2*28 THE MECHANICAL FUNCTIONS. 



it works with such diligence, that in the course of 

 an hour or two, it will have completed its task, and 

 formed an entire epiphragma. When the genial 

 warmth of returning spring has penetrated into 

 the abode of the snail, the animal prepares for 

 emerging from its prison, by secreting a small quan- 

 tity of a mucous fluid, which loosens the adhesion 

 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 re- 

 newed, 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.! 



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 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 

 frequently appear of different hues from the addi- 

 tion of colouring particles secreted at particular 

 periods by the mantle. 



* Gray, ibid. p. 214. 



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. 



