214 THE MECHANICAL FUNCTIONS. 



merely agglutinated together like the component 

 leaves of a sheet of pasteboard. 



It has long been the prevailing opinion among 

 naturalists that no portion of a shell which has 

 been once deposited, and has become consolidated, 

 is capable of afterwards undergoing any altera- 

 tion by the powers of the animal that formed 

 it. Very conclusive evidence has, in my opinion, 

 been adduced against the truth of this theory, by 

 Mr. Gray.* From a variety of facts, it appears 

 certain that on some occasions the molluscous 

 animal 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 un- 

 changed, 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 mechanical, rather than 

 vital ; and the shell itself as an extraneous inor- 

 ganic body, forming 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 worms 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 deposit is 

 produced. The worn edges of shells, and the frac- 

 tures, and other accidents which befall them, are 



* Philos. Transactions for 1833, p, 796, et seq. 



