CONSTRUCTION OF SHELLS. 3 



again, the land. Their instincts in general appear superior 

 to those of their relatives,, the undefended mollusca ; and 

 with regard to every other class of organized beings, there 

 subsists, besides several points of resemblance, a general 

 relation of a very peculiar kind. This is the relation of 

 inversion, the law of contrariety ; for as in other animals 

 the bones to which the muscles are attached lie within the 

 the body, in moUusks they are placed externally. The tes- 

 taceous coating forms an outer protective skeleton, and 

 furnishes to the muscles that fixed basis, without which, 

 mechanically, they could not act. All this most strikingly 

 evinces a pursuance of the same plan. 



But how, it may be asked, are the sliells of the mollusca 

 constructed ; and what arc their component parts ? Shells 

 may be regarded as epidermal in their character, being 

 formed upon the surface of a filmy cloak-like organ called a 

 mantle, and which answers to the true skin of other animals. 

 A slimy juice, consisting of a membranaceous tissue, conso- 

 lidated by an admixture of carbonate of lime, exudes from 

 the glands of this important organ, and thickening in suc- 

 cessive layers becomes hardened and moulded on the body ; 

 at first simple and unadorned, but subsequently embellished 

 according to the taste or inclination of the occupant. 



