Vertical Distribution of Individual Species. 
403 
Diaphanosoma brachyurum Sars. 
Table I, Appendix. 
This species belongs to the periodic Crustacea, its active de¬ 
velopment extending from the first of August to the middle of 
October. It is provided with very large antennae and is one of 
the most powerful swimmers among the limnetic Crustacea. It 
is also positive in its relations to light. In both these respects 
it resembles Diaptomus and its vertical distribution very closely 
agrees with that of the latter genus, although its numbers are 
very much smaller. In the early history of the species 
50 to 70 per cent, of the whole number are found in the upper 
stratum of the lake. The distribution becomes more equal dur¬ 
ing the decline of the species and at no time is there found any 
aggregation of individuals in the lower waters of the lake. The 
distribution of the small numbers present in the decline of the 
species is, however, quite irregular and the number in the 
upper part of the lake becomes smaller than that in the lower 
water. 
Marsh (’97, p. 216) suggests that the vertical distribution of 
Diaphanosoma is controlled by light rather than temperature. 
He finds it negative to light and thinks that it prefers cool 
water. In the laboratory Diaphanosoma moves toward the light 
along with Diaptomus , so that my observations would indicate 
that it is positive in its relations to light. I find also uniformly 
a larger percentage of adult animals in the upper meter by day 
than I find of the species of Daphnia. There is, therefore, noth¬ 
ing in my observations to confirm the idea that the species is 
negative in its relations to light. Since, however, the absence 
of Crustacea from the upper centimeters of the lake when the 
light is most intense, indicates a certain negative relation on 
the part of nearly all forms, it may well be that this species 
finds the light in the clear water of Green lake too strong, and 
responds to it more definitely than in lake Mendota. 
