The liver (f, fig. 347, g, .fig. 348) is a bulky organ enveloping the stomach and part of the intestine. It pours the product of' its secretion directly into the stomach by means of the biliary crypts. The liver alone con stitutes a very large portion of the visceral mass, and consequently of the body of the animal; it consists of a great number of fol licles connected together by means of lax and extremely delicate' cellular membrane ; this structure renders the organ very easily torn. We shall see by-and-bye that it is traversed in the greater number of mollusks by several muscles belonging to other parfs, an arrange ment which contributes to support and give it greater strength.
The exposition which has now been given of the structure of the organs of digestion, affords a ready explanation of all that bears upon this function in the conchiferous mol lusca. These animals not having the mouth armed with any hard part are unable to seize and swallow any kind of solid food, so that in general nothing more is found in their sto machs than segregated particles, proceeding without doubt from the decoinposition of aquatic animals and plants. The lips, and unquestionably the labial palps also, are de stined to give the animal perception of the aliment it takes. Once in the stomach, this aliment, impregnated with bile and probably also with a gastric juice secreted by the lining membrane of this pouch, is subjected to a first digestive elaboration ; it next passes the pylorus when it exists, and then traverses the intestinal canal and supplies to the absorbent system the elements necessary to the nutrition of the animal.
It does not appear that there is any par-: ticular system of absorbent vessels in the con chiferous Mollusca; the veins perform the office of absorbents, and they transmit with out any intermedium, and without their under going any glandular elaboration, the fluids ab sorbed to the general current of the circulation. After having thus had all the nutritious ele ments it contains abstracted, the alimentary mass, having reached the rectum, there com monly presents itself' under the form of minute globules ; it is soon afterwards expelled through the anus.
Organs y. circulation.—The organs of cir culation in the acephalous Mollusca consist of two vascular systems forming together a simple circuit, namely, a ventricle and an arterial system, and a venous system and two auricles. The ventricle in the majority of acephalous mollusca is single, symmetrical, situated in the dorsal median line of the body, and rests upon the rectum, which it embraces in its evolution (g, fig. 347, h, fig. 348) on
every side so closely, that the intestine appears to pass through it. It is to be presumed, however, that the intestine does not pass im mediately athwart the heart, but that this canal is only embraced so intimately by the central organ of the circulation, that it is impossible to separate without tearing them. The ventricle, which is regular and symmetrical in the greater number of the genera (a, fig. 349) is irregular and unsymmetrical in the Ostracean family, (a, jig. 350). It is generally elongated and fusiform; its parietes are thin, formed of muscular fibres variously interlaced, and often projecting in ternally. From either extremity issues one of the two main arteries of the body, the one superior giving branches to the whole of the anterior parts of the animal ; the other pos teriOr supplying branches to the principal vis cera,—the stomach, liver, intestinal canal, and ovary. Many superficial branches penetrate the mantle, and may be observed ramifying more especially upon the thicker parts which , constitute its edges.
When the back of the animal is very broad, and as a necessary consequence of this struc ture, the branchia3 of one side are at a consi derable distance from those of the other side, we find, as among the Archidae, that there are then two ventricles (a, a, fig.351,) and two auri cles (b, b, fig. 351) to secure the perfect per formance of the important business of circula tion. This interesting modification of the organs of circulation is of slight significance as regards the mere results of the function, for it still con tinues no more than a simple circuit, exactly as if it were effected by a single ventricle.
The auricles are two in number (b, b, figs. 349 & 351, i, fig. 348) in the whole of the genera of Conchifera except those of the family of the Ostracea, in which there is no more than a single irregular auricle (b, fig. 350), just as there is but one ventricle. The most general figure presented by the au ricles is the triangular. They communicate with the ventricle by one of the angles of the triangles, and they receive the blood of the branchim by the most extensive of their three sides. These organs are altogether membra nous; in their interior, however, we discover, with the aid of the magnifier, a great number of small fibrous fasciculi, by means of which the regular contraction of the ventricles appears to be effected.