GASTEROPODA. 



387 



Fig. 185. 



similar 



that 



Many Gasteropoda which 

 devour shell-fish and other 

 hard materials have a true 

 gizzard adapted to break in 

 pieces such food ; this is the 

 case with Thethys, 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 Limn<Eus 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 

 of Limnaeus. In Onchidium 

 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 

 plicae, affecting a longitudinal direction. 



There are some families in this class which 

 are provided with a still more elaborate appa- 

 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 Bullae (Akera) 

 the gizzard contains three plates of stony hard- 

 ness attached to its walls, and so disposed that 

 they are evidently powerful agents in the tritu- 

 ration of the food. In Eulla lignaria (jig. 186) 

 Fig. 186. two of these teeth are placej 

 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, 

 but similarly imbedded in 

 radiating muscles, whose 

 action must powerfully gr.nd 

 down the substances which 

 come under the influence of 

 this singular mill. In the other BuHa 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 Scyllaa (jig. 187, e) is, ex- 

 ternally, a strong fleshy cylinder, and when 

 this is opened there are iband, firmly im- 



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- 



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 oesophagus, 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 (jig. 188) they are found to have their 



Fig. 188. 



