FORMATION OP SHELLS. 



183 



118 



the bivalve tcstacca. A Clausiinn is another kind of co- 

 vering, serving also for protection, and consisting of a thin 

 spiral plate of shell, attached to the columella by an elastic 

 spring, by which the plate is retracted when the animal re- 

 tires into its shell. It thus corresponds exactly in its office 

 to a door, opening and closing the entrance as occasion re- 

 quires. An Epiphragma is a partition 

 of a membranous or calcareous nature, 

 constructed merely for temporary use. 

 It is employed for closing the aperture 

 of the shell during certain periods only, 

 such as the winter season, or a long con- 

 tinued drought. Fig. 118 exhibits the 

 lines which appear on the inner side of 

 the epiphragma, of the Helix pomatia, or 

 garden snail, and which indicates the succession of depositcs 

 by which it has been formed. 



It is remarkable in how short a time this 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 torpidity, 

 first, by choosing a safe retreat; and next by retiring com- 

 pletely within its shell, and then barricading its entrance by 

 constructing the epiphragma just described, and of which 

 the outer surfiice 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 mixture, 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 return- 

 ing spring has penetrated into the abode of the snail, the 

 animal prepares for emerging from ils prison, by secreting 

 a small quantity of a mucous fluid, which loosens the adhc- 



* Gray, Zoolog-lcul Journal, i. 214. 



