Juday—Diurnal Movement of Plankton Crustacea. 557 
the fact that the species exhibiting this phenomenon in its 
most typical form are found in as great abundance at a depth of 
a meter or two, or even deeper, as at the surface. In fact these 
forms- are usually more abundant some distance below the sur¬ 
face than at the surface. It means that some forms desert the 
upper strata' of water during the day but move up into these 
strata at night. In other words, they spread out into this 
region which is deserted by them during the day, for there is 
no general upward migration of the whole body of Crustacea. 
With rare exceptions, some individuals of each form continue 
their ascent until they reach the surface. Under this phenom¬ 
enon we also include instances where the Crustacea are found 
in very limited numbers near the surface in the daytime but 
occupy this region in much greater abundance at night. 
EXTEXT OF DIURXAL MOVEMENT. 
The foregoing results show a wide range in the extent of 
these daily migrations. The variation is no> greater, however, 
than one should expect from the great diversity of the ecolog¬ 
ical factors involved. Each lake possesses factors peculiar to 
itself and the Crustacea respond to these differences in environ¬ 
ment in a greater or lesser degree. As a result, the diurnal 
movement in each lake has certain individual characteristics. 
Thus in the various lakes, the zone deserted by Daphnia hya- 
lina during the day varies in thickness from a quarter of a 
meter to seven meters while there was no movement at all in 
one lake. Also the changes in the environment of a single lake 
during the seasons in which diurnal movements occur, may 
affect the extent of these migrations very materially. That is, 
there may be a seasonal variation in the extent of this move¬ 
ment. To mention only two instances, the movement of Daph- 
nia pulicaria changed from twelve meters in October to one 
meter in June in Oconomowoc lake and that of D. hyalina in¬ 
creased from two meters in July to six meters in September in 
Rainbow lake. 
The various forms of Crustacea are not equally sensitive to 
the-factors instrumental in the production of this phenomenon 
for they do not all show migrations of equal extent in the same 
lake. For example*, in Okauchee lake Daphnia hyalina showed 
a movement of two, D. pulicaria five, and Leptodora nine 
