The tongue of Oscatrio resembles that of the Patella, except in its armature, being fur nished on each side with a series of hooked and three-pointed scales, and another set of long, sharp, and recurved spines, whilst its centre is simply studded with tubercles. In Turbo pica the scales, which are cutting and denticulated, are arranged transversely along its surface.
The tongue of Buccinum (fig. 182, j), is placed at the extremity of the proboscis, form ing a most extraordinary apparatus, capable of destroying by its constant action the hardest shells; externally it resembles rather a mouth than a tongue, being divided into two lips, each of which is studded with sharp horny teeth. These lips are supported upon two cartilages which occupy the anterior half of the proboscis, and are moved upon each other by strong muscular fasciculi (h) in such a manner that the spines which arm the surface of the organ are alternately erected and depressed by their action, a movement the constant repe tition of which soon wears away the substances upon which it is made to act. This spiny tongue is situated just within the entrance to the msophagus (d), and besides acting upon foreign bodies will materially assist in pro pelling the food into that tube.
In other Gasteropods the tongue is short and merely an organ of deglutition : thus, in .Aplysia it is broad, heart-shaped, and studded with sharp points. In Onchidium and Doris, the surface is marked with transverse grooves, which are crossed at right angles by others of great fineness. And in the Snail and Slug, in which the surface of the tongue is similarly marked, the stria: are so delicate that they can only be seen with a microscope.
Alimentary canal.— W e shall commence our description of the intestinal canal of the Gasteropod Mollusca by the examination of the simpler forms which it presents. In the Snail (jig. 190), the whole alimentary tube (e, f, g, k) is thin and membranous. The stomach, which is merely a dilatation of the msophagus, is semitransparent, but studded with opaque points and internally folded into delicate longitudinal From this arises an intestine, of considerable length, without cmca, valves, or remarkable appearance inter nally, except near its termination, where the orifices of minute follicles may be detected ; the intestine having performed several con volutions enveloped in the masses of the liver, with which it is connected by cellulosity and numerous vessels, at last runs along the margin of the pulmonary cavity, close to the orifice of which it terminates. In Vaginulus the arrangement is nearly similar (fig. 189, g, h, i.) In Tritonia and Doris the structure of the digestive tube is equally simple, and in these as well as in the majority of the Gaste ropoda the only remarkable differences are found in the proportional size of the stomach and the length of the intestinal convolutions.
In Doris we find near the orifices by which the bile is poured into the stomach, an aper ture communicating with a round vesicle or cmcum, the inner surface of which is evidently glandular, and from its large supply of blood derived from one of the hepatic arteries, pro bably furnishing an abundant secretion ana logous to that of the pancreas. In Phasia nella the stomach is very voluminous and sacculated internally. In Buccinum the di gestive apparatus is more complicated in its structure. The oesophagus commences, as we have already seen, at the extremity of the pro boscis, and of course follows all the motions of that organ; when the proboscis is protruded in search of prey, the gullet is straight and adapted to the reception of food ; but when the proboscis is retracted within the body, the oesophagus is bent upon itself, so as to be partially contained within the proboscis, whilst the greater portion is folded beneath that organ in its retroverted state. After making another fold it dilates into a small crop, the lining of which is Micated in the direction of its axis, and to this succeeds the stomach, which is a moderately sized round cavity, irregularly ru gose internally. The intestine is very short, and has a small ccecum appended to its side ; it terminates in a capacious rectum, placed, as is invariably the case, in the vicinity of the respiratory cavity, and having its lining mem brane gathered into prominent longitudinal rugs. Many of the Gasteropoda are provided with several digestive cavities, resembling in some degree the stomachs of ruminating Mam malia. In Janthina, which is furnished with a proboscis like that of the Buccinum, the oesophagus arising from this terminates by a narrow slit in a membranous cavity or first stomach, to which succeeds a second, having thicker walls and plicated internally. The in testine is extremely short, terminating as usual in the neighbourhood of the respiratory cavity. In Pleurobranchns the resemblance of the stomachs to those of a ruminating quadruped is very strildng. The first stomach (fig. 185, a), which is membranous, receives the bile by a large orifice (b) placed near its communi cation with the second digestive cavity (c), which is smaller and more muscular ; to this succeeds a third (d), the sides of which are gathered into broad longitudinal lamella, pre cisely similar to those of a ruminant ; and to render the analogy still more perfect, a groove is found running along the walls of the second cavity from one orifice to the other, apparently subservient to rumination. The fourth stomach (e) is thin, and its walls smooth. This animal lives on Alcyonia and small Zoo phytes.