BIOLOGY OF PHYSA 23 



them to live in large numbers in the lake waters proper. Although 

 the beach of Lake Michigan at Frankfort offers no foothold 

 for Physa there is no reason to doubt that in other regions of 

 the lake there are pools and swamps in which the snail can live, 

 similar to those found upon Crystal Lake beach. These pools 

 are very likely to occur at a definite distance from the water's 

 edge since they must depend almost entirely upon extra high 

 waves, springs and rain for a water supply, but at the same 

 time must be out of the reach of the severe sweep of the waves. 

 Thus they will, through the future history of the lake, follow 

 down the beach as its waters diminish in volume, always tend- 

 ing to keep at a- certain definite distance from its edge. 



Ruthven (1905, p. 24) cites beach pool habitats of very similar 

 nature but very different origin in his study of a Lake Superior 

 beach. " There are numerous pools on this beach, in the angular 

 spaces formed by the removal, presumably by ice, of portions of 

 rocks between the joint planes. These pools are, as a rule, above 

 the reach of the highest waves, so that during the summer they 

 are not flooded by the cold water of the lake. Their tempera- 

 ture is thus higher than the lake water. In several readings 

 taken Aug. 6, 7 and 8, the temperature of the water in these 

 pools varied from 60° F. to 68°, which was 10° higher than the 

 lake water as it broke on the lower beach. The vegetation in 

 these pools is very scanty, consisting chiefly of algae which 

 line the sides. The small number of species is probably due to 

 their isolation, shallowness (6-18 inches), and to their smooth 

 sides which make it difficult for the plants to gain a foothold." 



The snails found in these pools were Physa ancillaria, Lym- 

 naea desidiosa, Planorhis parvus and occasionally Lymnaea 

 decollata. It is clear that if these habitats do not receive their 

 water supply from the lake, except in the time of severe storm, 

 their duration is likely to be relatively short. Since these pools 

 are entirely unshaded they are certain to become dry in seasons 

 of drought and the snails will perish. But on the other hand, 

 there will doubtless be found other pools lower down the beach 

 as the waters of the lake diminish, and they like the pools in 

 the sand above mentioned, will follow the lake in its life history 

 always keeping a definite distance from its waters. This kind 

 of habitat will cease when the character of the beach changes 

 by erosion, sinking, or elevation. 



