io THE LIFE OF THE MOLLUSCA 



The mandible is single, and placed on the upper 

 side of the mouth in the Limpets (Patellidae), one of 

 the Sea- Slugs {M gives punctilucens), the Land and 

 Freshwater Snails (Pulmonata, Plate III., Figs. 5 

 and 6). and the Elephant's-Tusk Shells (Dentaliidse). 

 Two kinds of Pond Snails (Limncea and Planorbis) 

 have three mandibles, an upper and two side plates 

 (Plate III., Fig. 7). The majority of the other 

 gastropods — for the carnivorous species are generally 

 without them — have two lateral plates. Many of 

 these are ornamented with most elaborate patterns, 

 and some have raised projections on them like a file 

 (Plate III., Fig. 8). 



The Cephalopoda also have a pair of mandibles, 

 upper and lower, resembling a Parrot's beak, save 

 that the upper fits within the lower one (Plate III., 



Fig- 4)- 



These mandibles are formed apparently of dense 

 chitine, strengthened in the case of Chcetoderma and 

 Nautilus with carbonate of lime. 



The radula is so important a feature as to call for 

 more detailed mention. With its supporting car- 

 tilages and muscles it occupies the position in the 

 mouth assumed by the tongue in the higher animals, 

 and much resembles that organ in general appearance. 

 The radula itself, however, consists of a series of 

 recurved teeth formed of dense chitine, attached in 

 transverse rows to a membrane of the same sub- 

 stance. In the Limpets (Docoglossa) the teeth 



