560 Wisconsin Academy of Sciences, Arts, and Letters. 
tomus and Epischura about the same as D. hyalina. Tbe order 
of leaving the surface in the morning was substantially the re¬ 
verse of the order of appearance in the evening. 
ECOLOGICAL FACTORS INVOLVED. 
Diurnal movement has been attributed to several causes. 
Food, temperature, and light have probably been cited most fre¬ 
quently. Each has been supported more or less strongly as 
being the chief cause of the phenomenon because of the different 
results obtained by various plankton observers. It has been 
considered a food phenomenon because the phytoplankton has 
been found most abundant near the surface, thus making this a 
good region for food. As a result the Crustacea move up into 
this region so that they may obtain an abundance of food. In 
general it may be said that these results do not support this 
idea. For example, some forms moved up from depths where 
food was nearly, if not quite as abundant as at the surface. In 
several instances, crustaceans rose from regions where the 
greater part of a catch was made up of phytoplankton and this 
phytoplankton consisted of the same plant forms as found at 
the surface. 
Then, too, some Crustacea, were able to obtain a sufficient 
amount of food at considerable depths to enable them to remain 
at these depths both day and night, while other forms nearer 
the surface, and thus in a region more plentifully supplied with 
food, moved up regularly at night. In Okauchee lake Daphnia 
longiremis remained below twelve meters both day and night 
from August to October while 1 other Crustacea much nearer the 
surface, hence in a, region where food was more plentiful, came 
to the surface at night. Also in Lake Mendota, D. pulicaria 
remained in the region of the thermocline both day and night 
where food was not nearly so abundant as near the surface 
while other forms near the surface moved up at night. Fuhr- 
man (7) states that it could not be considered a food phenom¬ 
enon in the lake on which he made observations because phyto¬ 
plankton'; was scarce at the surface. 
Considering the phenomenon from the standpoint of tempera¬ 
ture Ostwald (9) has given a very interesting theory to account 
for it. He believes that the descent is due to the lowering of 
the viscosity or the internal friction of the water (die innere 
