The Distribution in the Upper Meter. 
409 
half meter; at one, two, and three meters, and sometimes deeper. 
The results of these two methods were the same and can be 
stated in general as follows: 
1. On calm sunny days the upper ten centimeters of the lake 
may be almost devoid of Crustacea, as was the case on August 
1st, 2d, and 25th. At a depth of half a meter, however, the 
numbers become considerable and may be very great. On 
August 25th the total population of the water at this depth was 
at the rate of nearly 70,000 Crustacea per cubic meter, without 
including the nauplii, which numbered 18,000 more. At one 
meter the population was nearly 200,000 per cubic meter and 
below that depth the numbers rapidly declined. A large num¬ 
ber of similar observations were made on other days, and in one 
of the cases where the observations with the pump were ex¬ 
tended throughout the inhabited water the results have been 
diagramed and are shown in Fig. 33. 
2. The population of the upper meter is largely composed of 
immature Crustacea, the percentage of young varying in dif¬ 
ferent species. It is most marked in Diaptomus, Daphnia hya- 
lina , and D. retrocurva. Great numbers of young are found in 
the upper meter, as was the case on August 25th, and especially 
on September 8th, and the adults may be entirely absent. 
At the depth of a half meter a very few half-grown individuals 
are present, while they are fairly numerous at one meter and at 
the same depth the adults begin to appear. Below one meter 
by far the most conspicuous part of the population consists of 
adults, although the young may be present in numbers as great 
as the comparatively few adults. A similar relation of distri¬ 
bution holds for Daphnia retrocurva, although the proportion of 
this species in the upper meter by day seems to be smaller than 
that of its congener. The adults of Diaphanosoma approach 
nearer the surface when the sun is bright, than those of Daph¬ 
nia, but at least 75 per cent, of the individuals found between 
the half meter level and the surface are immature. The same state¬ 
ment is true for Diaptomus. Cyclops shows the least difference; 
females carrying eggs being regularly found in considerable 
numbers at half a meter, or even above that level, coming to 
the surface on cloudy days and occasionally in sunshine. Yet 
