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- 
hergiensis 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. 
