154 



SHELLS. 



LESSON XXXIL 



HABITATIONS ^-SHELLS PEARLS HABITS* 



A &REAT numbed Of the molluscous animals 

 dwell in a hard covering called shell. The colour 

 of shells is," in some instances, remarkably beauti- 

 ful, and their shape and make very curious. The 

 animal, as it increases in size, enlarges its dwelling 

 by adding fresh layers of matter at thelgdges ; 

 and, if the shell is injured, it is repaired again 

 with the greatest nicety. 



Such shells as are single are termed univalve 

 shells, and those which have two plates, as the 

 oyster, bivalve shells Bivalve shells are fastened 



