REMARKABLE CHANGES. 117 



the cavity of the shell, is greater as it approaches the 

 apex ; and where the spire is much elongated, or tur- 

 reted, as it is called, this deposition entirely fills the 

 upper part, which, in the early condition of the shell, 

 was a hollow space with thin sides. What then is the 

 purpose answered by this deposit ? Evidently to give 

 solidity and strength to a part which, by remaining in 

 its original state, would have been extremely liable to 

 be broken off by the action of the sea. 



In other cases, an interesting expedient may also be 

 observed. Instead of fortifying the interior of the apex 

 by a lining of hard shell, the animal suddenly with- 

 draws its body from that part, and builds a new wall 

 or partition across the cavity, so as to protect the sur- 

 face thus withdrawn. The portion of the shell thus 

 abandoned, being very thin and brittle, and having no 

 internal support, soon breaks off, leaving what is called 

 a decollated shell. 



The young of one genus, (magilus,) has a very thin 

 shell, of a crystalline texture ; but when it has attained 

 its full size, and has formed for itself a lodgement in a 

 coral, it fills up the cavity of the shell with a glassy 

 deposit, leaving only a small conical space for its body ; 

 and it continues to accumulate layers of this material, 

 so as to maintain its body at a level with the top of 



