340 
Birge—The Crustacea of the Plankton. 
Males are found during and after the spring and fall reproduct¬ 
ive periods, although in very small numbers, never exceeding 4 per 
cent, of the number of females and rarely being as numerous as this. 
Ordinarily it is only possible to find one or two males by careful 
search through the entire collection. These males are somewhat 
more abundant after the fall reproductive period than earlier, 
and may be found as late as the middle of December or even the 
first of January. It seems, therefore, that originally this spe¬ 
cies had two main reproductive periods, in the fall and spring. 
Each of these was probably closed by the production of males 
and the development of ephippia. The sexual reproduction has, 
however, almost entirely disappeared, and the species has prac¬ 
tically passed into a acyclic condition. 
Apstein (’96, p. 167,) finds that Daphnia hyalina is present 
from September to July, with a maximum in winter. This his¬ 
tory is so wholly different from that of the species as found in 
lake Mendota that no profitable comparison can be made. 
Daphnia pulex var. pulicaria Forbes. 
Figure 17.—Table G-, Appendix. 
The following table gives the average number of Daphnia 
pulicaria taken during the period of my investigation. From 
this and from the diagram it appears that the species was pres¬ 
ent in very small numbers during July and August, 1894; that 
it then entirely disappeared until the early part of July, 1895; 
it increased in numbers during the summer and autumn, in¬ 
creased greatly during December, and was present in consid¬ 
erable numbers during the winter. About the middle of April, 
1896, a period of rapid reproduction began, the species rising 
to a maximum in the latter part of May. At this time, and in 
the early part of June the males appeared and not infrequently 
numbered from one-third to one-fourth of the total catch. The 
females developed ephippia and the sexual eggs were produced 
early in June. The species rapidly declined after this date, al¬ 
though present in somewhat larger numbers early in September. 
Scattering individuals only were found from the first of Octo¬ 
ber through the winter of 1896-97. The species entered upon 
