844 
Birge—The Crustacea of the Plankton. 
same as~on the former occasion. The rise in numbers shown 
by the tables and diagram in the latter part of August and in 
September are therefore due to these unusual accumulations of 
the species and do not indicate a corresponding average rise in 
numbers extending over any considerable area of the lake. The 
case is wholly different with the increase which comes in late 
November and December. This is occasioned by a very rapid 
multiplication of the species. The brood-sacs contain from 5 
to 9 eggs. This ^reproductive period does not begin until after 
the temperature of; the lake has fallen below 10°, and mul¬ 
tiplication continues, although at a slower rate, throughout 
the winter. 
In the spring comes the main period of reproduction; and 
during May, 1896, the numbers were uniformly large, yet even 
here they were subject to very considerable variation. At the 
time of the maximum, the species was the most abundant of 
the limnetic Crustacea, with the exception of Cyclops , and since 
the individuals are so much larger than Cyclops, the species 
was the most important constituent of the crustacean plankton. 
It would seem necessary to suppose that the ephippial eggs 
deposited in June and July of one year remain unhatched for 
nearly a year. This is a very long period, and I have no direct 
observations which would make the conclusion certain. I am 
sure, however, that the species was practically absent from the 
plankton after August, 1894, since it was carefully looked for 
and only one specimen was found, and that in December. There 
was also no reproductive period in 1896 after the first of August, 
the increase in numbers in September of that year depending 
on an aggregation of individuals corresponding to that in 1895, 
there was no reproductive period during November or De¬ 
cember, and the species declined in number, so that it was not 
practicable to enumerate it in the plankton. The winter eggs of 
Diaphanosoma must remain unhatched from about Oct. 1 to 
June of the next year. 
The peculiar history of Daphnia pulicaria in lake Mendota 
is conditioned in great part by the fact that the species 
is unable to live in the cooler water of the lake below the 
thermocline. In lakes which are relatively plankton-poor, the 
