2 8 THE EPIPHRAGM 



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 species, 

 sometimes (Clausilia, Pupa, Flanorhis) consisting of the merest 

 pellicle of transparent membrane, while at others (Helix apcrta, 

 H. ipomatia) it is a thick chalky substance, with a considerable 

 admixture of carbonate of lime, with the consistency of a hard- 

 ened layer of plaster of Paris. Within these extremes every 

 variety of thickness, solidity, and transparency occurs. During 

 long hibernation several epiphragms are not unfrequently 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 rischer,^ the mollusc 

 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 becomes cal- 

 careous. 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 pulmonary orifice is then 

 opened, and a portion of the air within suddenly ejected, with 

 such force as to separate the viscid matter from the collar, and 

 to project it, like a bubble of air, from the aperture. The animal 

 then quickly withdraws farther into the shell, and the pressure 

 of the external air forces back the vesicle to a level with the 

 aperture, when it hardens and forms the epiphragm. In some 



^ Journ. dr. Conch, iv. p. 397; but the species observed is not mentioned. 

 - BiUl. Mils. C. Z. Harv. iv. p. 378. 



