174 THE MECHANICAL FUNCTIONS. 



effects the removal of large portions of its shell, when they 

 interfere with its own growth, or are otherwise productive 

 of inconvenience. We should at the same time regard these 

 cases in the light of exceptions to the ordinary rule, that a 

 portion of shell once formed remains ever after unchanged, 

 while it continues to be connected with the animal which 

 produced it. In a general way, indeed, we may consider 

 the connexion between the animal and the shell as m.echani- 

 cal, rather than vital; and the shell itself as an extraneous 

 inorganic body, formang no part of the living system: for 

 whatever share of vitality 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 worm.s are never filled up, nor the apertures of 

 the cavities so made covered over, unless the living flesh of 

 the animal be wounded; in which case an exudation of cal- 

 careous matter takes place, and a pearly deposite is produced. 

 The worn edges of shells, and the fractures, and other ac- 

 cidents 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 impregnated 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 surface 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 underneath the epidermis,* that is, between this 

 outermost layer of integument, and the subjacent corium, 

 which is incorporated with the mantle, and may be regarded 

 as forming one and the same organ. t 



* 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. 



■{"A secreting power is also, in some instances, possessed by the foot, as is 

 exemplified in some of the gasteropoda, where it forms an operculum, or 



