150 



THE VITAL FUNCTIONS. 



291 



(lelhieated in Fig. '294, which presents a view of the 



interior of the stomach of 

 that animal. The tooth a, 

 situated in the middle of 

 this frame, has a rounded 

 conical shape, and is smal- 

 ler than the others (b, c), 

 which are placed one on 

 each side, and which re- 

 semble 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 com- 

 pletely pulverizing the shells of the mollusca intro- 

 duced into the stomach. These teeth are the result 

 of a secretion of calcareous matter from the inner 

 coat of that organ, just as the outer shell of the 

 animal is a production 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 

 Crmu-Jisli, the gastric teeth are of a different shape, 

 and are more adapted to divide than to grind the 

 food. 



Among the gasteropodous Mollusca, several spe- 

 cies of Biillce have st omachs armed 

 with calcareous plates, which act 

 as cutting or grinding teeth. The 

 JBulla aperta has three instruments 

 of this description, as may be seen in 

 Fig. 295, which shows the interior of 

 llie stomach of that species. Similar 

 organs are found in the Bulla lig- 



