312 
Birge—The Crustacea of the Plankton . 
cea. The species present are those which are most easily avail¬ 
able as food, so that both in kind and quantity of food, the 
Crustacea find the most favorable possible conditions from early 
September to the latter part of November. Temperature is the 
predominant factor in influencing their development. 
In 1894 and 1896 Chydorus was present in great numbers. 
Both of these seasons were characterized by the great abundance 
of Aphanizomenon. In 1895 and 1897, when the predominant algae 
were almost exclusively diatoms, the number of Chydorus was 
extremely small. Diagram 10 shows the number of Crustacea from 
July to December, after subtracting Chydorus. It will be seen 
that the form of the curves is strikingly similar in all years, 
and that the numbers are extremely close for 1895 and 1896, 
with the exception of a great rise in late October, 1896, which 
was due to the sudden multiplication of Daphnia hyalina at that 
time. 
From the fall maximum the number declines, at first rapidly, 
and afterwards more slowly toward the winter minimum. The 
rapidity of the decline depends upon several factors. If a large 
number of young forms are produced late in the season, many 
of them die as well as their parents, and the decline in num¬ 
bers is correspondingly rapid. The number of the periodic 
species also exerts a great influence. In 1896, when Daphnia 
retrocurva was present in large numbers, its sudden disappear¬ 
ance at the close of its sexual period aided to cause a rapid de¬ 
cline in the total number of Crustacea present. The climatic 
conditions also exert a great influence. A rapid decline in tem¬ 
perature, accompanied by violent storms, causes the numbers to 
sink more rapidly than a more equable approach of winter tem¬ 
peratures. In any case the number of the Crustacea falls off 
rapidly during November, more slowly during December, and 
by the middle or last of that month the lake freezes and the 
winter conditions are fairly established. 
The different species of limnetic Crustacea enter the winter in 
very different conditions. Daphnia hyalina produces in the late 
fall large numbers of young, which serve to carry the species 
through the winter. The old individuals disappear during No¬ 
vember and December, very few lingering into January. Dur- 
