ZS THE EPIPHRAGM chap. 



tion or aestivation, or even during shorter periods of inactivity 

 and retirement, the object being, either to check evaporation of 

 the moisture of the body, or to secure the animal against the 

 cold by retaining a thin layer of slightly warm air immediately 

 within the aperture of the shell. 



The epiphragm differs widely in character in different spe- 

 cies, sometimes (^Clausilia, Pupa^ Planorhis^ consisting of the 

 merest pellicle of transparent membrane, while at others (^Helix 

 aperta^ H. pomatia) it is a thick chalky substance, with a con- 

 siderable admixture of carbonate of lime, with the consistency 

 of a hardened layer of plaster of Paris. Within these extremes 

 every variety of thickness, solidity, and transparency occurs. 

 During long hibernation several epiphragms are not unfre- 

 quently formed by the same individual snail, one within the 

 other, at gradually lessening distances. The epiphragm thus 

 performs, to a certain extent, the part of an operculum, but it 

 must be remembered that it differs radically from an operculum 

 physiologically, in being only a temporary secretion, while the 

 operculum is actually a living part of the animal. 



The actual mode of formation of the epiphragm would seem 

 to differ in different species. According to Fischer,^ the mol- 

 lusc withdraws into its shell, completely blocking all passage of 

 air into the interior, and closing the pulmonary orifice. Then, 

 from the middle part of the foot, which is held exactly at the 

 same plane as the aperture, is slowly secreted a transparent 

 pellicle, which gradually thickens, and in certain species be- 

 comes calcareous. Dr. Binney, who kept a large number of 

 Helix hortensis in confinement, had frequently an opportunity 

 of noticing the manner in which the epiphragm was formed.^ 

 The aperture of the shell being upward, and the collar of the 

 animal having been brought to a level with it, a quantity of 

 gelatinous matter is thrown out [? where from]. The pulmo- 

 nary orifice is then opened, and a portion of the air within 

 suddenly ejected, with such force as to separate the viscid mat- 

 ter from the collar, and to project it, like a bubble of air, from 

 the aperture. The animal then quickly withdraws farther into 

 the shell, iuid the pressure of the external air forces back the 

 vesicle to a level with the aperture, when it hardens and forms 



1 Journ. de Conch, iv. p. 397, but the species observed is not mentioned. 



2 Bull. Mus. C. Z. Harv. iv. p. 378. 



