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- 

 ch&ta of the plankton ; and to Mr. A. Hempel for my introduction to 

 the Rotifera. 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 oi^ttifflugia, 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 



