362 
Birge—The Crustacea of the Plankton. 
this is due to the increase in the temperature of the water or 
not, I find it difficult to decide. In each year, as will be seen 
by reference to Fig. 16, the number of this species fell off rap¬ 
idly and greatly at the close of the spring reproductive period, 
and this decline was followed by an equally rapid rise. So great 
a fall, followed by so great a reaction can hardly be attributed 
to the progressive rise of the temperature of the water, and it 
seems to me probable that this break in reproduction is due 
rather to a reproduction-pause following the imperfectly indi¬ 
cated sexual period. This species seems to have had originally 
two reproductive periods, which would naturally have been 
closed by the production of sexual eggs. There is left now 
barely a trace of sexual reproduction, but the break in the 
sexual reproduction is still indicated in the history of the 
species for spring and early summer. 
When reproduction again goes on rapidly during mid-sum¬ 
mer, the females produce only two summer eggs, which are 
large, transparent, and quite different in appearance from those 
laid in the spring. The number of eggs increases to four in 
early September if the temperature of the water has fallen from 
the summer condition. 
The period of rapid reproduction in the spring falls at a time 
when the temperature of the water is from 15° to 18° C. In 
the autumn the main reproductive period is not entered upon 
until after the lake has fallen to a temperature of 15° C. 
Daphnia pulicaria. The reproductive periods of this species 
are also limited by temperature. A high temperature exerts 
an effect more unfavorable than it produces on Daphnia hyalina, 
and the main periods of reproduction come earlier in the spring 
and later in the fall than do those of its sister species. Repro¬ 
duction also continues through the winter with considerable 
rapidity. The period of active reproduction in the fall begins 
after that of Daphnia hyalina closes, and the largest broods ap¬ 
pear in late November and early December, when the tempera¬ 
ture of the lake has fallen below 5° C. It is apparently not 
until the lake has fallen below 10° C. that eggs are produced in 
great numbers, and in the cold water of the late fall, the females 
deposit in the brood-sacs from five to nine eggs, and the birth 
