ORGANS OF DIGESTION. 369 



boscidian species are carnivorous and feed on living prey. 

 The mouth of the gasteropods is placed at the anterior 

 end of the body, is furnished above with one or more pairs 

 of tentacula and generally with a pair of eyes, and contains 

 often a pair of smooth horny lateral jaws, a fleshy tongue 

 supporting numerous recurved horny spines, or a long mus- 

 cular proboscis armed with numerous sharp recurved spines 

 at its extremity. The pharynx is generally a capacious 

 cavity furnished with distinct layers of circular and longi- 

 tudinal muscular fibres, and stronger fleshy bands to advance 

 and retract it. One or two pairs of salivary glands, con- 

 sidered by Meckel as pancreatic, extending along the sides 

 of the oesophagus, send their ducts into the mouth at the 

 base of the tongue. The oesophagus is longer than in the 

 acephalous mollusca, and is especially lengthened in the 

 carnivorous proboscidian gasteropods which mostly inhabit 

 turbinated shells. There is sometimes, as in the buccinum, 

 an enlargement or crop formed in the course of the oeso- 

 phagus, as is common in insects and cephalopods. The 

 stomach forms always a distinct cavity, often of great size, 

 and is sometimes divided in the phytophagous gasteropods, 

 as in the aplysia, into several compartments. The pyloric 

 end of the stomach receives by several wide ducts the 

 secretion of a large liver, and sometimes also that of one or 

 more pancreatic follicles. The interior of the stomach is 

 often provided with teeth, like the stomach of the Crustacea 

 and the gizzard of insects and sometimes with a pyloric 

 valve. The liver is of great size in this as in other mol- 

 luscous classes, its lobules are composed of more lengthened 

 tubuli biliferi than in the conchifera, and generally envelop 

 the intestinal canal as in these acephala. The gastric dart 

 with its coecal sheath is not developed in the gasteropods, 

 and the intestine, destitute of mesentery, is more lengthened 

 and more convoluted than in the conchiferous class, es- 

 pecially in those gasteropods which feed on vegetable sub- 

 stances. In the carnivorous species where the oesophagus is 

 lengthened, the stomach is generally small and the intestine 

 short and narrow. The intestine, supported only by vessels 

 and cellular tissue, is sometimes enlarged at its anal por- 

 tion to form a colon, but is without caecum or valvula 

 coli ; and instead of terminating, as in the articulated classes, 



PART IV. B B 



