254 THE MECHANICAL FUNCTIONS. 



species of Helix will construct this covering, 

 when circumstances occur to urge its completion. 

 On the approach of winter, the animal prepares 

 itself for passing that season in a state of tor- 

 pidity, first, by choosing a safe retreat ; and 

 next by retiring completely within its shell, and 

 then barricading its entrance by constructing 

 the epiphragma just described, and of which the 

 outer surface is represented in Fig. 119. Having 

 formed this first barrier, the animal afterwards 

 constructs a second, of a membranous nature, 

 situated more internally than the first, and at a 

 little distance from it. If at any other season, 

 while the snail is in full vigour, the experiment 

 be made of surrounding it with a freezing mix- 

 ture, it will immediately set about constructing 

 a covering for its protection against the cold ; 

 and 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 se- 

 creting a small quantity of a mucous fluid, which 

 loosens the adhesion that had taken place be- 

 tween the epiphragma and the sides of the aper- 

 ture ; and the former is, by the pressure of the 

 foot of the snail, thrown off. The whole of this 



* Gray, Zoological Journal, i. 214. 



