402 Birge — The Crustacea of the Plankton. 



Daphnia retrocurva Forbes. 

 Table H, Appendix. 



This species belongs to the periodic Crustacea and is present 

 in the lake from July to December. Its numbers during July- 

 are small and the proper history of the species does not begin 

 until the latter part of this month, or the early part of August. 

 In 1896, indeed, the numbers were very small until the decline 

 of D. hyalina in the middle and latter part of August gave an 

 opportunity for the presence of this species. 



In vertical distribution this species agrees very closely with 

 D. hyalina, as would be expected. In the early part of periods 

 of increase, from 45 to 60 per cent, may be found in the upper 

 level. This was the case in the latter part of July, 1895. It 

 was also true in late September and early October, 1896, although 

 the Crustacea moved rapidly downward so that the two- week 

 averages do not disclose the fact. In the old age of the broods, 

 as the numbers are declining, they are found chiefly in the 

 lower water of the lake. This was especially obvious in late 

 November and in December, 1895, when the species disappeared 

 quite slowly and lingered latest in the lower waters of the 

 lake. In 1896 the formation of the ephippia was nearly simul- 

 taneous on the part of all of the females and the species disap- 

 peared rapidly and completely in the early part of November, so 

 that this phenomenon of the old individuals lingering in the 

 lower water did not appear. 



Marsh (97, p. 210) finds the distribution of Daphnia Kahl- 

 bergiensis in Green lake very similar to that of D. retrocurva 

 in Mendota. He finds, however, a marked difference between 

 the vertical distribution by day and night, which I have not 

 seen. The fact, however, that D. retrocurva descends to a some- 

 what greater depth during the day than does D. hyalina seems 

 to indicate a greater sensitiveness to light than that of its con- 

 gener, although this sensitiveness does not lead to as great 

 movements as Marsh's observations would indicate for Green 

 lake. 



