SO LESSON IV. 



examine the coverings of the mollusca, are 

 there any parts of other animals that can be 

 compared to shells ? 



Child. The hard coverings of beetles are 

 something like them. 



Teacher. What strikes you as being the 

 principal difference between shells and the case 

 in which some insects are enclosed] 



Child. Shells are distinct from the mollusca, 

 as if they were their houses ; but the coverings of 

 insects fit the different members of their 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 mollusca 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 these two coverings, their 

 composition is not the same. You remember 

 learning in your lessons on lime,* the animal 

 substances of which it forms the principal part. 



Child. Yes, our bones consist principally 

 of carbonate of lirne, 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 



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



