2 JEAN DAWSON 



I. Introduction. 



The following ecological study of the snail, Physa, was made 

 principally in the region of Ann Arbor, Michigan. A large 

 number of streams and bodies of water were examined and 

 the following species were studied: Physa gyrina Say, Physa 

 gyrina hildrethiana Lea, Physa anatina Lea, Physa ancillaria 

 Say, Physa heterostropha Say, Physa elliptica Lea, Physa sayii 

 Tap. and Physa integra Hald. Wherever living snails were 

 found, special attention was given to the conditions in order 

 to analyse the stimuli to which they continually react. 



II. Local Distribution. 



No attempt was made to determine the distribution of Physa 

 in a geographical sense. Their occurrence in local bodies of 

 water is all that was sought. 



The dispersal of snails in bodies of water which have currents 

 and waves is relatively simple when one considers that not 

 only the parent snail itself, but its eggs, attached in masses to 

 weeds and other floating bodies, may be easily transported. 

 Thus the distribution in many lakes, and in river systems, may 

 be readily understood, but the manner of dispersal in certain 

 small isolated bodies of water, by such agents as man, animals 

 and floods alone is in many cases not entirely clear. Further 

 search shows that frequently an explanation may be found 

 in a study of the origin and history of the small bodies of water 

 themselves. A study of the physical processes which influence 

 lakes and rivers show^s that these systems are undergoing 

 continual changes, some of which lead to the formation of 

 small bodies of water which become cut off and isolated 

 from the parent body, or in other cases the lakes themselves 

 may gradually grow smaller and smaller until they become 

 ponds. 



In some cases before these ponds cease to be identified with 

 the parent waters, they may contain much animal life in common 

 which subsequently either dies or lives on in more or less abun- 

 dance, as the resulting environment decrees. While we may 

 account for the presence or absence of the snail in different 

 waters by these means yet they do not give us a clue as to why 



