206 



Marsh — Limnetic Crustacea of Green Lake. 



ference in vertical distribution in cold weather as compared 

 with that in warm weather I averaged the percentages in the 

 upper five divisions from June until September, 1896, — 7.96 to 

 17.96 inclusive, — and from November to April, 1895, — 24.94 to 

 3.95 inclusive, — with the following results: 



7.96 to 17.96 — warm weather 

 24 . 94 to 3 . 95 — cold weather . , 



0-5 



70.80 

 38.47 



5-10 



10.85 

 14.11 



10-15 



14.50 

 11.38 



15-20 



2.17 

 14.51 



20-25 



.48 

 10.17 



It is evident from these figures that there is a marked dif- 

 ference in the vertical distribution in warm and in cold weather. 

 Nearly 71 per cent, in warm weather are in the upper five 

 meters, while the upper fifteen include 96.15 per cent. In cold 

 weather, on the other hand, only 38. 47 per cent, are in the upper 

 five meters, and below that they are somewhat evenly distributed. 



To determine the difference between day and night I averaged 

 the five hauls in October, 1894, which were taken between six 

 p. m. and six a m., and compared them with ten hauls taken 

 in the same month between six a. m. and six p. m. The fol- 

 lowing was the result: 



15-20 



Night hauls 

 Day hauls . . , 



0-5 



5-10 



10-15 



24.57 

 29.27 



13.28 

 19.88 



26.84 

 18.72 



19.72 

 23.58 



It will be seen that the percentages are very similar, and I 

 infer that there is no appreciable diurnal migration. I con- 

 clude from this that they are not very sensitive to changes in 

 the amount of light. I take it, too, that while they are affected 

 by changes of temperature, they are not very sensitive to such 

 changes, or a larger proportion would be found in the warmer 

 deep water in the winter. C. Jluviatilis, in this respect, differs 

 very markedly from Epischura lacustris, which not only has a more 

 pronounced seasonal migration, but moves vertically in accord- 

 ance with diurnal changes of temperature in the surface water 



