42 Shell Life 



by a very remarkable organ peculiar to the 

 Mollusca. It is an almost transparent ribbon of 

 cliitin, whose upper surface bears a large number 

 of minute teeth, of varying forms, number, and 

 arrantrement, set in transverse rows, and each row 

 symmetrically divided into central, lateral, and 

 marginal teeth. The organ is variously known as 

 the tongue, the radula, the lingual ribbon, or the 

 odontophore. Only a small portion 

 is brought into action, the remainder 

 beino; coiled away at the back of the 

 ^^ mouth, ready for use when the teeth 



Portions of thetongue 



of a Whelk, with on the lorepart shall have been worn 



a row of teeth ^^^^ rpj^^^^ ^^^^j^ ^^ ^ ^,.^|g ^^^ f^^^ 



(greatly enlarged) 



and large in those species of car- 

 nivorous habit, whilst those of herbivorous tendency 

 have them small and numerous. Many of the 

 carnivorous mollusks make their tong^ues serve a 

 double office, for they have no jaws, and the whole 

 of the cutting and breaking up of their food has 

 to be done by the teeth ; this, no doubt, explains 

 their larger size. 



The number of these teeth to one tong^ue or radula 

 varies to a remarkable extent : thus, one of the Sea- 

 slugs (J^olis coronata) of our own coasts has only 

 about 17, whilst the large brown Sprinkled-snail 

 {Helix aspersa) of our gardens and hedges has 105 

 teeth in each transverse row, and no less than 135 

 rows — that is, a total of 14,175 teeth in one mouth. 

 There are, of course, many species with numbers 

 intermediate between yEolis and Helix; the large 

 Pond-snail {Limncea stagnalis), whose teeth are 

 shown in the fio^ure a below, has 111 such teeth in 



