62 MICHIGAN SURVEY. 1908. 



hardly account for 60 of the smaller ones, however, appearing in so short 

 a space of time. It may be taken as indicating a general and continued 

 migration in all directions within their bathytropic limits. 



It was noticeable that the large Limnaea emarginata and Limnaea 

 stagnalis, aside from the one specimen mentioned above, live at an aver- 

 age depth of 3 dm. and never deeper than 4.5 or 5 dm. To test their bathy- 

 tropism six of them were picked out of the deeper water by hand and 

 held in contact with the bottom in the shallow zone until they extended 

 their feet and attached themselves. At this time the water was very 

 quiet, moving just enough to cause a faint sound on the beach. But the 

 size of the shell of the two Limnacas is so large that they offer consider 

 able surface to the water and are consequently easily washed loose. Two 

 of the six swung a little from side to side and were then washed off and 

 carried by the undertow into water 3 dm. deep, where they again at- 

 tached themselves. A third, without being shaken by the waves, clun."^ 

 to the rock for some time, then suddenly let go its hold and drifted over 

 a low ledge into the deeper water. Two others immediately started to 

 crawl down the slope, and one in about fifteen minutes, the other in 

 about half an hour, had crawled over the ledge into water 3 dm. deep, 

 where they both remained stationary. The sixth remained attached, and 

 in three hours had crawled 2 dm. parallel to the shore, keeping at the 

 same depth. The next morning, twelve hours later, it had disappeared, 

 and of course could not be recognized in the deeper water. 



On August 4 two shells of Limnaea appeared in the shallow zone, but 

 it is not known whether they drifted or crawled up. They were there 

 at least three hours. After they were last observed a fresh breeze 

 sprang up from the east and the slight wave action caused by it prob- 

 ably washed them down. 



The level of the lake varies somewhat with the direction and inten- 

 sity of the wind, so that in front of the camp a strip of beach up to 

 5 dm. in width may or may not be covered with water. The smaller 

 shells, Physa sayii Tapp. and Physa sp., live in this zone in spite of the 

 fact that they are sometimes out of water. So far as observed they are 

 never exposed for any considerable length of time, so that. they do not 

 become dry. Then again the weathering of the rock has left bowl-shaped 

 hollows a centimeter or so across and about the same depth, and the 

 snails usually get into them. 



To summarize, the known facts bearing on the distribution of these 

 four species are as follows: 



1. Their lower limit is 4.5 to 5 dm. depth of water, governed possibly 

 by the water-pressure or the food supply. 



2. The upper limit is for Limnaea stagnalis and Limnaea emarginata 

 1.5 dm. of water, for Physa sayii and Physa sp. the shore-line. The 

 cleaner the rock and the less the wave action the shallower the water 

 which they may inhabit. 



3. Their horizontal distribution is controlled by (a) full exposure 

 to the sun; (b) a rock bottom; (c) a certain minimum of wave action. 



But two species of insects were collected which should properly be 

 considered here, caddice flies and stone flies. The larva cases of the 

 caddice flies were collected only in the outlet of a small stream 

 emptying into Rock Harbor, in, 1 — 1.5 m. of water (No. 163 or 



