EPIPHRA.&M OP HELIX ASPEESA. 519 



Close to the rim of the mantle, on the right side of the 

 animal, the disc is perforated by the respiratory orifice (o.r.) 

 leading directely into the respiratory chamber (c.r.). This orifice 

 is connected with the central aperture of the disc by a fissure 

 (fig. III.), which, like the central aperture, can be closed by the 

 approximation of its edges. 



When the epiphragm is about to be formed, the foot and head 

 of the snail are much contracted and entirely withdrawn deep 

 into the shell through the central opening in the disc, which 

 is then completely closed, leaving an even continuous surface for 

 the secretion of the epiphragm. 



Immediately over the site of the respiratory orifice the 

 epiphragm is perforated by a small aperture which afibrds access 

 from without to the atmospheric air, which even during the 

 period of repose may still be needed for respiration — a function 

 which during the dormancy of the animal is probably not quite 

 arrested *. 



Access to the atmospheric air is also probably connected with 

 the presence of an osphradium or olfactory organ, which may 

 perhaps be recognized in a small patch of modified ectoderm (o, 

 fig. II.) visible close to the edge of the respiratory orifice. 



It may also be noted that the perforation of the epiphragm 

 has an uneven edge, and gives the impression of having been 

 caused by the action of some solvent on the substance of the 

 epiphragm. When we bear in mind that it is in close proximity 

 with the respiratory orifice, that exit is given to the renal 

 secretion as well as to the contents of the alimentary canal 

 after this has received the secretion of the digestive gland (so- 

 called liver), we shall perhaps deem it not improbable that some 

 of these secretions have acted as a solvent on the epiphragm, 

 the orifice of which is situated exactly in the position best fitted 

 to bring it within reach of their action. 



When the conditions which call for the formation of an epi- 

 phragm are present, the snail seeks for some surface to which it 

 may apply the aperture of its shell in such a way as to exclude 

 the free access of the external air. This may be the shell of 



* I have never met with Helicc aspersa in a state of hybernation in which the 

 perforation of the epiphragm was not present ; and yet I can find no pubhshed 

 account of it. In conversation, however, with Col. Godwin-Austen, whose 

 researches among the terrestrial Gastropods have contributed so largly to our 

 knowledge of these animals, I found that its existence was well known to him. 



