53 



external surface of the sac at this point forms a perfect 

 circle in transverse sections (Text-fig. 5). 



The cells of the lateral and basal walls of this 

 circular sheath are of medium length. The cells of the 

 dorsal wall are of extraordinary size and extend down 

 into the cavity, forming a deep ridge, which extends for 

 some distance from the blind end. This odontoblast 

 ridge lies, of course, in the gutter formed by the radula. 

 Thus one sees that the cells of the lateral and ventral 

 walls are directed towards the basal ventral plate of the 

 radula, whilst the dorsal ridge cells are directed towards 

 the teeth, both median and lateral. 



Fig. 5. 



As one passes towards the pharynx the depth of all 

 the cells decreases, the dorsal ridge passes away and 

 gradually the ordinary epithelial type is reached 

 continuous with the epithelium of the pharynx. 



The nuclei of the odontoblast cells are oval, more 

 or less elongated and contain numerous small granules. 

 The most characteristic feature of the odontoblast, how- 

 ever, is the free end of the cell, The secretion leaves it 



