MORPHOLOGY OF THE HELICINID^. 767 



The buccal cavity occupies the snout, in front of the tentacles. 

 It is a simple funnel-shaped cavity bounded by a rather thick 

 muscular wall, the internal sui-face of which is thrown into about 

 19 or 20 longitudinal folds. The cavity is lined by a layer of 

 rather long columnar epithelial cells which secrete a thick cuticle. 

 Dorsally the buccal cavity is prolonged backward into a little 

 glandular diverticulum which lies above the median part of the 

 cerebral commissure. The buccal cavity is separated from the 

 pharynx by a constriction, deepest on the dorsal side, where the 

 cerebral commissure lies in it. In a surface view, before dis- 

 tui'bance of the various parts, this constriction is not visible 

 from above, as it is covered over by the anterior salivary glands 

 shortly to be described, and muscle-fibres pass from the walls of 

 these glands to the walls of the buccal cavity and of the snout. 

 Consequently the cerebral commissure seems to be embedded in 

 the buccal mass. 



The passage from the buccal cavity to the pharynx is narrow. 

 The pharynx is a relatively spacious sac, of which the cavity 

 is continued posteriorly into the oesophagus above and into the 

 radular sac below. Beneath and at the sides of the anterior 

 end of the radular sac lie the odontophoral cartilages, the 

 anterior ends of which pi'oject forward into and occupy the 

 greater part of the lower moiety of the pharyngeal cavity. It 

 will readily be understood that, in consequence of the projec- 

 tion of the anterior ends of the odontophoral cartilages into 

 the pharyngeal cavity, the latter extends round them both at 

 the sides and below. Below the cartilages the pharyngeal 

 extension forms a broad flattened diverticulum, reaching back 

 nearly to the posterior ends of the anterior cartilages, as far 

 as the point marked x in fig. 3 (PI. XXX.). Latei-ally, the line 

 of attachment of the pharyngeal wall to the anterior odonto- 

 phoral cartilages is roughly indicated in the same figure by the 

 curved line running upward and forward from the point x towards 

 the opening of the oesophagus. It results from this arrangement 

 that in an oblique section, such as is represented in fig. 4 

 (PI. XXX.), the pharynx appears to give oW two posterior diver- 

 ticula, lying outside the anteinor ends of the odontophoral 

 cartilages. The inner walls of these diverticula are thin and 

 composed of a single layer of cubical epithelial cells : they are 

 continued round the anterior and upper edges of the cartilages 

 into the lining membrane of the radular sac. The outer wall of 

 each diverticulum is strengthened by a thin plate of cartilage, 

 too small and transparent to be recognized in dissection, but 

 readily recognizable in sections. These lateral pharyngeal carti- 

 lages serve for the attachment of muscles, one set of which run 

 forward to be inserted on the walls of the snout, the other set 

 run backward and are inserted on the odontophoral cartilages ; 

 the former are protractors, the latter retractors, of the walls of the 

 pharynx. 



A portion of the epithelial lining of the outer wall of each 



53* 



