SHELLS. 25 



bodies like a skin, and seem to be a part of the 

 animal itself. 



Teacher. The coverings of insects are united 

 to their different members ; but shells are at- 

 tached to the mollusks only in one or two places 

 by muscles. You must have had the opportunity 

 of observing how they adhere in one spot to their 

 shell. 



Child. Yes, you are obliged to cut the 

 oyster away from the shell. 



Teacher. Besides the difference which you 

 have observed in the covering of insects and 

 mollusks, their composition is not the same. 

 You remember learning in your lessons on lime,* 

 the animal substances of which it forms a prin- 

 cipal part. 



Child. Yes, our bones consist principally of 

 carbonate of lime, and I think you said that 

 shells were also a calcareous substance. 



Teacher. Shells are composed of carbonate 

 of lime, with the addition of a small portion 

 of an animal substance called gelatine : the 

 covering of insects is a phosphate of lime. The 

 substance of shells is testaceous, having the 

 hardness of baked earthenware, in latin, test a ; 

 and hence the mollusca enclosed in them are 

 called testaceous mollusca : the covering of in- 

 sects is crustaceous, having the hardness of crust, 

 in latin, crust a. 



Child. But whence is the carbonate of lime, 



* See Lessons on Objects, page 173, Sixth Edition. 



