BIOLOGY OF PHYSA 5 



in the order of succession found in passing from the headwaters 

 down stream. By this method the description of the conditions 

 is greatly simpHfied and emphasis may be placed upon the rela- 

 tive influence of the dominant forces which change the envi- 

 ronment. No effort is made to describe any given river from 

 its source to its outlet, but rather to discuss in general the various 

 situations in which Physa occurs in the definite order which 

 the snail presents in going from the source toward the mouth 

 of a river. 



I. Springs and Brooks. Active springs and small brooks 

 resulting from them do not seem to be promising waters for 

 Physa. They throw light upon the problem at hand only through 

 the absence of Physa in them. I have examined many hillside 

 springs and brooks about Ann Arbor as well as those in other 

 parts of the state, and I have seen Physa in such waters but 

 once. This was in a box sunk in the ground on the banks of 

 Chelsea Pond near Chelsea, Mich. Here, apparently feeding upon 

 algae, was found Physa gyrina hildrcthiana Lea. 



a. Brooks. The absence of snails from springs and from such 

 brooks as do not become creeks is probably due to their isolation. 

 The absence of Physa from springs is well illustrated by one which 

 was found flowing down the side of a steep clay bluft' of Lake 

 Michigan at Frankfort. The water flows from an ujjper stratum 

 of the bluff, but is lost before reaching the lake. In no case 

 was Physa found in such waters and indeed it is difficult to 

 see how snails could be carried there. The absence of the 

 snail from brooks is illustrated by those flowing down the 

 morainic hills of Crystal Lake near Frankfort, Mich. One of 

 these rushes down a steep hillside fully 90 feet above the level 

 of the lake and on account of the swiftness of the current, there 

 is little opportunity for the snail to become dispersed into these 

 waters from the pools and shallow waters of the lake below. 



b. Brook-ditches as Habitats. With the tiny brook may be 

 placed the brook-ditch. By a brook-ditch is meant a stream 

 that has had its bed artificially lowered. Its course is fairly 

 straight and its banks steep, presenting uniform conditions 

 on both sides of the stream. The ditch depends largely upon 

 surface drainage for water and in consequence varies greatly 

 in depth and swiftness of current during the year. 



Kavanaugh swamp has many of these ditches presenting 



