240 THE MECHANICAL FUNCTIONS. 



no part of the living system : for whatever share 

 of vitahty it may have possessed at the moment 

 of its deposition, all trace of that property is 

 soon lost. Accordingly we find that the holes 

 made in shells by parasitic worms are never 

 filled up, nor tlie apertures of the cavities so 

 made covered over, unless the living flesh of the 

 animal be wounded ; in which case an exuda- 

 tion of calcareous matter takes place, and a 

 pearly deposit is produced. The worn edges of 

 shells, and the fractures, and other accidents 

 which befall them, are never repaired, except as 

 far as such repairs can be made by the addition 

 of materials from the secreting surfaces of the 

 mantle. It is found that shells may be impreg- 

 nated with poisonous metallic salts, such as those 

 of copper, without any detriment to the animals 

 they enclose. 



The power of secreting the materials of shell 

 does not usually extend to the whole of the sur- 

 face of the mantle, but is generally confined to 

 the parts near the margin, composing what is 

 termed the collar. The calcareous substance is 

 always poured out imderneath the epidermis,* 

 that is, between this outermost layer of integu- 



* Mr. Gray considers the external membrane of the shell, or 

 epidermis, as formed by the outer edge of the plates of animal 

 substance, which have scarcely any calcareous matter in their 

 composition, and which are soldered together into a membranous 

 coat. 



