308 
Birge—The Crustacea of the Plankton. 
ice was made March 27th; in 1896, March 29th. The first col¬ 
lection in water was made April 12th, 1895, April 4th, 1896. 
In general, the lake opens either wholly or over the greater 
portion of its surface about the 1st of April. The period im¬ 
mediately following the opening of the lake seems to be a time 
of trial for most of the limnetic Crustacea. The temperature of 
the water increases very slowly at first, or, indeed, may be 
lowered temporarily; and the surface is, of course, agitated by 
gales which are so frequent in April. 
During the spring Cyclops ordinarily increases in numbers 
with a rapidity dependent on the rise of temperature in the 
water, and upon the reproductive condition of the species at the 
time of the disappearance of the ice. Diaptomus and D. hyalina 
do not begin to rise in numbers until after the first of May, as 
may be seen by reference to Pigs. 8 and 9. During April these 
species are wont to decline in number, so that the smallest 
catches made during the year ordinarily come in the latter part 
of April or the first of May. Cyclops, however, increases with 
great rapidity. Reference to the diagrams and tables will show 
that in 1895 Cyclops increased more than fourfold in number 
during two weeks, and that this increased number was nearly 
quadrupled during the next two weeks. In 1896 Cyclops ad¬ 
vanced with even greater rapidity and about two weeks earlier 
than in 1895. In each year the increase in Cyclops was about 
a month in advance of that of Diaptomus or Daphnia hyalina , and 
in 1896, about two weeks ahead of the multiplication of Daph¬ 
nia pulicaria. The spring maximum is reached during the 
month of May, either in the first or the latter part of the 
month, according to the temperature. At the maximum the 
population of the lake consists largely of Cyclops , about 70 per 
cent, of the total in 1895, and 80 per cent, in 1896 consisting of 
this species. 
The multiplication of the Crustacea and rotifers during the 
spring seems to be more rapid than that of the algae, and in late 
spring at the time of the maximum, the algae are far less 
numerous with respect to the Crustacea than at any other season 
of the year. In a word, the eaters multiply in excess of the 
food. This undue multiplication of the Crustacea puts a check 
