a backward and forward movement takes place, the sharp teeth rasp- 

 ing off small particles of food. 



" The horny jaw lies in the upper part of the mouth, and cuts 

 off large pieces of food, such as leaves or other vegetation. The food 

 is then acted upon by the radula, being pressed against the roof of the 

 mouth, while the teeth rasp off small pieces, which are then swallowed. 

 As fast as the teeth on the front end of the radula are worn out they 



Diagram of animal of land snail, Helix, removed from its shell. The snail 

 is in the position of crawling, aod the organs are in their natural position, 

 bs, buccal sac; e, eye-peduncle; f, foot; 1, lung; Iv, liver; m, mouth; me, 

 orifice of mucous gland; mg, mucous gland; n, nerve ganglia in head and 

 foot; oe, ossophagus; t, tentacle. (Hyatt.) 



are replaced by new ones which are pushed forward from the core 

 of the radula sac. In this way the teeth are being constantly worn 

 out by use and being replaced by new ones. 



" The radula is one of the most important features in the classi- 

 fication of the Mollusca, and a knowledge of its characteristics is quite 

 essential to any one who would seriously study the subject, as it is 

 present in all classes except the clams or bivalves." 



Having explained to us the function of the radula and its position 

 in the animal, with a small pair of tweezers, Professor Parker deftly 

 removed the jaw and radula of the white-lipped snail, placed it in 

 a drop of Canada balsam in the center of a microscope slide, carefully 

 lowered a cover -glass over it, and slipped it on the stage of the 

 microscope. 



After adjusting the eye -tube, he motioned us to come and look 

 through it. This we did in turn, and saw a flat background covered 

 with peculiarly shaped teeth. The Professor changed the objective on 

 the microscope to one of higher power and again asked us to look. 

 This time we could make out only a few teeth, but each one was 



40 



