36 



MOLLUSCA. 



usually much coiled, and as a rule bend forwards to open in front 



on the right side in the mantle cavity. The anus, however, is some- 

 times on the dorsal surface behind. 



Many of the higher Gastropoda 

 possess an invaginable proboscis, the 

 invagination beginning at the base; 

 others possess one which is retractile 

 from the point. The mouth is bounded 

 by lips, and leads into a buccal cavity 

 armed with hard masticating structures, 

 and receiving the ducts of two salivary 

 glands. The buccal cavity leads into 

 the oesophagus, which is followed by a 

 dilated stomach, usually provided with 

 a csecal appendage. The stomach opens 

 into an intestine, which is usually long 

 and much coiled, and surrounded by a 

 very large, multi-lobed liver. The liver 

 occupies nearly all the upper part (upper 

 coils) of the visceral sac, and pours its 

 secretion into the intestine and also 

 into the so-called stomach (fig. 511). 

 The arrangement of the digestive canal 



and of the liver presents in details many essential modifications ; one 



of the most remarkable is that offered by the intestine with its 



hepatic caeca of the Phlebenterata 



(fig. 512). The terminal portion of 



the intestine is distinguished by its 



size, and may be called the rectum. 

 The armature of the buccal 



cavity consists partly of jaws 



placed on the upper wall, partly of 



the so-called lingual ribbon (radu- 



la), placed on a tongue-like pro- FlG<513 . -Longitudinal section through the 



jection of the ventral surface of buccal mass of Helix (after W. Keferstein). 



O, mouth ; Mh, buccal cavity ; M, muscles; 

 the buccal cavity. Sd> radula . Z n, lingual cartilage ; Oe, 



The jaws consist either of a single oesophagus; *7, jaws; z, sheath of 



, , radula. 



curved horny plate, placed close 



behind the edge of the lip, or of two lateral pieces of very different 

 form, between which, in some Pulmonates, there is an unpaired 

 piece. There are no lower jaws; but on the floor of the buccal 



PIG. 512. Alimentary canal of 

 JEolis papillosa (after Hancock). 

 Bm, buccal mass ; Oe, oesophagus; 

 M, stomach, L, liver sacs, which 

 enter the dorsal appendages; A, 

 anus. 



Mh- 



