216 
Marsh—Limnetic Crustacea of Green Lake. 
0-5 
5-10 
10-15 
15-20 
August, 1893. 
48.30 
39.27 
52.01 
38.28 
69.07 
30.60 
39.89 
16.28 
17.89 
9.84 
August, 1896..... 
19.60 
15.46 
16.54 
8.23 
September-October, 1894. 
October, 1894, day. 
15.82 
6.01 
October, 1894, night.. 
I do not think that the number of collections is large enough 
to draw inferences final in character in regard to the vertical 
distribution of Daphnella , especially since the total number in 
any collection is small. It would appear, however, that the up¬ 
per five meters are more densely populated in September and 
October than in August and that the number is also greater in 
the upper five meters in the night time than in the day time. 
I do not feel like speaking in any dogmatic way in regard to 
the interpretation of these facts, but I venture to suggest that 
Daphnella is, in its vertical distribution, controlled rather by 
light and darkness than by changes of temperature. If it were 
very sensitive to changes of temperature the fact that it is 
found in greater numbers near the surface at night than in the 
day time, and also in greater numbers in September and Octo¬ 
ber than in August would indicate a liking for cool water: but 
if this liking were very pronounced, it would seem that it 
would migrate deeper in August. If we suppose light to be 
the controlling factor, we would explain the greater number 
near the surface in September and October by the greater num¬ 
ber of cloudy days in those months. Very likely the solution 
of this problem is not so simple as my speculations would indi¬ 
cate, and a satisfactory result can only be reached by a care¬ 
fully conducted investigation in the laboratory of the behavior 
of the animal under different conditions of light and tempera¬ 
ture. It may be noticed that Apstein (’96, p. 79) states that 
the time when the larger numbers are found at the surface, co¬ 
incides with the time of total maximum numbers, a conclusion 
quite the opposite of what my observations would indicate. It 
does not appear, however, that his conclusions were based on 
any large number of exact observations. 
