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 r 

 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. 



