Juday—Diurnal Movement of Plankton Crustacea. 545 
during the third half hour after sunset and were not present in 
a surface catch made an hour and a, half before sunrise. They 
were not found in June. 
PEWAUKEE LAKE. 
This lake is the shallowest of the ten on which observations 
were made for diurnal movement. The eastern half of the 
lake scarcely exceeds two meters in depth. This part was orig¬ 
inally a swamp and the water is maintained at its present depth 
by means of a dam about two meters high at the outlet. The 
whole of the eastern, half supported a dense growth of the larger 
forms of aquatic plants and the entire lake' was the richest of 
the ten in phytoplankton.’ 
The observations were made in the western half of the lake 
which has a maximum depth of thirteen meters. They con¬ 
sisted of a single day and a single night series. 
Daphnia hyalina .—There was no diurnal movement of this 
species. 
Ceriodaphnia .—This form 1 showed no movement. 
Diaphanosoma brachyurum .—This species was at a depth of 
one meter in the day catches but was present in the surface 
catch at night. 
Diaptomus .—In the daytime, this form was at a depth of one 
meter but. it came to the surface at night. 
Cyclops .—The young of this form were at the surface and 
adults at a depth of one meter in the day catches. The latter 
came to the surface at night. 
Nauplii .—There was no diurnal movement of the nauplii. 
LAKE GENEVA. 
A complete set of observations was made in August, 1900. 
This lake was comparatively poor in phytoplankton. 
Daphnia hyalina .—Both young and adults were absent from 
the upper five meters in the day catches. The former reached 
the surface during the second half hour after sunset and adults 
half an hour later. Both left the surface about one hour be¬ 
fore sunrise. 
Diaphanosoma brachyurum. ^Neither young nor adults were 
found above five meters in the day catches but both came to the 
35 
