SHELLS. 27 



then proportionably small ; but as the animal 

 increases in size, it adds to the dimensions of 

 its shells by additions made at the opening, and 

 to its thickness by a succession of layers de- 

 posited within. 



Child. Do the mollusks always construct 

 their shells of the proper shape ? 



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 shall excite your 

 admiration of the little animal, when I tell you 

 that he not only constructs, but also adorns his 

 habitation. 



Child. But whence can he procure such 

 beautiful colours ? 



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 

 before it is hardened, form its diversified tints. 



Child. It seems very wonderful that so 

 many shells should have the same pattern. 



Teacher. It is indeed most wonderful. I can 

 tell you however how it is supposed to be effected . 

 The pores containing the colouring matter are 



