168 THE VITAL FUNCTIONS. 



which are placed one on each side, and which 

 resemble in their form broad molar teeth. When 

 these three teeth are brought together by the 

 action of the surrounding muscles, they fit 

 exactly into each other, and are capable of 

 grinding and completely pulverising the shells 

 of the mollusca introduced into the stomach. 

 These teeth are the result of a secretion of cal- 

 careous matter from the inner coat of that organ, 

 just as the outer shell of the animal is a pro- 

 duction of the integument : and at each casting 

 of the shell, these teeth, together with the whole 

 cuticular lining of the stomach to which they 

 adhere, are thrown off, and afterwards renewed 

 by a fresh growth of the same material. In the 

 Craw-fish, the gastric teeth are of a different 

 shape, and are more adapted to divide than to 

 grind the food. 



Among the gasteropodous Mollusca, several 

 species of Bullce have stomachs armed with 

 calcareous plates, which act as cutting or grind- 

 235 ing teeth. The Sulla aperta has 



three instruments of this descrip- 

 tion, as may be seen in Fig. 295, 

 which shows the interior of the 

 stomach of that species. Similar 

 organs are found in the Sulla 

 lig7iaria. The Aplysia has a con- 

 siderable number of these gastric teeth. An 

 apparatus of a still more complicated kind is 



