Factors Determining the Annual Distribution. 363 



of these broods is followed by a marked rise in number. As the 

 lake cools and freezes, reproduction still goes on, though more 

 slowly than at the earlier date and more slowly than in Cyclops. 

 Yet, during the winter of 1895-6, when Daphnia pulicaria was 

 abundant, it was always possible to find females bearing in the 

 brood-sac eggs in various stages of development. Active repro- 

 duction begins again in the spring, as soon as the ice has dis- 

 appeared. The temperature of the water rises so rapidly and 

 uniformly at this season that it is impossible to state the opti- 

 mum temperature, but the large spring broods were produced 

 shortly after May 1st, when the lake had reached a temperature 

 somewhat over 12° C. The maximum number of the species was 

 found about the middle of May, at a time when the maximum 

 rate of reproduction was past. Males appeared in the latter 

 part of May, and the ephippia were ripe early in June and were 

 deposited before the middle of that month. After this date the 

 species rapidly declines, but lingers for a time in the cool bot- 

 tom water of the lake. The numbers become so few in late July 

 and in August that no fair average can be given. They did not 

 entirely disappear, however, in 1896, as they did in 1894, and 

 it was always possible to find a few individuals in each catch 

 by careful search. 



This species is confined to the cool water of the lake during 

 the warm season of the year. In plankton-poor lakes it occupies 

 the whole region below the thermocline. In lake Mendota this 

 region is not inhabitable except at the very top, and the species 

 is confined to the narrow zone which includes the thermocline. 

 It is probable that this unfavorable influence on the life of the 

 species is the cause of its disappearance or great reduction in 

 number during the warm season of the year. 



The relations of Chydorus to temperature are less definite 

 than those of the regular plankton Crustacea. I have already 

 said that Chydorus is a littoral form, which occupies the lim- 

 netic region only under favorable conditions. These seem to be 

 rather determined by food than by temperature. The active 

 life of the species, however, lies from the first of June to the 

 vlast of October, and the maxima may fall at almost any time 

 within these limits. In 1894 the species was practically absent 



