326 Birge—The Crustacea of the Plankton. 
Apstein (’96, p. 179), finds that D. graciloides has its 
maximum in lake Ploen in winter and in the Dobersdorfer 
See in summer. Its relations in the latter lake agree very- 
well with those of the same genus in lake Mendota. He con¬ 
cludes from the striking difference in the two lakes that tem¬ 
perature has no effect on the species. Marsh, who finds that 
D. minutus has its maximum in Green lake in September and 
October (’97, p. 192), also thinks that temperature affects the 
genus very little. I am unable to agree with this conclusion, 
so far as the form studied by me is concerned. It is the first 
of the perennial Crustacea to slacken its reproductive activity 
in the autumn, and this occurs when food is at its maximum. I 
can attribute this check only to the fall in temperature. Indeed, 
my observations show that the reproductive activity of D. Oregon - 
ensis is more promptly checked by the decline of temperature 
than is that of any other of the perennial species. 
Cyclops. 
Figures 15, 21. —Table E, Appendix. 
There are two species of Cyclops which are at times conspicu¬ 
ous in the plankton of lake Mendota, C. brevispinosus Herrick 
and C. Leuckartii Sars. C. pulchellus Koch was rarely seen. 
C. brevispinosus is by far the more numerous and is practically 
the only species except in summer. From October to May only 
scattered individuals of any other species are met, but during 
summer brevispinosus declines and Leuckartii may be as numer¬ 
ous as it or even more so. The numerical relation has not been 
determined because of the great labor involved in discriminating 
the species, especially in the immature examples which always 
constitute by far the greater part of the catch. 
Cyclops brevispinosus is the most abundant species of limnetic 
Crustacea at almost all times, and at its maximum is far more 
numerous than any other species ever becomes. It is the only 
abundant Copepod which reproduces under the ice; Daphnia 
pulicaria among the Cladocera has the same habit. 
The winter numbers are as follows, stated in thousands per 
square meter of surface: 
