FORMATION OF SHELLS. 255 



process of construction has to be renewed, on every 

 occasion when another covering is required.* 



One great use of these coverings is to prevent 

 evaporation from the surface of the body of the 

 annual. It is thus that Snails, Bulimi, &c. may 

 be preserved for months, and even years in a 

 torpid, but living state, ready to be restored to 

 the active functions of life, when sufficient water 

 is supplied. t 



The enlargement of bivalve shells is conducted 

 on the same principles as that of univalves ; the 

 augmentation of bulk taking place principally at 

 the outer margin of each valve, and correspond- 

 ing with the growth of the included animal. 

 The order of succession in which the layers are 

 deposited is clearly indicated by the lines on the 

 surface, which frequently appear of different 

 hues from the addition of colouring particles se- 

 creted at particular periods by the mantle. 



The shells of Oysters and other acephalous 

 mollusca which adhere to rocks, are often 

 moulded, during their growth, to the surfaces to 

 which they are applied. The mantle, being ex- 



* An epiphragma differs from true shells in having no adhe- 

 sion in any part to the animal which formed it. 



t A remarkable instance of this apparent reviviscence of snails, 

 which had lain for many years in a dormant state in a cabinet 

 of shells, and which crawled out on being accidentally put into 

 warm water, is recorded in the Philosophical Transactions for 

 1774, p. 432. 



