The organization of the Conchifera is simple enough. The researches of anatomists have shown that these animals are provided digestion, circulation, respiration, with organs of creneration 0 5 and (in the greater number) of locomotion; with a skin or envelope common to the whole of these organs ; and a nervous system bringing the different systems into mutual .relation with each other.
Of the organs of digestion.— In the Con chifera, as amono other animals, these organs begin at the oral aperture. This aperture(a, fig. 346) placed at the anterior part of the animal is deeply hid den between the foot (b, fig. 346), and the anterior retractor mus cle (c) in the Dimyaria, and under a kind of cowl formed by the mantle in the Mono myaria. The mouth is in the form of a trans verse slit, compnsea between two lips, ge nerally thin and nar row, as in almost all the Dimyaria, or lo bated and digitated, as in some of the Monomyaria, (a, fig.
348). The lips ex tend on either side in the form of two flat tened smaller appendages, more or less elon gated, occasionally truncated, streaked or laminated on their internal surface, and to which the title of labial palps has by general consent been given, (d, fig. 346, c, fig. 348.) The mouth in the Conchifera never presents any part that is hard. In the greater number of these animals it terminates without any intermediate passage in a stomach, the form of which is subject to but little variety. When there is an cesophagus (a, fig. 347), it is variable both in point of length and capacity; it has nothing constant, relatively to the other distinctive characters of the groups established among the conchifera generally : thus it either occurs or is wanting indifferently among the individual members of the' dimyarian and mo nomyarian families.
The stomach (6, fig. 347, d, fig. 348) is a membranous .pouch, commonly pear-shaped, sometimes globular, rarely elongated and narrow. When the cesophagus exists, it opens into the upper part of the stomach ; but when that canal is absent, the mouth termi nates directly in the stomach. Examined internally, the stomach presents several de pressions irregularly dispersed over its surface, by means of which the bile is brought into its cavity ; it is on this account that these minute depressions have received the name of the biliary crypts. The intestine (c, fig.
347, e, 348) arises from the posterior N.vall of the stomach, and a very singular ap paratus is occasionally found in its vicinity (d,fig. 347), the use of which is not yet de termined. It consists of a small appendage which may be compared to the vermiform process of the ccecum in the higher animals; it communicates with the stomach, and is filled by a horny process or stylet of different lengths and thickness, according to the genera and species examined. The anterior extremity of this body is attached to the parietes of the stomach by means of small extremely thin and irreffular auricular processes (oreillettes). It is to be presumed that quantities of the food may fall during the act of digestion between the parietes of the stomach and the horny body, by it to be pressed or bruised in some particular manner. Yet when those conchiferous ani mals which are furnished with the apparatus just mentioned, are examined by dissection, no particle of food is found in such a position. We may therefore be allowed to conjecture that this part accomplishes some other purpose in the economy of the conchifera. Whatever this may be, it must, we should imagine, be connected with the function of digestion.
The intestinal canal in the conchiferous Mollusca is generally slender, cylindrical, and from one extremity to. the other almost always of the same diameter. Mier having made a variable number of convolutions within the substance of the liver and the ovary, -the in testine comes into relation with the dorsal and median line of .the animal's body. It con tinues in this direction to the posterior extre mity, there to terminate in the anus (e, fig. 347, f, fig. 348); the whole of this dorsal part of the intestine is named rectum. The rectum is generally longer in the Dimyaria than in the Monomyaria, because the anus is found above the superior adductor muscle in the former, whilst in the Monomyaria the rectum twists round behind the central muscle to terminate in an anus which floats between the edges of the mantle.