410 
Birge—The Crustacea of the Plankton. 
while it is not easy to determine the exact proportions of young, 
it is very obvious that the majority of the immature Cyclops are 
near the surface. 
3. A far larger proportion of Cyclops is usually obtained from 
the upper five or ten centimeters than comes from any of the other 
forms of limnetic Crustacea, and it may be present at the very 
surface on hot, calm, sunny days, as on Sept. 13. 
4. The nauplii are found in considerable numbers in the up¬ 
per water during the day and frequently extend to the very sur¬ 
face, yet ordinarily the number at the surface is only a third, 
or even a smaller fraction of that found at one-half meter. 
Older nauplii may be found in large numbers at the surface and 
confined to the upper one-half meter. 
5. In windy and cloudy weather the Crustacea approach nearer 
to the surface, the numbers of Diaptomus and Cyclops being es¬ 
pecially increased by the change in the condition of the sky. 
Daphnia hyalina also may come nearer the surface. But the num¬ 
bers of these species during the day in the upper ten centimeters 
are always decidedly smaller than at one-half meter, so far as 
my observations extend. 
6. 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 sur¬ 
face, 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. 
