BIOLOGY OF PHYSA 37 



around the aperture. It is therefore concluded that it is by 

 chance if the snail is hermetically sealed and that this is due 

 to the position the snail happened to be in when the clay grew 

 so hard that movement was no longer possible. 



The farmer who owned the land said that the swamp had been 

 completely dry since earl}^ June. I have no reason to doubt 

 this general statement as there were no signs whatever of young 

 snails and probably the water dried up before they could de- 

 velop. No other snails were found living. The specimens of 

 Physa gyrina hildrethiana were not sealed; the bivalves could 

 not bury themselves in such stiff clay and both species were 

 dead. 



Swamp C I Swamp C^ is a few rods away from Swamp C^ 

 in the same hayfield and represents the next stage in drying 

 up of Swamp C'. It has the same clay basin but is shallower 

 and has logs and sedges in it. Careful search revealed but a 

 few fragments of decaying shells which serve to enable a pro- 

 phecy to be made concerning the fate of snail life in Swamp C^ 



Swamp C ^ This swamp is very much like Swamp B\ in its 

 physical makeup, but had no mollusca except Sphaerium which 

 were buried in the peat. Fragments of shell were found but no 

 whole ones. This indicates that none of the snails had suc- 

 ceeded in surviving some severe drought during the past few 

 years. Another swamp was found that gave the next stage in 

 their life history series, namely, one that had become too dry 

 even for the bivalves whose decaying shells in the dry peat 

 told the story of a long, severe drought. 



Conclusion Concerning Desiccation. The following snails have 

 only a slight burrowing reaction if any at all: Physa gyrina 

 hildrethiana, Physa elliptica, Planorbis parvus Say, Lymnaea palus- 

 tris, and Lymnaea desidiosa. Baker (191 1) p. 39. Sphaerium bur- 

 rows in the softer substratum but cannot in the stiff clay. The 

 Lymnaea forms a heavy mucous epiphragm and in consequence has 

 in any case a better chance of living through drought, which prob- 

 ably accounts for their presence in many waters to the exclusion 

 of other snails. It was observed that all the above named snails 

 lived for a time upon almost any kind of substratum providing 

 it was moist. All may live for a few months when their apertures 

 are turned down and buried in the clay, even though they do not 

 form an epiphragm. Even in the clay, Lymnaea has the advan- 



