(1293). Our next investigations must be concerning the internal anatomy of the Conchiferous Mollusca* In the Oyster, the general disposition of the body resembles that of the Pecten described above; and the mouth, enclosed between two pairs of delicate lips, occupies a similar position at the termination of the branchial lamellae. In this well-known mollusk the oesophagus is extremely short, so that the mouth appears to open at once into the stomachal cavity (fig. 248, a), which is imbedded in the substance of the liver (d), - the biliary secretion being poured into the stomach itself through several large orifices, represented in the figure. A very peculiar arrangement exists in the stomachs of many-genera, the digestive cavity being prolonged in one direction, so as to form a lengthened caecum, or blind sac-culus, wherein is lodged a cartilaginous styliform body, the use of which is not easy to conjecture, although its office is no doubt connected in some way or other with the preparation of the food. The liver is proportionately of large dimensions, and is at once recognized by its greenish or, in some cases, dark chocolate colour; it is entirely separable into masses of secerning follicles loosely connected together by a delicate cellulosity. The intestine varies considerably in extent, and, as a necessary consequence, in the arrangement and number of its convolutions. In the Oyster it is comparatively short, bending twice upon itself, and winding around the stomach and adductor muscle (b, c, d,f); its termination (g) projecting between the folds of the mantle upon the opposite side of the body to that where the mouth is situated, and so disposed that excrementitious matter is cast out beyond the influence of the ciliary currents.

In Pecten we have already noticed that it performs sundry gyrations through the visceral mass, as well as about the muscle that closes the shell (fig. 247, c, n, m); while in the Cockle-tribes it even penetrates the base of the foot and winds extensively through its muscular substance (fig. 253.) In the greater number of the Con-chifera, but not in the Oyster-tribe, there is a very remarkable circumstance connected with the course of the intestine, the object of which is involved in obscurity: the rectum, at some distance from its termination, passes right through the centre of the ventricle of the heart, its coats being tightly embraced by the muscular parietes of that viscus.

(1294). The position of the branchiae in the Ostracean family has been already described; it now remains therefore to notice their intimate structure, and the arrangement of the vessels connected with respiration and the circulation of the blood. The branchial fringes are, of course, essentially vascular in their composition, being, in fact, made up of innumerable delicate parallel vessels enclosed in cellular tissue of extreme delicacy, and exposing a very extensive surface to the influence of the respired medium. The countless branchial canals through which the blood is thus distributed terminate in large vessels enclosed in the stems to which the fixed extremities of the vascular fringe are attached (fig. 249, f, g, h, i); these communicate extensively with each other, and, ultimately uniting in two principal trunks (e, k), pour the purified blood derived from the whole branchial apparatus into the auricle of the heart.

Alimentary canal of the Oyster (Ostrea edulls.)

Fig. 248. Alimentary canal of the Oyster (Ostrea edulls).

(1295). The heart in the Oyster (fig. 248, n, o) is situated in a cavity between the folds of the intestine and the adductor muscle, in which position, from the dark-purple colour which it exhibits, it is at once distinguished. It consists, in the species we are more particularly describing, of two distinct chambers - an auricle and a ventricle. The auricular cavity (fig. 249, b), the walls of which are extremely thin, and composed of most delicate fasciculi of muscular fibres, receives the blood from the respiratory apparatus, and by its contraction transmits it through two intermediate canals (c) into the more muscular ventricle (d), whence it is propelled through the body by the ramifications of the arterial system (n, o,p).

(1296). The above description of the circulatory apparatus as it exists in the Oyster is applicable in all essential points to every family of Conchiferous Mollusca; but there are important modifications in the structure of the heart and arrangement of the blood-vessels, met with in different genera, which now demand our attention. Most generally, in consequence of the broad and dilated form of the animals, instead of a single auricle, such as the Oyster has, there are two auricular cavities, one appropriated to each pair of branchial lamellae, and placed symmetrically on the two sides of an elongated fusiform ventricle, into which both the auricles empty themselves, till the course of the blood is similar to what we have described above.

(1297). A still greater modification is found to exist in those species most remarkable for their breadth. In Arca, for example, there are not only two auricles, but two ventricles likewise, placed upon the opposite sides of the body; that is, there is a distinct heart appropriated to each pair of gills, - each receiving the blood from the branchiae to which it belongs, and propelling it, through vessels common to both hearts, to all parts of the system.

Heart and respiratory system of the Oyster.

Fig. 249. Heart and respiratory system of the Oyster.

(1298). We must now, before entering upon the description of other families of Conchipera, examine the character of the locomotive apparatus with which those possessed of the power of moving about are furnished. The instrument employed for this purpose is a fleshy organ appended to the anterior part of the body, called the foot: but of this apparatus, for obvious reasons, no vestige is met with in the fixed and immoveable Oyster; and even in the Scallop we have seen only a rudiment of such an appendage. When largely developed, as in Mactra (figs. 250 & 251), the foot forms a very important part of the animal, and becomes useful for various and widely-different purposes. In structure it almost exactly resembles the tongue of a quadruped, being entirely made up of layers of muscles crossing each other at various angles - the external layers being circular or oblique in their disposition, while the internal strata are disposed longitudinally. In the Cockle-tribe (Cardium) this organ attains to a very great size; and on inspecting the figure given in a subsequent page, representing a dissection of the foot of Cardium rusticum (fig. 253), the complexity of its muscular structure will be at once evident, and the disposition of the several layers composing it more easily understood than from the most elaborate verbal description.