36 LESSON V. 



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, from 

 test a, the Latin for baked earthenware. 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 mollusca is small, the shell is propor- 

 tionably so ; but as the animal increases, it adds 

 to the dimensions 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 pattern as that which appears on the 

 shell. Shells are either terrestrial, fluviatile, or 



LESSON V. 



Teacher. Let us now consider in what situa- 

 tion the mollusca are placed. They are, as you 

 know, exposed to the dashing of the waves, 

 borne by the violence of storms against, rocks, 

 and carried down rapid rivers. You can readily 



