Juday—Diurnal Movement of Plankton Crustacea. 565 
tion, however, for any of the migrating forms. The accom¬ 
panying diagrams, Figures 1 and 2, bear upon this point. The 
transparency of the water was determined by means of a 
Secchi’s disk and the line marked Tr., representing transpar¬ 
ency, shows the depth at which the disk just disappeared from 
Oct25 Apr24 
1300 ISO/ 
Apr 25 
/so/ 
Hay3 
1301 
Hay 15 Hay 28 June/O June 27 
!30! ISO/ /so./ /so/ 
_ 
Di Cy. 
~D.hr 
Q< Cy. 
-D.hr 
OiD-fr 
Cy. 
- Di - 
D/.Cy. 
~D.hr 
j_1 
Cy. 
-Di— 
-D.fr- 
- Tr - 
-D.hr 
-Tr~ 
-Tr — 
-Tr — 
-Tr - 
—Tr — 
-Tr — 
—Tr — 
4 
Fig. 2.—Vertical distribution of Crustacea in Lake Mendota. For explanation 
see text, p. 565. 
view. The position of Daphma hyalina (D. h.), Diaptomus 
(Di.), and Cyclops (Cy.) is indicated in the figures and shows 
the upper limits of adults in the daytime. Daphma hyalina 
is omitted in two cases, as it was not found in Green lake and 
only young were taken in Oconomowoc lake in October. These 
figures show that the depth to which, the adults of these genera 
descended in the daytime was practically independent of the 
degree of transparency of the water. This was true not only 
of the different lakes but also of the same lake at different times. 
In Okauchee lake, the transparency was fifty per cent, greater 
in October and June than in August, yet the day position of 
Daphma hyalina was the same in the three months. Ini Ocono- 
