The Distribution in the Upper Meter. 400 



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. G-reat 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 



