344 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 ephippiai 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 



