304 Birge—The Crustacea of the Plankton. 
than three in winter nor more than eight in the period from 
July to October. 
In the study of this rhythm of development, three facts may 
well be noticed in the first place. First, the number of Crus¬ 
tacea in lake Mendota is to a singular extent dependent upon 
the perennial forms. In other lakes it often happens that the 
periodic forms are the dominant members of the summer popu¬ 
lation. Of these forms, Bosmina is practically entirely absent 
from lake Mendota; Diaphanosoma appears in small numbers 
only; and Daphnia retrocurva only rarely equals in number the 
related species, Daphnia hyalina. There is, therefore, no great 
increase in numbers in summer dependent on summer forms 
alone. Indeed, the influence of the periodic species is not 
greatly felt until September, and the shape of the developmental 
curve would not be greatly altered, were the periodic species 
omitted. 
Second, Chydorus occupies a peculiar place among the plank¬ 
ton Crustacea. It is properly a marginal form, and appears in 
the limnoplankton only under favorable conditions. Apstein 
has connected its presence in the limnetic region with that of 
Chroococcaceae. My observations seem to connect its abundance 
in the limnoplankton with an abundant development of these and 
similar plants. In other words, it seems true for lake Mendota that 
periods when the diatoms and Ceratium are the only abundant algae, 
are periods when Chydorus is present in small numbers; while 
in periods when the Schizophyceae or Anabcena abound, Chydorus 
is also abundant. The maxima of this species, therefore, have 
occurred without close reference to temperature or season, and 
may come at any time from June to late October. These maxima 
are also very irregular in amount, number, and duration. 
Chydorus , also, is peculiar in the limnoplankton on account of 
its small size. It contains little more animal matter than a good- 
sized nauplius, and decidedly less than an embryo Daphnia. 
While, therefore, a great abundance of one form of plankton 
Crustacea usually affects unfavorably the number of other spe¬ 
cies, Chydorus appears to be more independent of the presence 
of other forms. It seems, as it were, superposed on the regular 
limnoplankton, rather than a part of the general limnetic life, 
