viii TEETH OF THE RADULA 21 5 



radula as a whole is to tear or scratch, not to bite ; the food 

 passes over it and is carded small, the effect being very much the 

 same as if, instead of dragging a harrow over the surface of a 

 field, we were to turn the harrow points upwards, and then drag 

 the field over the harrow. 



The radula itself is a band or ribbon of varying length and 

 breadth, formed of chitin, generally almost transparent, some- 

 times beautifully coloured, especially at the front end, with red 

 or yellow.^ It lies enveloped in a kind of membrane, in the floor 

 of the mouth and throat, being quite flat in the forward part, but 

 usually curving up so as to line the sides of the throat farther 

 back, and in some cases eventually forming almost a tube. The 

 upper surface, i.e. the surface over which the food passes, is covered 

 with teeth of the most varied shape, size, number, and disposition, 

 which are almost invariably arranged in symmetrical rows. 

 These teeth are attached to the cartilage on which they work by 

 muscles which serve to erect or depress them ; probably also the 

 radula as a whole can be given a forward or backward motion, so 

 as to rasp or card the substances which pass over it. 



The teeth on the front part of the radula are often much 

 worn (Fig. 112), and probably fall away by degrees, their place 

 being taken by others successively pushed up from behind. At 

 the extreme hinder end of the radula the teeth are in a nascent 

 condition, and there are often as many as a dozen or more 

 scarcely developed rows. Here, too, lie the cells from which the 

 teeth are originally formed. 



The length and breadth of the radula vary greatly in difterent 

 genera. In Littorina it is very narrow, and several times the 

 length of the whole animal. It is kept coiled away like a 

 watch-spring at the back of the throat, only a small proportion 

 of the whole being in use. I have counted as many as 480 rows 

 in the common Littorina littorea. In Fatella it is often longer 

 than the shell itself, and if the radula of a large specimen be 

 freshly extracted and drawn across the hand, the action of the 

 hooks can be plainly felt. In Aero]je, the Tm'binidae generally, 

 and Haliotis it is very large. In Turritdla, Aporrhais, Cylichna, 



^ The substance both of the jaw and I'adula is neither crystalline nor cellular, but 

 laminated. Chitin is the substance Avhich forms the ligament in bivalves, the 

 'pen' in certain Cephalopoda, and the operculum in many univalves. Xeither 

 silica nor keratine enter into the composition of the radula. 



