50 JEAN DAWSON 



and mantle could be seen ; and gradually they retreated farther 

 into the shell leaving a hardened curtain of mucus stretched 

 across the aperture. 



It was observed that all species of snails did not form an 

 epiphragm. Some had formed entire epiphragms, some nearly 

 perfect ones, and others had mere pieces of the dried mucus 

 sticking to the edge of the aperture. Most of the shells were 

 sealed to the glass wherever any part of the shell about the 

 aperture came in contact with it. Physa was rarely observed 

 to form an entire epiphragm and it was equally rare to find 

 Lymnaea palustris not doing so. Pilsbry (1896, p. 96). 



Probably the physical properties of the mucus, which may 

 vary with the individual and the species, together with the 

 size and shape of the aperture of the shell, determines in a large 

 degree whether or not an epiphragm is formed. It has already 

 been pointed out that with the aid of its epiphragm a snail 

 may withstand quite a severe drought, even when the aperture 

 of its shell is not sealed in clay. Of course, an entire epiphragm 

 is of great value, and there is no doubt that it was this that 

 enabled the snail Lymnaea palustris mentioned in the descrip- 

 tion of Swamp A\ (p. 34) to survive its nineteen dead mates, when 

 they were left on the top of a large stone by the retreat of 

 the water. 



The ease with which some of the Lymnaeid<s form an epi- 

 phragm accounts for the occurrence of this species in some small 

 ponds and ditches to the exclusion of other species. Yet in 

 seasons of extreme drought few of these snails succeed in with- 

 standing the severe conditions. Possibly the survivors are 

 those whose epiphragms most securely sealed them in their 

 shells. This presumably accounts for their scarcity in ponds 

 in years following a severe drought. A snail whose aperture 

 is not wholly sealed may withstand adverse conditions but 

 only those hermetically sealed can withstand a more severe 

 drought. This same economic use of mucus is cited by Poulton 

 (vol. 34 of Nature, p. 617) for slugs. He placed a number 

 of slugs in a box with some earth, forgot about them and left 

 them in his study for several months in the hottest weather. 

 He thought, of course, to find them dead and was surprised 

 to find the majority living encysted in mucous capsules. Some 

 had died immediately but the rest were curled up and living. 



