V. BULLA. The animals of this genus differ from the preceding, in the absence of tentacula, and in the foot be hind being furnished with a membranaccous appendage.
The body is oblong—becoming a little narrower in front. Below, the foot is broad, thin, and waved on the margin, expanded on each side behind, and capable of being turned upwards. At the posterior part of the foot, there is a broad membranaccous appendage, separated from it by a groove, part of which folds upwards, arid a part is spread over bodies, like the foot. It assists in closing the mouth of the shell, and in its position and use is analogous to the operculum, in the following order. Above the foot, in front, but separated from it by a groove, is a flat fleshy expansion, which Cuvicr terms the tentacular disk, considering it as formed by the union of the inferior and superior tentacula. In the centre of the disk, in the Bulla hydatis, (Linn. Trans. vol. ix. tab. 6. f. I.) Montagu observed two eyes. Between this portion of the back and the posterior extremity, is the dorsal plate or shell, forming the genus Bulls of con chologists. This shell is covered by the integuments in some species, while in others it is exposed. But in all, the part containing it is partially concealed by the animal, by means of the reflected margins of the foot, and its appendage. Along the right side of the body there is a groove formed by the foot, and its appendage on one side, and the dorsal plate and tentacular disk on the other. The branchim are situated in a cavity under the shell or dorsal plate, and resemble those of the Aplysia. Behind the gills, in the lateral groove, is the anus, and in front of these, the orifice of the united or gans of generation. The penis is removed as in the Aplysia, and connected by a similar slit.
The mouth is, as usual, in front, above the foot and beneath the tentacular disk, both of which serve as lips. The cheeks are strengthened on each side by a corneous plate. The tongue is well developed in some, as the B. ampulla, while in the B. aperta it is reduced to a small tubercle. The gullet is large, and in the B. ligniaria makes two folds before entering the gizzard. This last
organ is fortified by three testaceous plates, convex and rough on the inner surface, and attached to strong mus cular walls. These plates exhibit in the different spe cies considerable varieties or form and markings. The intestine, before terminating in the anus, makes several convolutions in the substance of the liver. The salivary glands exhibit considerable differences. In the B. anz pulla, they are long and narrow, and their inferior ex tremity fixed to the gizzard. In the B. aperta and ligna ria, they are short, with the extremity free. In the B. hyclatis they are long, unequal, and the extremity of the one belonging to the left side is forked. The liver forms a part of the contents enclosed in the spire of the shell. It envelopes the intestine, and empties the bile into its pyloric extremity.
The auricle and ventricle appear to occupy the same relative position as in the Aplysia, but the structure of the arteries is unknown. The organs of generation have also so near a "resemblance as to forbid a detailed description. Some species are said to eject a coloured fluid like the Aplysia, from the lid of the branchim. A gland is observed in the Bulla lignaria, similar to the Aplysia, in which it is probable the fluid is prepared.
The species of this genus have not been sufficiently investigated in a living state. When preserved in spi rits, it is impossible to form a correct idea of their true appearance, as exhibited when alive in sea-water, since they usually exist as a shapeless mass. Cuvicr has given delineations of such preserved species, but they bear no resemblance to the figures of Montagu, of the same species, taken from living objects.
AI. Lamarck is inclined to subdivide this genus into two, distinguishing those in which the shell is conceal ed by the term Bullxa, from such as have the shell in part exposed, which he retains in the genus Bulla. The Bulla aperta of British conchologists is an example of the former, and B. lignaria of the latter. The shells of the genus Bullcea are thin and white—tbose of Bulla stronger, more opake, and covered with an epidermis, which, after the death of the animal, is easily detached.