Marsh — Limnetic Crustacea of Green Lake. 



197 



tion of this, as I hope in a later paper to treat more fully upon 

 its life history after further researches. 



So far as I know there have been no preceding observations 

 on the seasonal distribution of Epischura. Its nearest European 

 relative is Heterocope, and this is stated by Apstein to occur 

 from the latter part of Juty into November, its maximum period 

 being in the summer. He does not record any time of the ap- 

 pearance of the larval forms. 



In its vertical distribution, Epischura is largely confined to 

 the upper regions. While laboratory experiments would seem 

 to indicate that it avoids bright light, the averages of my col- 

 lections apparently show that it is more largely controlled by 

 the conditions of temperature. In my collections of August, 

 1893, I found 81 per cent, in the upper ten meters. The average 

 of the collections of 1894, extending from the latter part of Sep- 

 tember to the last of November was 53.11 percent, in the upper 

 five meters and 19.52 per cent, from five to ten meters, thus 

 making 72. 63 per cent in the upper ten meters. In order to com- 

 pare the distribution at different seasons, I computed the aver- 

 age percentages in the collections from the surface to five meters, 

 and from five meters to ten meters for June, July and August, 

 1896, and from November, 1894 to April, 1895, with the following 

 results : 





0-5 



5-10 



0-10 



Winter, 24.94-3.95 



42.53 

 60.55 



14.18 

 28.51 



56.71 



Summer, 7.96-17.96 



89.06 







This would seem to indicate that Epischura prefers the warmer 

 water, although it is by no means absent from the cold water of 

 the surface in the cold season. It occurred to me that if Epischura 

 were, to a large extent, controlled in its vertical distribution 

 by conditions of temperature, there might be a diurnal migra- 

 tion caused by the cooling of the surface water at night, for the 

 surface responds quickly to changes in atmospheric temperature. 

 To determine whether any such effect would be produced, I com- 



