106 FORMATION OF SHELLS. 



to the body of the animal, or rather a continuation 

 of the tendinous fibres that form the ligaments^ by means 

 of which it is fixed to the shell. They also show, that 

 the shell itself owes its hardness to the earthy particles 

 perspired through the vessels of the animal, which gra- 

 dually incrust the meshes formed by the filaments of 

 which this membranaceous substance is composed. 



The eminent naturalist, Reaumur, established two 

 facts in reference to the mollusca ; namely, that the 

 growth of a shell is simply the result of successive 

 additions made to its surface ; and also, that the mate- 

 rials forming each layer, so added, are furnished by the 

 organized fleshy substance, which he termed the skin 

 of the animal, but which is now known by the name 

 of the mantle ; and not by any vessels or other kind 

 of organization belonging to the shell itself. 



To illustrate the process of forming a shell : — If the 

 portion of the shell of a living snail be removed, which 

 can be done without injury to the animal, since it ad- 

 heres to the flesh only in one point, there is formed, 

 in the course of twenty-four hours, a fine pellicle, re- 

 sembling a spider's web, which is extended across the 

 vacant space, and becomes the first stratum of the new 

 shell. This web is found, in a few days, to have in- 

 creased in thickness, by the addition of other layers 



