38 COMPAKATIVE DENTAL ANATOMY 



4) are but modifications of the limbs translated 

 from the locomotive series and set apart for spe- 

 cial mouth organs. In the higher crustaceans the 

 stomach is provided with calcareous plates or 

 stomacholiths, with molar-like prominences for 

 grinding food by means of the powerful muscles 

 which move them. These are interesting struc- 

 tures, as they show how similar functions may 



! 

 Fig. 4. — X-ray of the claw of Lobster, modified, for dental purposes. 



develop analogous structures in dissimilar parts 

 which have no homology whatever. 



In the Mollusca we find that the bivalves (the 

 clams, oysters, mussels, etc.) are entirely without 

 head or dental apparatus. The other groups, 

 however, present some form of dental structure 

 in most of their members. In the Cephalopoda 

 the organs of mastication include a corneous beak, 

 resembling that of a parrot, but reversed; within 

 the oral opening there is a fleshy tongue, which 



