10 



pendence which approach closely that found in young fish and the 

 large insect larvae organisms not always regarded as planktonts. 



The plankton is thus a composite assemblage of organisms whose 

 association depends in varying degrees upon their relation to their 

 common habitat, the open water. In actual practice, all the organ- 

 isms found in the open water are regarded as within the scope of 

 plankton investigations, and justly so, for by virtue of their pres- 

 ence they become more or less involved in the complex interrela- 

 tions which pertain to the flux of matter, the succession of species, 

 and the food relations which exist through the changing seasons in 

 the aquatic environment. 



In our own investigations it has been our purpose to include all 

 the organisms found in our collections ; that is, all which our meth- 

 ods of examination give us a sufficient means of investigating. 

 Naturally, the bacteria are to large extent excluded from our con- 

 sideration, though they properly belong to the plankton, and in the 

 processes of nitrification and denitrification play an exceedingly im- 

 portant part in the economy of aquatic life. 



THE COMPOSITION OF THE PLANKTON. 



The composite character of the plankton is especially marked in 

 streams , as, for example , in the Illinois River, owing to the mingling 

 of organisms from a great variety of tributary sources backwaters, 

 lakes, ponds, pools, marshes, swamps, brooks, rivers, canals, sewers, 

 drains, and industrial wastes. Few lakes possess so varied a supply, 

 and in none can the proportional effect of these contributions exceed 

 that of the stream. Added to this contributed assemblage, and in 

 some seasons predominating over it, is the indigenous or autono- 

 mous plankton of the stream itself. 



The component organisms of the plankton of the Illinois River 

 number 528 forms, including only those which have been identified 

 from collections made in the main stream and including both 

 species and well-defined forms or varieties. Species found thus far 

 only in the backwaters are not included, though there is little doubt 

 that they occur also in the main stream. No effort has been made 

 to build up merely a long list of species, but only to identify, so far 

 as possible, the common and recurring forms. Neither has any 

 attempt been made to establish new species or revise those already 



