480 Birge — Vertical Distribution of Pelagic Crustacea. 



nearly one-half of the Crustacea in the upper 3 m. during the 

 summer, and observations continued into the early fall show the 

 same result. It is not necessary to count the Crustacea in order 

 to see the difference in number from different levels. The six 

 bottles of a series can be placed in order with certainty, except in 

 those cases where the bottom level and that next above it con- 

 tain almost nothing. The total amount varied, but that from 

 the upper level was always the largest. 



As confirming the accuracy and sufficiency of the method em- 

 ployed, we may briefly refer to results obtained later in the 

 year — in October. As soon as the temperature of the lake be- 

 came uniform from top to bottom the Crustacea became pretty 

 uniformly distributed, showing an arrangement wholly different 

 from that of the summer months. One example is added: 



Number of crustocea caught. 





Oct. 17, 9 A. M. 



Diaptomus. 



192 

 234 

 186 

 282 

 2-28 

 164 



Cyclops 



D. hyalina. 



Temp. 



0—3 ... 



Meters. 



732 

 666 



1093 

 9T8 

 720 

 696 



4890 



702 

 6? 8 

 576 

 300 

 480 

 564 



0.10 ra 

 12 85° C 



3-6 



6—9 ., 







9—12... 







12-15 





15—18 . 



18 m 



13 10° O 









Total 



1364 



3280 











Other catches were about the same in proportion, though dif- 

 fering in numbers, as would be expected. This disclosure of the 

 change in distribution, which came on rather gradually during 

 some two weeks, together with the concordance in the result of 

 observations taken about the same time, seems to give addi- 

 tional confidence in the results obtained by our methods. 



A second peculiar fact of distribution, also of uniform occur- 

 rence, is the singular vertical position of Daplniia pulicaria dur- 

 ing July. If little confidence could be placed in our methods, 

 this curious result could hardly have been reached. D. pulicaria 

 did not differ widely in number from D. hyalina, yet the distri- 

 bution of the two species was as distinct as possible. An un- 



