864 
Birge—The Crustacea of the Plankton 
during the latter part of August, rising rapidly to a maximum? 
in September (Fig. 20), and then declining slowly until late- 
October, when it fell off more rapidly and finally disappeared, 
with the exception of occasional scattered individuals. In 1895- 
it reappeared in May, reached a small number which it main¬ 
tained about six weeks, rose rapidly to a maximum in July,, 
and then declined to a small number which was maintained with 
approximate constancy from the latter part of August, through^ 
the autumn and winter, declining, but not quite disappearing,, 
in the following April. In 1896 the species was much more- 
abundant than in either year, a fact which I have connected- 
with the greater abundance of Aphanizomenon during that 
season. The species had a great development from July to the* 
middle of October, reaching its maximum early in September. 
There was also a minor maximum in early July and one in the- 
first half of October. It appears, therefore, that the maxima of* 
this species have come in July, 1895, in September, 1896, and! 
in October, 1894, and that in other years these months have 
been marked by the presence of very small numbers of the' 
species or its total absence in other years. It is, therefore,, 
impossible to say more on the relation of temperature than that 
the maxima fall in the warm season of the year. During the* 
winter of 1895-6, when the species was regularly present, re¬ 
production went on, as was evidenced by the regular presence' 
of eggs in the brood-sac of the females. 
Summing up these results of temperature, it may be said that 
in lake Mendota, temperature exerts a greater control over the- 
number of the plankton Crustacea than does food. The number- 
of the Crustacea falls off in autumn, while food is still abundant; 
reproduction is checked in winter, although the food present 
would permit reproduction; and the reproductive periods of the- 
perennial species are arranged rather with reference to temper¬ 
ature than to food supply. 
If I were to sum up my impressions as to factors affecting 
the numbers, I should state them as follows: 
1. Food sets an upper limit to number. 
2. The algae of the upper strata of water determine the de¬ 
velopment or failure of the young broods. 
3. Temperature determines the rhythm of reproduction. 
