IVIV.J THE COMMON SNAIL. 287 



c. The levators ; delicate muscles arising just above 



the protractors, and passing upwards to be in- 

 serted into the cephalic wall, near the bases of the 

 smaller tentacles. 



d. The depressors, small muscles underlying the pro- 

 tractors, and passing obliquely backwards. 



2. Carefully remove one half of the buccal mass, cutting 

 to the near side of the middle line. Examine under 

 water and note — 



a. The odontophore, a cushion-shaped elevation of 

 the floor of the mouth, completely covered in 

 mucous membrane. There overlies it % yellowish 

 ribbon-shaped band — the lingual ribbon or radula; 

 follow this back into its sac (see Sect. D. x. c) 

 and note that as that is approached it assumes a 

 whitish colour and membranous texture. 



b. The radula. Remove this bodily, with a pair of 

 forceps j transfer it to a glass slide, cover in water 

 and examine under a low power. Note the 

 presence of an immense number (between twelve 

 and thirteen thousand) of chitinous teeth. Those 

 which are functional can be readily distinguished 

 by their yellow colour and sharp pointed cusps. 

 Examine that portion of the radula which lay 

 within the sac. Note the transparent immature 

 teeth, becoming simpler and more papilla-like as 

 the hindermost border is reached. Examine in 

 like manner the front end. It is beset by teeth 

 whose cusps are worn down, and reduced, in many 

 cases, to the condition of mere functionless rudi- 

 ments. 



