HYALEA. — The two fins are supported upon a fleshy neck, enclosed between the two lobes of the mantle (fig. M. 3, c), which latter (fig. 114. 3, g, h, k) correspond accurately with the valves of the shell, beyond the edges of which they protrude all around, and which they cover with a thin epidermis.
The position of the hranchim Cuvier ob served not to correspond with what he had, erroneously, believed it to be in Clio,—namely, the surface of the lateral fins ; for in Hyalea he could not discover any vascular net-work in those organs, even with a microscope ; and thus indirectly confirms the correctness of Eschricht's views upon this point. He, there fore, sought for thein elsewhere, and, " on breaking the shell, he found them to be situ ated between the two lobes or the mantle at the bottom of the outer space between them on each side, so that the lateral fissures of the shell have apparently the function of admitting the surrounding element to the branchial organs. These latter are composed of little lamince, resembling those of patellm, phyllidia, &v., which surround the body so as to form a sort of elliptical belt, not placed transversely, but parallel to the course of the dorsal surface (fig. 114, 5, 6. p, q,r, s). The other viscera occupy- the arched and rounded portion of the shell, or the interior of the cervieal region, and are enveloped in a kind of peritoneum of a blackish colour. On plac ing the animal upon its flat valve, or ventral surface, the heart iS seen to be situated on the right side, at the inner border of that portion of the bronchial band marked t in fig. 114. 5. A cylindrical muscle (u, fig. 114. 5 and 7) is attached to the intermediate point of the shell, and traverses the viseeral mass to be inserted into the neck by four tongue-like processes. The action of this muscle will be to retract the creature within its shell.
In front of the four branchim is situated the penis, upon which lies the cesophagus, and this in turn is surmounted by the brain — these organs filling up the thickness of the neck.
The cesophagus (v, v, fig. 114, 5 and 6) is long and slender, and the mouth, according to Cuvier, is a simple anterior opening, in the interior of which a few wrinkles only are perceptible, representing the tongue. The cesophagus dilates into a kind of membranous crop (w, w, fig. 114. 7 and 9), which is suc ceeded by a muscular gizzard (x, x, fig. 114. 7 and 9) of a cylindrical shape, the walls of which are of tolerable thickness. Both these cavities are furnished internally with longitudinal folds, and these are thicker and more numerous in the crop than in the gizzard (fig. 114. 9). The intestine (y,y,) is slender, and of the same diameter throughout its whole length, which is considerable. It makes two convolutions in the interval between the lobes of the liver (z x,fig. 111. 7). The anus is situ ated on the right side of the neck beneath the corresponding lateral fin : the liver is of no great bulk, and forms a nearly globular mass.
The organs of generation consist of an ovary which occupies the greater portion of the right side; of an oviduct of moderate length ; of a ,testicle which is almost as large as the ovary, and of a common deferent canal. The penis is here, as in Clio, an organ al together distinct from the testicle : it is situ ated, as already said, beneath the cesophagus, where it is folded upon itself; and when pro truded, issues through an orifice placed in the front of the neck and a little below the mouth. It is represented in situ in fig. 114. 7, and detached in 8.
The brain (fig. 114. 7, r) is large, flat, and of a square form, slightly narrowed pos teriorly : the nerves issue principally from its angles, two of them going to join a double ganglion situated beneath the cesophagus.
The salivary glands appear to be wanting.