120 REMOVAL OF WHORLS. 



other ; but they wholly disappear as the animal ap- 

 proaches maturity. In other cases, the animal removes 

 exterior portions of shell formerly deposited, when they 

 are in the way of its farther growth, and when the 

 mouth of the spire is advancing over the irregular sur- 

 face of the preceding whorls. In other instances, no 

 such power of destroying portions of shell previously 

 deposited appears to exist ; and each successive whorl 

 is moulded on the one which it covers. 



It has long, indeed, been a prevailing opinion among 

 naturalists, that no portion of a shell which has been 

 once deposited, and has become consolidated, can be 

 altered by the power of the animal which formed it. 

 It appears, however, that on some occasions the crea- 

 ture removes large portions of its shell, when they pro- 

 duce inconvenience ; but these cases must be regarded 

 as exceptions to the general rule. The connexion 

 between the animal and the shell is mechanical rather 

 than vital ; and the shell itself must be considered as 

 an extraneous body, forming no part of the living sys- 

 tem. Whatever share of vitality was possessed at the 

 time of its deposition, all trace of that property is soon 

 lost. Thus the holes made in shells by parasitic worms 

 are never filled up, nor even the apertures of the cavi- 

 ties so made covered over, unless the living flesh of 



