33 LESSON IV. 



that there should be such a variety of shapes m 

 shells ? 



Teacher. The shape of the shell is deter- 

 mined by that of the animal itself. 



Child. But the mollusca do not always con- 

 tinue the same size. 



Teacher. Very true. They are quite minute 

 when they come out of the egg, and the shell is 

 then proportionably small ; but as the animals 

 increase in size, they add to the dimensions of 

 their shells by additions made at the opening, 

 and to its thickness by a succession of layers 

 deposited within. 



Child. Do they always construct their shells 

 of the proper shape 1 



Teacher. Yes, the carpenter with his rule 

 and compass is not so exact and unerring as 

 these heaven- taught builders. But is form the only 

 point that attracts your attention in these shells ? 



Child. Oh no ; they have most beautiful 

 colours and markings ; these cannot belong to 

 the carbonate of lime. 



Teacher. No, and again I must excite your 

 admiration of the little animal, which, while he 

 constructs, also adorns his habitation.. 



Child. But whence can he procure such 

 beautiful colours 1 



Teacher. He is furnished in himself with all 

 that is necessary both for the constructing and 

 beautifying his shell ; his skin you remember is 

 full of pores ; these contain colouring fluids, 

 which, penetrating the calcareous substance be- 

 fore it is hardened, form its diversified tints. 



