214 



FUNCTIONS AND POSITION OF RADULA 



CHAP. 



tion of the jaw, it comes within the province of the radula, the 

 front part of which perhaps co-operates to a certain extent with 



. . — ■'^ 



Fig. 110. — Jaws of Sepia : A, in situ 

 within the buccal mass, several of 

 the arms having been cut away ; 

 B, removed from the mouth and 

 slightly enlarged. 



Fig. 111. — Patella vulgata L., show- 

 ing the normal position of the 

 radula, which is doubled back in 

 a bow; the shell has been re- 

 moved, and the whole visceral 

 mass is turned forward, exposing 

 the dorsal surface of the muscular 

 foot: gi\ longitudinal groove on 

 this surface ; i, i, intestine ; /, 

 liver; m, m, mantle edge; mu, 

 muscles (cut through) fastening the 

 visceral mass to the upper sides of 

 the foot ; ov, ovary; r, radula ; w./, 

 upper or dorsal surface of the foot. 



the jaw in performing the biting process. The function of the 



like Turritella, Harpa, or Struthiolaria, or where the radula is almost filmy in 

 its transparency, like those of Actaeon and the small Scalaria. 



" When once the radula is laid out, the mounting is commonly easy. 

 Canada balsam makes it too transparent. Fluids may be used, and are almost 

 necessary for thick radulae like those of large Chitons; but the best general 

 medium is glycerine jelly. It runs under the cover glass by capillary attraction, 

 and may be boiled (though only for a moment) to get rid of air bubbles. It 

 should then be left unfinished for several weeks. If cracks appear, the reason 

 is either that the jelly is a bad sample, or that it has been boiled too long, or 

 (commonly) that the object is too thick ; and there is not often any difficulty in 

 remounting. I have no serious complaint of want of permanence against the 

 medium, if I may speak from a pretty wide experience during the last twenty 

 years." 



