to the excretory organs. The vein of the mantle, at one extremity, is connected with the excretory organ. This organ on each side has a few small veins entering the auricles, but the mass of its blood enters the branchial artery. The branches of this vessel are found on the outer side of the inner branchial lamina, and on the inner side of the external one. The corresponding venous ramuscules enter three veins, one lying between the two internal branchia, which sends its blood to the two others, situated at the superior margin of the external lamina. These last form the auricles. There is nothing remarkable in the distribution of the arteries. Poli found two ventricles in the Arca; there are two likewise in some other genera, the shells of which have their beaks remote. There are never more than two real auricles; but dilatations at the commencement of the aorte have been so called. In the Cardium echinatum, where there are two posterior aorta, each at its commencement is considerably di It is rather red in Teredo, according to Home. lated, and the cavities thus formed have strong muscular columns on their internal parietes. The communication between the auricles, effected by a transverse vein in the Pecten, Spondylus, &c., is more complete in the oyster, where they are united into one, but there are still two auriculo-ventricular openings. The veins, then, do not all enter the branchial artery, some joining the branchial vein, where it forms the auricles. In the Pecten we see the visceral blood circulating through the excreting organs, which return it to the branchial arteries. The veins between the excreting organs and the sinus, may be the channels by which the former receive blood from, or remit it to the latter. The first supposition is perhaps the correct one, and in this case, this part of the circulation in the Anadonta, &c., only differs by the visceral blood entering the sinus before it circulates through the excreting organs; and by more of the blood of the sinus going to them. This distribution is something like a portal system. There is a free passage from the veins of the mantle into the auricles and sinus in the Pecten, and into the sinus and tissue of the excretory organs in the Anadonta, &c.1 RESPIRATORY SYSTEM. As is known to zootomists, the branchia of the Lamellibranchiata are ciliated for the purpose of exciting currents in the water. In the Monomyaria and Arcace@ there are no orifices or siphons to the mantle for the inlet and exit of the water &c. In the Pecten, Spondylus, and Lima the branchiæ of each side are situated on a triangular membrane, at a distance from those of the other. The two branchia of the same side are not distinct from each other, and their processes are disunited, and do not form a continuous membrane as is generally the case, but are kept in contact with each other by lateral processes. In the oyster the branchia have not only their processes conjoined into a membrane, but the several lamine are united at their bases. In the Arca, Pectunculus, &c., the branchia of each side are separate from those of the other; as they are in Modiola, Mytilus, Lithodomus, and other genera; but in these latter, the water enters by the posterior fringed extremity of the mantle, and makes its exit by the separate orifice situated higher up. There is a valve be 1 Treviranus considers the blood from the branchial veins to pass through the excretory organs before it enters the auricles. Vanderhoeven was aware that the venous blood circulated through them, in opposition to the erroneous opinion of the former, according to Prof. Grant. Vid. Lancet. low this orifice, and another above, to regulate the currents in the proper direction. The orifice is lengthened into a tube in the Lithodomus; and in the Pinna the anal valve has taken a lengthened ligulate form. The Uniones only differ from this in having the branchie united, and the water appears to make its exit by insinuating itself between them and the foot, and so through the orifice. In the Cardium we see two distinct openings behind; the water enters by the lower one, distends the mantle, and this orifice being then closed by the valve, it gets between the foot and the branchia, and is discharged through the superior orifice. In this animal the posterior extremity of the branchie is united to the septum between the orifices, so that the two respiratory cavities only communicate by the side of the foot. In the Cyclas both the short tubes open internally below the branchia, and there is no passage required between the latter and the foot. The water in this and similar cases has only access to the spaces left internally between the lamine of the branchia (oviducts of Home) from behind: it is over these spaces that the secretory organs and oviducts commonly open. We find this last disposition, only with the siphons more developed, in Donax, Tellina, Psammobia, and many others. There is in these commonly a valve between the internal orifices; but the water is frequently ejected from both tubes. In the Mactra, Cytherea, Venus, and Venerupis, the tubes are more muscular than in the Tellinidæ, &c., and they are more or less united; the branchia are in these often medianly separated. In the Solen, Hyatella, Mya, Pholas, Teredo, &c., a different disposition takes place. Here the branchia are prolonged into the inferior siphon, and as they are not separated from the base of the foot within, nor from the mantle without, the water drawn in through the inferior orifice must make its exit by the same, or by the anterior opening. But water is likewise drawn in by the other, and so gets access to the interior inter-laminar spaces of the branchia; and by this superior siphon the ova, fæces, and secretions are discharged. Here the branchia are often very long, and the siphons very muscular. We sometimes find small supplementary branchiæ, as in the Psammobia, Pholas, &c., or the external pair may be shortened in front, as in Mya, Venerupis, and many other genera. In the Pandora the only appearance of the external We see this also in the Tunicata. By one orifice water enters the respiratory sac, by the other it is drawn into the external meshes of the branchia. The water drawn in by each opening must make its exit by the same. Those writers who say the contrary must be incorrect, unless the water pass through the stomach and intestine. lamine consists of two very narrow strips at the base of the others; this is the case also, according to Blainville, in the Osteodesma, allied to the former. Though it is not by the action of the orifices or siphons, or by the relaxation of the shell-muscles, and the opening of the valves, that the water is drawn into the mantle, yet these accompany the occurrence; and though the water commonly escapes in a continuous stream from the action of the cilia, a sudden ejection of it frequently takes place, accompanied by a closing of the valves and a contraction of the siphons. These latter parts have frequently at their extremities circles of ciliated fringed pro cesses. (To be continued.) ART. VI.-Observations on the Lamellicorns of Olivier. (Continued from Page 24.) COUNTRY. BY THE ARRANGEMENT OF AUTHORS. |