MICHIGAN ACADEMY OF SCIENCE. 35 



II. Compai-ative Extend of Shallow Littoral Conditions. In most places in 

 Douglas Lake, the shallow littoral zone formed only a narrow shelf fringing 

 the shore, while in the pools and swamps there were no other conditions 

 present. This resulted in two important differential effects on the lake 

 fauna: (1) the free immigration of the deep water forms of the lake, and (2) 

 the fact that the shallow littoral forms were not completely dependent on 

 the water above them for food and air. 



The shelf of the lake was open to the immigration of the deep water forms, 

 and the opposite was also true, and the migrations of the deep littoral forms 

 of group 4 certainly showed to what extent competition between the in- 

 habitants of the different zones was possible. On the other hand, the food 

 and air supply of the shelf was added to constantly by the contributions of 

 the wind-developed currents which brought the plankton and air in from 

 the entire surface of the lake, while the pool and swamps forms were com- 

 pletely dependent on the water above them for these necessities. This 

 lack of aeration by change in the water might have been one of the factors 

 that caused the predominance of air-breathing forms in the pools altho its 

 influences must have ):)een largely counterbalanced by other methods of 

 aeration. It certainly seemed as if the food supply must have had a strong 

 effect in the elimination of the larger species from the latter habitats. It 

 would take a rather large pool, for instance, to supply enough food for a 

 self-reproducing body of clams, and they certainly would not be anywhere 

 near so abundant as in the same area on the shelf. 



///. Constancy in Amount of Water. The lake had practical^ a con- 

 stant water supply, but in the pools the amount of water was subject to 

 great variation, while in the swamps comparative dessication was the dom- 

 inant feature during much of the time. As a result, the pool forms had to 

 be more able to resist change in the amount of water with its attendant 

 dilution and concentration of the mineral content and other substances. 

 In addition, the shells of the swamps needed adaptations for the resistance of 

 drought, and these were found to be present. All of the shells of the swamps 

 were able to live for some time out of water, either burrowed in the mud 

 or up on the water plants, with an epiphragm closing the aperture. 



In the cases of Lijmnaea palustris and Aplexa hypnoruui, the egg-masses 

 must have been able to resist even greater dessication, as the bottoms of 

 pools, which had been dry for some time, were dug up without the discovery 

 of any living shells, and yet the juvenile shells appeared in great numbers 

 during the next rainy season. Specimens of the Sphaerium occidentale were 

 also found, in which the animals were all dead, but the damp decaying 

 bodies contained numbers of living, juvenile specimens, protected within 

 the shells of the viviparous parents. Experimental data, on the abilities 

 of different molluscs and their eggs to resist dessication and stagnancy of 

 the water, would be very interesting and important in this connection. 



IV. Isolation. Lakes are usually connected with other lakes and with 

 large streams by at least temporary streams, and can receive their fauna 

 in this way; Douglas Lake was no exception in this regard (6). The pools 

 and swamps, on the other hand, were usually more completely isolated 

 from other similar habitats. The ponds, it is true, could receive some of 

 their fauna from or thru the parent lake before complete separation from it, 

 but it was often very difficult to find signs of how this would be accomplished; 

 the nearest specimens of Plajiorhis trivolvis to those of the Sedge Point Pool 

 were found in the mouth of a brook three miles away, and these latter were 

 totally different in form and appearance. A single specimen of Physa 



