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: 



