SHELLS. 27 



in size, it adds to the dimensions of its shell by 

 additions made at the opening, and to its thick- 

 ness by a succession of layers deposited 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 

 arranged in the skin of the mollusks with the 

 same undeviating regularity as the spots upon the 



