Many Casteropoda which _devour shell-fish and other hard materials have a true gizzard adapted to break in pieces such food ; this is the case with Thclhys, an animal whose mouth is totally desti tute of dental organs, but their want is supplied by a fleshy gizzard resembling that of a bird, having its interior lined with a dense cartilagi nous membrane, like that which lines the gizzard of graminivorous fowls, and in its cavity shells of Mollusca and Crustaceans are found comminuted by its action. In Limnens we find a gizzard strictly analogous in structure to that of a granivorous bird : it presents two dilatations, one at the cardiac, the other at the pyloric extremity, whilst the middle portion is occupied by two strong mus cles, united at the sides by tendinous bands. The gizzard of Planorbis is precisely bedded in its muscular walls, twelve horny plates, which are extremely hard and as sharp as the blades of a knife; their edges are dis similar to that of Limureus. In Orichidiinit the muscular gizzard is followed by two other stomachs, the lining membrane of that which immediately succeeds it being gathered into large folds, which must greatly retard the pas sage of the aliment; while the third cavity, which is short and cylindrical, is likewise lined with a membrane folded into more delicate pliete, affecting, a longitudinal direction.
There are some families in this class which are provided with a still more elaborate ratus for the preparation of their food, their stomachs being armed internally with teeth variously disposed, and on many accounts extremely curious. In all the Bullm (Akera) the gizzard contains three plates of stony ness attached to its walls, and so disposed that they are evidently powerful agents in the ration of the food. In Bulk lignaria (fig. 186) two of these teeth are placel on either side of the gizzard, into the cavity of which they project, and are united to each other by strong muscular bands ; the third piece is smaller than the other two, hut similarly imbedded in radiating muscles, whose action must powerfully grind down the substances which come under the influence of this singular mill. In the other Buthe the structure of the gizzard is the same, but the bony plates differ slightly in form and arrange ment. In all, however, the fragments of shells and other hard substances found in it attest the efficacy of the apparatus.
The gizzard of Sculling (fig. 187, e) is, ex ternally, a strong fleshy cylinder, and when this is opened there are found, firmly im posed in the direction of the axis of the organ, and as they project considerably into its cavity, their action upon the contents of the gizzard must be sufficiently evident.
Aplysia, however, furnishes us with the most curious form of these stomachal teeth. The cesophagus, which is comparatively narrow at its commencement, soon dilates into a capa cious crop, which is generally found filled with pieces of fucus and the fragments of shells. To this crop succeeds a short cylindrical gizzard with strong and muscular walls, and after the gizzard we find a third stomach which leads to the intestine. On opening the gizzard and third stomach (fig. 188) they are found to have their interior armed in a manner which is probably unique. The sides of the gizzard (b) are covered with pyramidal plates of a rhomboidal figure, the apices of which resemble the tubercles found upon the grinding surfaces of the human molar teeth. Of these there are twelve larger plates arranged in quincunx, besides several smaller ones placed near the entrance of the organ. These teeth are of a horny nature and formed of laminae parallel to their bases : their adhesion to the surface of the lining membrane is so slight that they are detached by the slightest effort, without leaving any trace of membrane or other bond of union, the place of their attachment being only indi cated by a smooth and prominent surface, corresponding in shape to the base of each tooth. The apices of all these teeth meet in the centre of the gizzard, and whatever passes through that cavity must be bruised by their action.
The third stomach (d) is armed with teeth of a totally different nature. These are little conical hooks (c) attached to one side of the organ only, and as little adherent in the dead animal as are the pyramids of the gizzard towards which their points are directed. In the figure many have fallen off, leaving slightly elevated spots indicative of the place of their attachment. Near the pylorus is a large aper ture communicating with a ccecum of consider able size (f), evidently identical with the spiral ccecum of the Cephalopoda both in its position and relation to the insertion of the biliary canals (e), forming, as in Fishes, the rudiment of a pancreas. From the orifice of the ccecum a ridge is prolonged into the com mencement of the intestine (g).
Accessoryglands.—The auxiliary chylopoietic secretions found in the Gasteropoda are gene rally only two, the salivary and the hepatic. In some rare instances already adverted to, as in Doris and Aplysia, we may likewise add the pancreatic furnished by the cceca, which in those genera terminate in close vicinity with the ducts issuing from the liver, and which, from every analogy, represent the pancreas of vertebrate animals.