Juday—Diurnal Movement of Plankton Crustacea . 561 
Beibung des Wassers) through its increase in temperature. 
This lowering of the internal friction of the water through its 
increase in temperature, affects the speed of sinking of the 
plankton and thereby calls forth a descent of the pelagic forms 
during the early hours of the day. An increase of the internal 
friction by cooling will not, however, cause a movement toward 
the surface. Diffusion streams and active wandering bring 
about the ascent during the second half of the night, toward 
morning. The active wandering varies according to the swim¬ 
ming ability of the different forms. My observations do not 
lend support to this theory. The forms showing diurnal move¬ 
ment came to the surface early in the evening instead of toward 
morning. In fact, they came to the surface so early in the 
evening that the water had cooled comparatively little so that 
the internal friction at this time was near its minimum for the 
day. In the morning, also, many Crustacea left the surface an 
hour or two before sunrise. This means that they descended 
during a period while the water was still cooling and therefore 
while the internal friction was increasing instead of decreasing. 
With very few exceptions, the migrating Crustacea left the sur¬ 
face in the morning before the sun had affected the tempera¬ 
ture of the water in the least. Therefore temperature can not 
be regarded as either a direct or an indirect cause of the diurnal 
movement noted in these observations. As 1 stated above, how¬ 
ever, temperature did affect the extent of these daily migrations 
in a very marked degree in some instances. Waldvogel (14) 
states that in the two lakes on which he made observations the 
Crustacea ceased their diurnal movements during the strongest 
warming of the water in August and did not resume them again 
until the water had cooled below 20° C. In this instance tem¬ 
perature acted as an inhibitory factor rather than as a cause of 
migration. Burckhardt (4) says also that temperature is not 
the cause of the phenomenon because he observed diurnal migra¬ 
tions in January during a snow storm and at this time the water 
of the lake was practically of a uniform! temperature. Parker 
(11) found that the migration of Labidocera aestiva did not de¬ 
pend primarily on temperature. 
A number of observers have attributed the diurnal movement 
of Crustacea to light. That is, the forms showing this phe¬ 
nomenon are negatively phototropic and therefore move down 
36 
