PTEROPODA AND GASTROPODA. 



557 



Stomach of Aplysia. 



coat3 of the second stomach, or'gizzard {fig. 205.), are thickened, and 

 the interior callous 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 complexi- 2^2 



ties relate to the low organised cha- 

 racter of the food of the Aplysice : 

 the sea-weed on which these Mol- 

 lusks subsist, after coarse masti- 

 cation and commixture with the 

 salivary secretions, 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 (a) with the hepatic 

 secretion, and with the fluid, pro- 

 bably analogous to pancreatic 

 juice, which is secreted by a single long blind glandular sac {b\ 

 communicating with the beginning of the intestine. A similar simple 

 form of pancreas is present in some species of Doris, and other fuci- 

 vorous genera of Gastropods, as Tritonia and Scyllcea, which like- 

 wise have horny gastric teeth. In the BullcBa aperta *, in which 

 the tongue is not armed with teeth, the stomach is surrounded with 

 three large horny plates, concave externally and convex towards the 

 cavity. In the Bulla lignaria^ 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 trans- 

 verse muscular fibres attached to their circumference, except at the 

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

 interposed 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, which was sold, whilst the deception 

 lasted, at fabulous prices, will not soon be forgotten by conchologists. 

 In regard to the number of cavities, the most complex stomachs 

 in the Gastropoda are those of Oncliidium, which has three longi- 

 tudinally plicated gastric compartments, and of Pleurobranchus, in 

 which they resemble those in the Ruminants. The first stomach 

 (fig. 206. a), which is membranous, receives the bile by a large orifice 

 (6) placed near its communication with the second digestive cavity 



Preps. Nos. 493. 494. 



t Prep. No. 492. 



