112 INTRODUCTION TO ZOOLOGY CHAP. 



teeth in one row of the radula of PlanorMs, a very common 

 pond snail, is shown in Fig. 63. 



When the snail is feeding, the lips are retracted, and the 

 cushion and rasp pushed forwards until the latter can be 

 scraped against the surface on which the snail is feeding. 

 If the snail is feeding on the Algae which have covered the 

 sides of the tank, the passage of the tongue over the glass is 

 indicated by the clean tracks left where the Algae have been 

 scraped away. 



Naturally the teeth on the rasp rapidly become worn down 

 by this scraping action, but just as rapidly are they replaced 

 by the pushing forward of fresh teeth from behind. The 

 formation and growth of the new teeth occur at the hinder 

 end of the rasp, which is lying in the regenerative " tongue 

 sac " ; the whole structure, as it is renewed from behind, 



FIG. 63. Radula of Planorbis. 

 A few of the teeth from one transverse row ; c, central tooth of the row. 



gradually grows forward, and so a constant supply of teeth is 

 forthcoming. 



The sense of sight t in spite of the presence of 

 eyes, does not seem at all keen, the keenest senses 

 being those of touch and of smell (the tactile and olfactory 

 senses). The whole body seems sensitive to touch, parti- 

 cularly the tentacles, which are also the seat of the special 

 sensory cells thought to be olfactory in function. 1 The 

 sense of taste is probably located in the upper lip, which, in 

 some forms, is curiously lobed. There is no proof that a 

 purely auditory sense is present, apart from the tactile sense, 

 which would be affected by any strong vibration due to a 

 sound. The sensitiveness of water snails to various stimuli 

 has not, however, been very fully worked out. 



1 J. W. Taylor, Monograph of the Land and Fresji Water Mollusc^ of the 

 British Isles, vol. i. pp. 224-30. 



