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 Dlaptomus 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. 



