GASTRIC TEETH. 



123 



mouth, or entrance into the alimentary canal, for in many- 

 animals we find this office performed by interior organs. 



294 Among the inferior classes, we 



find examples of this conforma- 

 tion in the Crustacea, the Mol- 

 lusca, and above all in Insects. 

 Thus, there is found in the sto- 

 mach of the Lobster, a cartilagi- 

 nous frame-work, in which are 

 implanted hard calcareous bodies, 

 having the form, and performing the functions of teeth. 

 They are delineated in Fig. 294, which presents a view of 

 the interior of the stomach of that animal. The tooth a is 

 situated in the middle of this frame, has a rounded conical 

 shape, and is smaller than the others (b, c,) 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 

 pulverizing the shells of the mollusca introduced into the 

 stomach. These teeth are the result of a secretion of calca- 

 reous matter from the inner coat of that organ, just as the 

 outer shell of the animal is a production of the integu- 

 ment: and at each casting of the shell, these teeth, together 

 wnth 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, se- 

 veral species of Bullx have stomachs armed 

 with calcareous plates, which act as cutting 

 or grinding teeth. The Bulla aperta has 

 three instruments of this description, as 

 may be seen in Fig. 295, which shows the 

 interior of the stomach of that species. 

 Similar organs are found in the Bulla lig- 



