208 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



2. Paired postero-dorsal muscles, connected dorsalLy in 

 the middle line and continuous in direction with those of the 

 oesophagus. 



3. A single, stout basal mass of muscle situated on the 

 ventral side of the odontophore. 



4. A circular band of muscle acting as a sphincter round 

 the buccal orifice. 



[Open the buccal mass by a median longitudinal incision 

 along the dorsal side.] 



An internal view of the bulb shows that it consists of a 

 thick walled chamber, from the postero- ventral portion of 

 whose floor arises a pu] ley-like structure, the odontophore. 

 The food entering the mouth is forced upwards over the top 

 of the odontophore and leaves the bulb by the dorsally situated 

 oesophagus, coming under the action of the radula or tooth 

 ribbon as it does so. The interior is divisible into five 

 regions : — 



1. The oral region beset with longitudinal ridges as 

 already noted. This part, which is eversible, ends abruptly 

 posteriorly where the sphincter muscle causes a slight con- 

 striction and the jaws are fixed. 



2. The mandibular region. On the walls of this region 

 are affixed paired flat horny plates, the so-called mandibles. 

 (Fig. 9, mand.) Dorsally the plates are connected by semi- 

 cartilaginous tissue so as to appear to be continuous ; ventrally 

 they merge into the epithelium of the mouth. They are 

 composed of dark brown chitinous rods packed side by side, 

 but with one free end curved and projecting slightly into the 

 cavity. The rods have been called " stick cells." The 

 mandibles have no masticatory function, but help to stiffen 

 the buccal orifice and to form a plate against which the radula 

 can work. 



