SHELLS. 31 



fits the members of their bodies, and has more 

 of the nature of a skin. The substance also 

 differs ; that of shells is a carbonate of lime, with 

 a small portion of an animal substance called 

 gelatine, and is termed testaceous ; the case of 

 insects is a phosphate of lime, and is called 

 crustaceans. The carbonate of lime, of which 

 the shells are formed, is secreted in the glands 

 of the mantle, and oozing out, takes the form of 

 the animal, and gradually hardens. When the 

 mollusk is small, the shell is proportionably so ; 

 but as the animal increases, it adds to the dimen- 

 sions of its abode by additions at the opening, 

 and to its thickness by layers from within. The 

 colour and markings with which the shells are 

 diversified, are produced in the pores of the 

 mantle, and are there arranged in the same pat- , 

 tern as that which appears on the shell. Shells 

 are either terrestrial, tiuviatiie, or marine. 



LESSON VI. 



Teacher. Let us now consider in what situa- 

 tions shells are placed. They are, as you know, 

 exposed to the dashing of the waves, borne by 

 the violence of storms against rocks ; and car- 

 ried down rapid rivers. You can readily imagine 



