7 
species and groups through the collections of 1894-1899 were drawn 
up on uniform folio sheets, and the annual totals and averages com- 
puted therefrom. With the data in these forms it is possible to 
turn at once to the statistics of the plankton of a given day, or to 
the seasonal distribution of any desired species. ° 
ACKNOWLEDGMENTS. 
I am indebted to Prof. S. A. Forbes, Messrs. E. B. Forbes, F. W. 
Schacht, and R. W. Sharpe for many suggestions concerning the 
Entomostraca; to Prof. Frank Smith for assistance with the Oligo- 
cheta of the plankton; and to Mr. A. Hempel for my introduction to 
the Rotijera. The identification of the cosmopolitan species of the 
fresh-water plankton of the Illinois River was greatly facilitated by 
the most excellent library of the Illinois State Laboratory of Natu- 
ral History, the accumulation of many years’ careful selection by 
its director, Prof. S. A. Forbes. The literature of fresh-water fauna, 
and to a large extent of its flora also, is very fully represented therein. 
The excellent Laboratory collection of identified Entomostraca 
from European specialists was also of great service. 
I am indebted to Mr. R. E. Richardson for valuable services as 
clerical assistant, and for substantial help in organizing the great 
mass of data resulting from the enumerations. 
Except as noted in the discussion in subsequent pages, I hold 
myself responsible for all of the identifications of the species re- 
corded. The enumeration is also all my own work, with the excep- 
tion of that of the nauplii, of two species of Difflugia, and of Pedi- 
astrum in about one third of the collections, in which I had the 
assistance of Mr. R. J. DeMotte, and that of the commoner Rotifera 
in a few of the collections, which were counted by Mr. Richardson. 
DEFINITIONS. 
The term “plankton” was used by Hensen (’87) to designate 
“Alles was im Wasser treibt.” It was applied by him only to that 
assemblage of marine organisms which float passively in the open 
sea, without active recourse to shore or bottom, and unable by their 
own efforts materially to change their location. The term has 
since been extended also to assemblages of organisms in fresh water 
which bear a similar relation to open water. This fresh-water 
