Nauplii—Daphnia hyalina. 
335 
larvae, or to the failure of Cyclops to produce eggs. It will be 
seen that the nauplii were exceedingly numerous throughout 
the winter and into the spring, and during the month of May a 
certain relation can be traced between the numbers of nauplii 
present and those of immature Cyclops —-the nauplii decreasing 
in number as the Cyclops increase. It is evident further that 
the death rate of these larvae during the winter must be very 
low, or that the losses are balanced by the production of young 
which develop to this stage, without going further until the 
warming of the water in the spring. 
During the month from the middle of July to the middle of 
August numerous determinations were made, from which it 
appeared that the maximum and minimum numbers of the 
nauplii vary in about the same ratio as do those of the adult 
Crustacea. In July, out of six observations the maximum was 
3.8 times the minimum, and in August the maximum was 3.4 
times the minimum. The largest number observed was 2,040,000 
per sq. m. of surface on July 18th. A larger series of observa¬ 
tions would undoubtedly have shown, in the spring of 1897, 
numbers equal to this. 
Daphnia hyalina Leydig. 
Figure 16.—Table F, Appendix. 
The autumn numbers in both years show a decline to a minimum 
which extends throughout the winter and until the first or mid¬ 
dle of May. In 1895 this minimum was established in Novem¬ 
ber, but in 1894, not till late December or January. In 1895 
there was no marked reproductive period in the autumn. This 
was apparently due to the continuation of summer conditions, 
until near October 1, and the sudden change at that time. 
The final reproductive period of this species lies at the end of 
October or early in November. After the close of this period, 
the old females rapidly decrease in number, and almost, or 
wholly, disappear before the first of January. The young grow 
somewhat rapidly until they have reached about half the mature 
size, and after that, grow very little or none at all until the fol¬ 
lowing spring. Reproduction is practically wholly absent during 
the winter, although it occasionally happens that a single female 
can be found in March, having eggs in the brood-case. 
