300 



LECTURE XXII. 



122 



stomach of Aplysia. 



thickened, and the interior cal- 

 lous lining is beset with firm 

 horny processes, some in the 

 form of hooks or canine teeth, 

 others in that of rhomboidal 

 plates or molar teeth. These 

 complexities relate to the low 

 organised character of the food 

 of the AplysicE : the sea-weed on 

 which these Mollusca subsist, 

 after coarse mastication and com- 

 mixture with the salivary secre- 

 tions, is macerated in the crop, 

 conveyed to the stomach, there 

 pierced by the gastric spines, 

 percolated by the solvent juices, and pounded by the horny plates. 

 The chyme is then mixed in the duodenum (o) with the hepatic secre- 

 tion, and with the fluid, probably analogous to pancreatic juice, which 

 is secreted by a single long blind glandular sac (6), communicating 

 with the beginning of the intestine. A similar simple form of pancreas 

 is present in some species of Doris, and other fucivorous genera of 

 Gastropods, as Tritonia and Scyllcea, which likewise have horny gastric 

 teeth. In the Bullcea aperta * the stomach is surrounded with three 

 large horny plates, concave externally and convex towards the cavity. 

 In the Bulla lignaria f the gastric triturating plates are calcareous : 

 two of these plates present an irregular triangular form, with the 

 angles rounded off, slightly concave externally, and convex towards 

 the gastric cavity : they are united together by strong transverse 

 muscular fibres attached to their cix'cumference, except at the upper 

 part of the gizzard, where a third valve of an oblong form is inter- 

 posed between the two lateral ones. The imposition of the name 

 Gioenia upon the large gastric plates of the Bulla, as the valves of a 

 new bivalve shell, will not soon be forgotten by conchologists. 



In regard to the number of cavities, the most complex stomach m 

 the Gastropoda is that of the Onchidiuni, wliich has three longi- 

 tudinally plicated gastric compartments. 



The intestine, after performing a few convolutions in the substance 

 of the liver and generative gland, always more numerous, and of 

 greater width in the herbivorous than in the carnivorous Gasteropods, 

 terminates, with a few exceptions, at or near the entry of the respi- 

 ratory cavity on the right side of the body. The anus has a median 



Preps. Nos. 493, 494. 



t Prep. No. 492. 



