42 JEAN DAWSON 



weeds were present. This pond was found to be subject to 

 drought. Evidently Lymnaea, through its abihty to form an 

 epiphragm, survived while other species were not so favored. 



The current belief is that snails can withstand drought by 

 burrowing into the ground when a pond dries up. In many 

 of the swamps that were dry when examined, most of 

 the snails were dead, hundreds of dead snails lay upon the 

 moss and ground. Neither dead nor living shells were found 

 buried. 



In ponds and swamps subject to severe drought, the forma- 

 tion of an epiphragm and the sealing in clay are the only effectual 

 means of the continuance of snail life. Small bodies of water 

 reach in time, the stage where severe droughts may occur yearly, 

 and snails in order to live must be able to form an epi- 

 phragm or the substratum must contain more or less clay. 

 Physa, therefore, has but little chance in a pond subject to 

 drought unless there is more or less clay, but Lymnaea has a 

 good chance whether peat, sand or clay prevails. Physa inhabit- 

 ing swamp waters differ greatly in appearance from the same 

 species found in the open pond. The difference is so great that 

 Mr. Bryant Walker identified the specimens at once as swamp 

 forms without a previous knowledge of the locality from which 

 they were taken. 



Lewis (1855, p. 120) remarks upon the variation of Lymnaea, 

 due he thinks to such conditions as locality and temperature. 

 " I have found that all these shells which abound equally in 

 the Erie Canal and the Mohawk River at this place, have local 

 peculiarities, by which I can determine at a glance, almost, 

 which of the two localities a certain number of shells may have 

 been taken from." 



The most formidable condition for Physa in small bodies of 

 water may be said to be too many pond weeds and too much 

 decaying debris. While drought may be said to be disastrous 

 to Physa in bodies of water that have a loose substratum, and 

 dry up completely, yet it is essential for a pond optimum 

 that it is sufficiently severe to kill the pond weeds but not the 

 snail. These conditions which enable Physa to withstand drought 

 are seen to furnish the basis for the popular opinion that it 

 prefers ponds and ditches that dry up. .. J 



