442 Birge — Vertical Distribution of Pelagic Crustacea. 



no observations as yet show the time when this very distinct 

 tendency of Crustacea toward the upper part of the water begins 

 or the date at which it has become established. 



In observations made on lake Plon, and printed in the 3d Report 

 of the Biological Station (p. 137), Zacharias gives a few obser- 

 vations regarding the number of Crustacea and their distribution 

 in depth at that lake. He finds that Cyclops is in September by 

 far the most numerous genus, having a maximum number of over 

 400,000 per square meter of surface. In August, however, he 

 notes that only 140,000 per square meter was present. As the 

 opening of our dredge was ¥ V of a square meter, the average 

 number of this genus caught by us during July would be at the 

 rate of 50,000 per square meter and the maximum would be over 

 136,000. Since the coefficient of the dredge can hardly be lower 

 than four, the number of Cyclops per square meter of surface is 

 not smaller in July in lake Mendota than that noted for August in 

 the far-deeper lake Plon. No figures are given by Zacharias for 

 Diaptornus, which was our most numerous species. Bosmina he 

 finds abundant, while no pelagic Bosmina or Ceriodaphnia has 

 been found in lake Mendota. Hyalodaphnia is present in very 

 large numbers, ranging from 62,000 to 94,000 per square m. 

 A corresponding species of Hyalodaphnia is found in lake Men- 

 dota in July and later in very small numbers. Only a few scat- 

 tering individuals were noticed in our counting, the number being 

 quite too small to use in the study of the Crustacea. 



In observations on the Crustacea from different depths, Zach- 

 arias found the Crustacea distributed as follows: 



August 19. 



— 10 m 

 — 20 m. 

 — 40m 



Hyalo- 

 daphnia. 



540 

 510 

 660 



Copepoda. 



840 

 1,350 

 1,620 



Bosmina. 



?50 

 100 

 100 



Aug. 31— Cyclops 

 (3 obs ). 



690 

 2,190 

 3,510 



690 

 1,500 

 1,320 



These figures indicate that almost or quite all of the indi- 

 viduals of Daphnia and Bosmina were confined to the upper 10 m., 

 and so far his observations agree substantially with our own. 

 They indicate also that the Copepoda extend to very considerable 



