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&re the only abundant algae, 

 are periods when Chydorus is present in small numbers; while 

 in periods when the Schizophyceae or Anabama 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, 



