BIOLOGY OF PHYSA 25 



shallow waters Physa may live. Presumably the island will 

 be ultimately connected with the mainland on both east and 

 west shores, thus dividing the present lake into two smaller 

 lakes. The separation of the lakes will moderate the severity 

 of waves and currents and this will in time alter present shore 

 conditions. It is seen that Physa demands practically the 

 same conditions for a habitat in a lake and pond as those in 

 streams, although they may be formed in very different ways. 



3. Ponds and Swamps. In this paper ponds are considered 

 separately from lakes because although lakes tend eventually 

 to become ponds, not all ponds were originally lakes. In the 

 ■ present study the largest ponds include bodies of water less 

 than about 50 feet in diameter. This includes all the later 

 stages of lake life and might include all small bodies of water 

 cut off from the lake. The pond and swamp waters are, as 

 in the river and lake, aerated by the overlying atmosphere. 

 Whipple and Parker (1902) speak of the process by which water 

 obtains air as breathing. According to the analysis given 

 in their paper, and also those by Hoppe Seyler (1896) the upper 

 layers of water are saturated with oxygen and contain but 

 little carbonic acid, especially away from the cities. This 

 is true for the shallow and for the upper layers of the deeper 

 water. The oxygen decreases and the carbonic acid increases was 

 the deeper layers of the water are reached. Birge ( 1 908) . Whipple 

 and Parker also find that waters that are agitated take oxygen 

 from the air and liberate much of their carbonic acid. The upper 

 layers of some water, especially those that contain water plants, 

 are found often to be super-saturated with oxygen. According 

 to these results it appears that Physa as a rule lives in those 

 parts of lakes and streams that are saturated with oxygen and 

 contain but little carbonic acid. 



There is another element, however, which, according to the 

 authors above cited, tends to lower the oxygen content and 

 increases the carbonic acid in many small bodies of water, namely, 

 the presence of organic debris. In quiet waters, undisturbed 

 by waves and currents, decaying vegetation occurs and 

 takes up the oxygen from the water and liberates carbonic acid. 

 In the small lagoons and behind bars and near the blind end 

 of spits (as described for Crooked Lake) such conditions prevail 

 and it is worthy of note few or no Physa occur there. 



