222 THE MECHANICAL FUNCTIONS. 



the real steps by which these changes have taken 

 place.* 



Changes equally remarkable are observed to 

 occur in the interior of the shell at diflferent stages 

 of its growth. On the inner surface of the 3Iilra, 

 the Volute, and other shells of a similar kind, there 

 is deposited a layer of a hard semi-transparent cal- 

 careous material, having a vitreous appearance. t 

 The thickness of the layer, which thus lines the 

 cavity of the shell, is greater as it approaches the 

 apex ; and where the spire is much elongated, or 

 fnrritecf, as it is called, \ this deposit entirely fills 

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

 shell, was a hollow space with thin sides. The 

 purpose answered by this deposit is evidently to 

 give solidity and strength to a part which, by re- 

 maining in its original state, would have been ex- 

 tremely liable to be broken off by the action of the 

 sea. 



In other cases a different expedient is adopted. 

 The animal, instead of fortifying the interior of the 

 apex by a lining of hard shell, suddenly withdraws 

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

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

 face thus withdrawn. That portion of the shell, 



* According to Bruguiere, there is reason to believe that tlie ani- 

 mal of the Cyprcea, after having completed its shell, in the manner 

 above described, still continuing to grovi', and being incommoded 

 for want of space, quits its shell altogether, and sets about forming 

 a new one, better suited to its enlarged dimensions. It is stated, 

 also, that the same individual is even capable of forming in succes- 

 sion several shells. De Blainville, however, considers it impossible 

 that the living animal can ever quit its shell. Malacologie, p. 94. 



t This is the substance represented at d, Fig. 107, p. 210. 



I As in the genera Turritella, Terebra, Cerithium, and Fas- 

 ciolaria. 



