1902 Snails hi Wmter 27 



genera have adopted this idea ; perhaps the snails most often 

 found are various Hyalinia.^ There is, too, a simple way of 

 obtaining what appears to be practically the same thing as 

 this subterranean retreat — burrowing under stones which are 

 in contact with the ground. So many snails habitually 

 retire every day to such situations at all times of the year 

 (presumably for the cool damp), that it affords a convenient 

 winter resort. As a matter of fact, one generally finds that 

 snails seem to have burrowed into the ground from under 

 stones ; but whether this is because we generally look for 

 them in such places, or because they burrow from where 

 they find themselves at the onset of cold weather, or because 

 the surface layer of earth is frozen hard elsewhere, is a point 

 that cannot be discussed here. I am not aware that the 

 winter habits of such a subterranean species as the minute 

 Ccecilioides acicula have been worked out ; presumably it 

 has none, since in its natural haunts (or what are currently 

 believed to be such) there is little or no winter. 



Since the power of increasing heat production under the 

 stimulus of cold is practically absent in the Mollusca, the 

 only way in which they can combat the winter is by de- 

 creasing their heat loss. The method by which, in all prob- 

 ability, a snail is liable to lose most heat is by evaporation 

 from the free surface of its body. To escape this unneces- 

 sary loss the body is retracted within the shell, and thus an 

 area is left exposed which is equal only to the aperture of 

 the shell. Moreover, the surface is retracted below the edge 

 of the mouth, whereby it is the less exposed to ventilation. 

 And at least two more devices are often resorted to : the 

 mouth is applied to some flat impervious surface and ce- 

 mented on to this round its edge, and a diaphragm of this 

 cement substance is thrown all across and entirely closes 

 the aperture.^ 



^ One occasionally finds in several species what appear to be attempts at making 

 a sort of cocoon similar to that seen in the "shelled-slug" Testacella. 



'^ I confess that I have never been able to satisfy myself of the actual existence 

 of the "air-hole" in the epiphragm which is often stated to be present. The 

 point, nucleus, or whatever it may be called, which is said to mark the site of this 

 aperture, seems more likely to be the last point with which the foot was in contact, 

 when the epiphragm was not yet of solid consistence. The epiphragm is apparently 

 readily pervious to a slow exchange of gases and aqueous vapour over its whole 

 extent. 



