Juday—Diurnal Movement of Plankton Crustacea. 555 
great variations in the time of leaving the surface in the morn¬ 
ing. 
Bosmina. 
This form occurred in seven lakes and it showed diurnal 
movement in three of them. There was a movement of one 
meter in two lakes and four meters in the third. 
Bosmina was irregular in its time of arrival at the surface, 
the limits being the second half hour before sunset and the sec¬ 
ond half hour after sunset. The difference in the time of its 
leaving the surface in the morning varied from more than an 
hour and a half before sunrise to more than half an hour after 
sunrise. 
• Chydorus. 
This form was found in four lakes but it showed no diurnal 
movement in any of them:. 
Epischura lacnstris Forbes. 
Epischura was present in eight lakes but only one individual 
was taken in two sets of observations in one lake. It was not 
found in the day catches of three lakes so that the extent of its 
movement in them could not be determined. In the other four, 
the migration ranged from two to twelve meters. 
It came to the surface during the first and second half hours 
after sunset and disappeared again from the fourth to the first 
half hour before sunrise. 
Limnocalanus macrurus Sars. 
Limnocalanus was found only in the two deep lakes, Green 
and Geneva. Only one individual, however, was taken in all the 
catches made in the latter lake. In Green lake, this was the 
predominant crustacean' below a depth of fifty meters. It 
showed an, apparent movement of thirty-five meters in this lake. 
Only one individual was found above fifty meters in the day 
catches but several were present in the night catch made just 
below the thermocline, fifteen meters. The thermocline seemed 
to prevent a further upward movement for not a single indi- 
