310 Birge — The Crustacea of the Plankton. 



different seasons. In 1894 it was only a small fraction of the 

 total number of Cyclops present, while in 1896 it was quite as 

 numerous as Cyclops brevispinosus. In 1896 Chydorus developed 

 in great numbers in the latter part of June and early July. 

 This development coincided with the presence of great quantities 

 of Aphanizomenon. In 1895, which was characterized by a pre- 

 dominance of diatoms among the plankton algae during the sum- 

 mer, there was no marked development of Chydorus until au- 

 tumn. The second cause of this midsummer increase is the re- 

 newed reproductive activity of the perennial species, especially 

 Daphnia hyalina. These species has a marked reproductive period 

 and maximum in the spring,- (Fig 16) at which time from five to nine 

 eggs may be produced. After the production of the spring 

 broods the reproduction is greatly checked, and the species de- 

 clines rapidly in number; but when the summer temperature of 

 the water has been established, the species again reproduces, so 

 that its numbers increase rapidly. Only two eggs are, however, 

 regularly produced at oace during the summer. 



The result of these additions of new forms and increase of 

 old ones gives a marked rise of the total number of the Crus- 

 tacea in late June and early July. This rise was very feeble 

 in 1894, owing to the wholly peculiar condition of the vege- 

 tation, as stated elsewhere. 



From this mid-summer maximum all of the species, except 

 Chydorus, usually decline steadily and somewhat uniformly until 

 the middle or the last of August. Three possible causes may 

 be assigned for this decline: first, the exclusion of the Crustacea 

 from the deeper water of the lake; second, the increased tem- 

 perature of that part of the lake inhabitable by them; third, 

 the great development of Ceratium, which regularly becomes a 

 predominant alga during this period, and which is much less 

 available as food than the diatoms and Schizophyceae. Cera- 

 tium exerts a more unfavorable influence on the number of the 

 Crustacea from the fact that the young Crustacea are quite un- 

 able to eat it. It is so large and its shell is so hard that they 

 cannot master it, yet Ceratium occupies, with its enormous 

 swarms, the upper strata of water, which naturally belong to 

 the young Crustacea. While, therefore, the adult Crustacea may 



