184 THE MECHANICAL FUNCTIONS. 



sioii that had taken place between the epiphragma and the 

 sides of the aperture; and the former is, by the pressure of 

 the foot of the snail, thrown off. The whole of this 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 animal. 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 augnlentation of bulk 

 taking place principally at the outer margin of each valve, 

 and corresponding 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 frequent- 

 ly appear of different hues from the addition of colouring 

 particles secreted 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 

 exceedingly flexible, accommodates itself to all the inequali- 

 ties it meets with, and depositing each successive layer of 

 shell equally on every part, the figure of the surface is as- 

 sumed, not only by the valve in contact with it, but also by 

 the other valve, which is formed by the opposite surface of 

 the mantle,J and which during its formation was immediate- 

 ly superposed on the thin edge of the other valve, while it 

 was deflected by the irregular surface on which it grew. 

 As the enlargement of the shell proceeds, it was necessary 

 that the muscle, which closes the valves, and is attached to 



• An epiphragma differs from true shells in having no adhesion in any part 

 to the animal which formed it. 



■f A remarkable instance of this apparent reviviscence of snails, which had 

 lain for many years i)i a dormant state in a cabinet of shells, and whicli 

 crawled out on being accidentally put into warm water, is recorded in the 

 Philosophical Transactions for 1774, p. 432. 



4: Defrance, Annales des Sciences Naturellcs, ii. 16. 



