Factors Determining the Annual Distribution. 363 
<of these broods is followed by a marked rise in number. As the 
take 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¬ 
on um 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 
ifound about the middle of May, at a time when the maximum 
rate of reproduction was past. Males appeared in the latter 
ipart 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 
tand 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 
Fby careful search. 
This species is confined to the cool water of the lake during 
dhe warm season of the year. In plankton-poor lakes it occupies 
the whole region below the thermocline. In lake Mendota this 
iregion 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 
Hast of October, and the maxima may fall at almost any time 
within these limits. In 1894 the species was practically absent 
