SHELLS. 25 



Teacher. The coverings of insects are united 

 to their different members ; but shells are attached 

 to the mollusks only in one or two places by 

 muscles. You must have had the opportunity of ob- 

 serving how they adhere in one spot to their shell. 



Child. Yes, the oyster must be cut away 

 from the shell. 



Teacher. Besides the difference which you 

 have observed in the covering of insects and mol- 

 lusks, their composition is not the same. You 

 remember that you learnt 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, called in latin, test a; and hence 

 the mollusca enclosed in them are called testa- 

 ceous mollusca : the covering of insects is crus- 

 taceous, having the hardness of crust, in latin, 

 crust a. 



Child. But whence is the carbonate of lime, 

 of which shells are composed obtained ? Is it 

 found in the sea ? 



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



