BIOLOGY OF PHYSA 11 



this is that the snails are all young. This condition is found 

 in a creek which drains Steere's Swamp, in Honey Creek, about 

 five miles west of Ann Arbor, and also in the Railroad Creek. 



3. Occasionally in the bottom of Honey Creek there are found 

 small banks of Elodea over which flow, about twelve or eighteen 

 inches of quite rapid water. Upon these banks live a small 

 number of small young Physa gyrina hildrethiana. 



4. In the background of Fig. 3 is a portion of a deserted 

 creek channel, and w^here this joins the present stream bed, 

 there is back water from the creek for a distance of 8 to 10 feet. 

 Here Char a and Spirogyra grow in abundance. The water 

 stands five to eight inches above the water weeds where it joins 

 the creek, but a few^ rods back there is less and less water until 

 the weeds project through its surface. Snails are found in this 

 habitat in numbers except where there is but little water above 

 the mass of weeds. 



5 . A creek on the west side of Lima Center furnishes a good 

 example of an ox-bow pool habitat. Fig. 6. This habitat is 

 becoming captured by water weeds. Solid masses of weeds 

 grow in patches here and there, and in these masses no snails 

 are found, but in the less obstructed portions Physa gyrina, 

 Physa anatina Lea, and Lymnaea palustris are found in about 

 equal abundance. 



3. River as a Habitat. The Huron, the largest stream 

 at Ann Arbor, was studied in detail. It is relatively small, 

 being about 150 feet in width and two to six feet in depth above 

 the dam at Ann Arbor. Its flood plain is bounded on both 

 sides for miles along its course by high hills. Above Ypsilanti 

 the river meanders from side to side forming bayous and ox-bow 

 ponds. It is actively cutting and carries a load of silt. The 

 boulders and heaps of coarse gravel deposited here and 

 there in its bed tell of greater work during the spring freshets 

 when it floods its banks and flows swiftly at twice its normal 

 width. In waters like this, Physa must be well dispersed, yet 

 the only place I found it living or dead was among some water 

 weeds in the slack water above a dam. These snails were 

 all young and were living upon the weeds and not upon the 

 bottom. The following are examples of habitats found in waters 

 cut oft' from the river. 



I. The Huron river above Ypsilanti ofters an example of a 



