Juday—Diurnal Movement of Plankton Crustacea. 551 
ing especially increased by the change in the condition of the 
sky. Daphnia liyalina also may come nearer the surface.” 
5. “At night, the population of the upper meter changes in 
character. The young, instead of being concentrated in swarms 
in this layer, become more evenly distributed and the adults, 
which were found below the one-meter level, rise toward the 
surface. Leptodora and larval Corethra have been regularly 
taken at the surface in considerable numbers at night. During 
the day, these animals are rarely, if ever, found close to the 
surface, although they may be abundant enough above the three- 
meter line. It would appear, therefore, that these animals 
move toward the surface at night, together with the Crustacea 
on which they feed. Epischura seems to have the same habit” 
(Birge 2b. pp. 407-410.) 
MOVEMENTS OF THE DIFFERENT FORMS OF CRUSTACEA. 
Daphnia hyalina Leydig. 
This species was found in nine of the ten lakes. Two varie¬ 
ties of individuals, those with round and those with pointed 
helmets, were found in three lakes. There was a diurnal move¬ 
ment of the adults in eight of the nine lakes, ranging in extent 
from a quarter of a meter to seven meters. With one exception, 
the young did not show so great a movement as the adults for 
they were nearer the surface in the daytime. Where the extent 
of migration of the adults did not exceed a meter, young were 
usually found at the surface in the daytime, and in some cases 
they w r ere at the surface when the adults showed a movement 
of two meters. 
This is a very small movement as compared 1 with that ob¬ 
served by Burckhardt (4) for this species in Vierwaldstattersee, 
a lake which is deeper than any of these nine Wisconsin lakes. 
He found diurnal movements of twenty to thirty meters in 
March, while a maximum of fifty meters was noted. 
The variety having round helmets did not show diurnal move¬ 
ment in Bainbow lake while that with pointed helmets did. 
The reverse of this was true in Okauchee lake and there was 
a movement of both varieties in Hagawicka lake. 
In the eight lakes in wdiich there was movement, the time 
of arrival at the surface varied from the third half hour before 
