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 Avere 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 ari-anged 

 in sj^mmetrical 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 for- 

 ward 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 differ- 

 ent 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 Patella 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 Aerope, the Turbinidae generally, 

 and Haliotis it is very large. In Turritella, Aporrhais^ Cylichna^ 



1 The substance both of the jaw and radula is neither crystalline nor cel- 

 lular, but laminated. Chitin is the substance which forms the ligament in 

 bivalves, the ' pen ' in certain Cephalopoda, and the operculum in many uni- 

 valves. Neither silica nor keratine enter into the composition of the radula. 



