ANATOMT AND PUTSIOLOGY OF MOLLUSCA 201 



gardens, and even on live animals, including those of their own 

 species. In snails the mouth lies in a large head bearing 

 various sense-organs. Just inside the mouth a short gullet 

 leads to a tough pad upon which is a file-like rasping organ 

 called the radula (Fig. 194). The teeth in one cross-row are not 

 all alike as in a file, but vary in a complex way that differs in 



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Fig. 194. — The snout of a snail cut vertically and lengthwise to show the 

 mouth and rasping organ. 1, Dorsal wall of head ; 2, mouth ; .3, jaw ; 4, 

 radula ; 5, cartilage of tongue ; G, muscular wall of pharynx ; 7, muscles run- 

 ning from pharynx to ventral wall of head ; 8, space in head for withdrawal 

 of tongue; 9, pocket for radula; 10, ccsophagus ; 11, opening to salivary 

 gland ; 12, fold behind radular pocket. From Lang. 



different species ; the successive cross-rows are, however, very 

 similar. The teeth are composed of a tough, hard substance 

 (chitin) , but the front rows, which are those that are in action 

 at any time, eventually wear off and are replaced by new 

 teeth that push forward. During feeding, the hard edges of 

 the mouth or " jaws " are protruded, and the food is dragged 

 into the mouth and cut into small pieces which then pass on to 

 the fleshy pad, where they are rasped or grated by the radula. 

 Thence the food passes through an oesophagus to an enlarged 



