210 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



however, may differ in various individuals according to age. 

 The broadest and oldest part of the radula is that spread over 

 the anterior face of the rotella. Here the teeth are dark brown 

 in colour while those further back are paler and softer. The 

 posterior portion of the radula is hidden within the radular 

 sac (Fig. 9, rod. s.), where the new rows of teeth are formed 

 to replace those worn out by constant use. This replacement 

 goes on throughout the life of the animal. 



5. The pharyngeal canal. Dorsal to the odontophore 

 the cavity of the bulb becomes laterally compressed and an 

 incipient separation of the channel so formed occurs. This is 

 the pharyngeal canal and the ingrowing projections that will 

 later cut it off from the buccal cavity are the pharyngeal 

 crests. (Fig. 22, ph. can. and ph. cr.) These crests bear 

 groups of " stick cells " similar to those forming the mandibles. 

 The canal passes insensibly into the oesophagus. 



Oesophagus and Crop. The oesophagus is the posterior 

 continuation of the pharyngeal canal. (Fig. 8.) At first it 

 is a narrow thin-walled tube which passes through the nerve 

 ring. After emerging from the ring it swells into a voluminous 

 chamber, the crop (Fig. 8, cr.), which becomes spirally coiled 

 when the hunchback attitude is assumed by the animal. 

 The walls possess a thin muscular coat, and internally have 

 rugose folds. The lining endothelium is ciliated but not 

 glandular. The crop is called the first stomach by earlier 

 writers. It is capable of great expansion. 



The Gizzard. This is sharply marked off from the preceding 

 region by an annular constriction which occurs at the point of 

 junction, and by a pronounced difference in musclature. The 

 gizzard consists of two portions. It is thick walled, possessing 

 a dense coat of circular muscle fibres covering the anterior 

 portion and of longitudinal fibres in the posterior portion. 

 When slit open the two chambers are seen more plainly. 

 These are called by some authors the first and second triturating 



