52 THE BOOK OF NATURE STUDY 



the " radula." It is a horny ribbon covered with numerous fine, 

 backwardly directed teeth, and capable of being worked to and 

 fro against the jaw above by special muscles. At each bite the 

 upper lips are retracted, the jaw brought forward, and then the 

 radula pushed upward and backward with a rasping stroke. 

 The vegetable matter caught between the radula and jaw is thus 

 torn off. This process is most easily observed in water-snails 

 when they are browsing upon the algae that grow upon the glass 

 sides of a fresh-water aquarium. If small land-snails, whose skin 



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FlG. 32. Portion of radula of snail, magnified ; and three individual teeth of same, 



more highly magnified. 



is nearly transparent, be under observation, the action of the 

 radula may be rendered visible by focussing with a magnifying 

 glass the light from a candle flame upon the side of the animal 

 remote from the observer. As the front part of the radula is 

 constantly being worn away by the friction which it encounters 

 so it is steadily renewed by growth from behind, the whole horny 

 sheet slowly sliding forward over the floor of the mouth. 



Every snail is hermaphrodite, i.e. it contains both male and 

 female organs, but pairing is probably always necessary for the 

 fertilisation of the eggs. Prior to the act of mating a remarkable 

 bayonet-like dart of calcareous material is discharged by the 

 snail into the body of its mate. The form of the dart differs 



