CH. VIl] SNAILS AND SLUGS 211 



composed of organic and calcareous matter. When the 

 animal is about to form the epiphragm it retires within 

 the shell and exudes from the collar of the mantle a 

 quantity of mucus charged with calcareous matter. With 

 this the mouth of the shell is filled. A small quantity of 

 air is then discharged from the respiratory aperture and 

 separates the film of calcified mucus from the body of the 

 animal. The pressure of the discharged air causes the 

 still flexible epiphragm to bulge outwards. Almost at the 

 same moment the animal retreats further within the shell 

 and the pressure of the outside air forces the epiphragm 

 back, making it flat or even concave 1 . The whole structure 

 then speedily hardens, but remains at all times porous, so 

 that air can diffuse through. It is however insoluble in 

 and but slightly pervious to water. At times several 

 epiphragms are found one within the other as the snail 

 retreats farther and farther into its shell : in such case 

 each succeeding epiphragm is more delicate and less 

 calcareous than its predecessor. 



During periods of drought a "summer epiphragm" 

 is frequently formed. This is a far more delicate structure 

 than that just described, being usually transparent and 

 but feebly calcified : it often exhibits a white calcareous 

 spot opposite the respiratory aperture, and this spot is 

 usually perforated. Probably a small aperture for re- 

 spiration exists in all epiphragms, both "winter" and 

 "summer." Some species of fresh-water snails, notably 

 of the genus Planorbis, close the shell-mouth by a firm 

 white epiphragm when the streams and pools in which 



1 Binney, Bull. Mas. C. Z. Harv. iv. 



142 



