550 Wisconsin Academy of Sciences, Arts , and Letters. 
mg series of the 12 th, it left the surface during the first half 
hour before sunrise and, on the 25th, half an hour earlier. 
Diaptomus.—There was no diurnal movement of this form 
in October and April. In May, both young and adults showed 
a movement of half a meter on the 3rd and also on the 15th. 
On the latter date, young came to the surface about sunset and 
adults half an hour later. On the 17th, adults left the surface 
during the second half hour before sunrise and young were still 
present in the surface catch made one hour after sunrise. On 
the 28th there was no movement of either young or adults, as 
both were found at the surface in the daytime. In June, young 
were found at the surface in the daytime and adults at a depth 
of a quarter of a meter. The latter came to the surface an hour 
or more before sunset. On one occasion, they did not leave the 
surface until the second half hour after sunrise and, at another 
time, they were still found in the surface catch made an, hour 
and a half after sunrise. 
Cyclops .—This form was regularly found at the surface in 
the daytime and there was comparatively little increase in num¬ 
ber at night. 
Nauplii .—There was no diurnal movement of the nauplii. 
The above results show that the upper meter of Lake Men- 
dota is well populated in the daytime and that the movement, 
therefore, is confined within very narrow limits. In this 
respect, they are in complete agreement with the results ob¬ 
tained by Birge (2b). With the exception of Corethra larvae 
which were not found in my observations, the following state- 
ements of his results apply equally well to mine: 
1. “On calm sunny days, the upper ten centimeters of the 
lake miay be almost devoid of crustacea. At a depth of half a 
meter, however, the numbers become considerable and may be 
very great.” 
2. “The population of the upper meter is largely composed 
of immature crustacea, the percentage of young varying in dif¬ 
ferent species.” 
3. “A far larger proportion of Cyclops is usually obtained 
from the upper five or ten centimeters than comes from any of 
the other forms of limnetic crustacea and it may be present at 
the very surface on hot, calm, sunny days.” 
4. “In windy and cloudy weather, the crustacea approach 
nearer the surface, the numbers of Diaptomns and Cyclops be- 
