CHAP. X 



MOLLUSC A 



127 



The structure of the shell, and the mode of growth, are 

 essentially similar to that of Limnaea and of all other snails. 

 The natural length of life of land snails seems somewhat 

 greater than that of pond snails ; Helix aspersa lives five or 

 six years, though, as a rule, not more than two or three 

 annual lines of growth are to be seen on the shell, possibly 

 for the reason that growth ceases in the second or third 

 year of life. 



Land snails have no true operculum ; before they hiber- 

 nate, they retire to some sheltered corner, often burrowing 

 into the loose soil ; then the mouth of the shell is closed by 



FIG. 80. The Common Garden Snail (Helix aspersa). 



a thin membranous plate or epiphragm, formed from a 

 discharge of mucus and calcareous matter from the collar 

 or thickened rim of the mantle. This mucus at first fills the 

 mouth of the shell, and is often separated from the body by 

 a discharge of air from the lung, becoming convex at first, 

 but sinking in again, and gradually becoming flat or concave. 

 It hardens into an opaque plate, which, however, remains 

 permeable to air. 



A somewhat similar epiphragm is often formed in the 

 summer during a temporary drought, but in this case it is 

 much thinner, less calcified, and often perforated in the centre 

 of a specially calcified opaque spot opposite the breathing 

 aperture. 



