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ON THE RADUL^ OF THE BRITISH HELICIDS. (Part II.) 

 By Rev. E. W. Bowell, M.A. 



Bead Wth December, 1908. 

 [The terminology proposed in this paper has been revised in 

 accordance with suggestions made by the President and other 

 members of the Society.] 



It is necessary to add to the number of technical terms used for 

 describing the parts of the radula, because the microscopic methods 

 and apparatus now at our disposal give a much more perfect view of 

 the structures in question. It will further be desirable to define the 

 terms now employed. 



The radula is an oblongate membrane disposed with its long sides 

 roughly parallel to the sides of the snail's body. It is, in the forms 

 now under consideration, enclosed in a muscular organ called the 

 odontopJiore, which is the atrium of the digestive system. In its front 

 portion the odontophore also carries the maxilla ( = jaw; the 'machoire' 

 of French authors), which seizes the food, and perhaps also assists in 

 holding it in position while it is being rasped into small pieces by the 

 radula. The radula is provided with regularly disposed rows of unci 

 or hooks, the front and upper portions of which have long been called 

 teeth. It is not yet certain whether these unci bear any relation to 

 the parasphenoidal teeth of such forms as Batrachosefs among the 

 vertebrates. 



The unci are arranged with their free cutting points directed towards 

 the oesophagus of the animal. These points are called cones. Where 

 more than one cone occurs on each uncus, the terms mesocone, ecto- 

 cone, and endocone will naturally be used. In some cases it may be 

 advantageous to draw a distinction between endocone plus mesocone, and 

 bifid mesocone or bifid endocone ; but the truth of such determinations 

 can only be settled by sections showing the groups of cells in the 

 radular sac (the infolded inner end of the radula) from which the unci 

 originate. 



Riicker ' has shown the mode of development of the unci in Helix 

 pomatia. The mechanical difficulties in sectioning such material are 

 considerable, but it seems certain that each uncus in each longitudinal 

 row is a cast of the same matricial cell or group of cells, situated on 

 the lower wall of the radular sac. (This accounts for the regularity 

 with which any asymmetry in development is repeated throughout 

 the length of the radula.) A question which seems to call for settle- 

 ment is, how is this cast produced ? Is it («) a membrane detached 

 from the top of the matricial cell or cells ? (Microscopic examination 

 of unsectioned, but stained, radulae favour this interpretation.) Or is 

 it (i) a chitinous structure actually secreted by all the cells which 

 surround it in the radular sac, and only owing its definite form to the 

 impress of these strangely enlarged and persistent matricial cells? 



' Bericht d. Oberhass. Ges. Xatur. u. Heilkiinde, 1883, vol. xxii. 



