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. 



