15 



The Entomostraca include the largest fresh-water planktonts, 

 and in every respect constitute an important element of our river 

 plankton. They form the final link in the food cycle which con- 

 nects the nutrients in solution in the water and in decaying detritus 

 with the fish and other aquatic vertebrates. They include numer- 

 ous species, some of which are adventitious. All of the Ostracoda 

 belong to this latter class. The Cladocera furnish some of the 

 most important eulimnetic species and a large number of adventi- 

 tious forms, while the Copepoda are almost wholly eulimnetic. 



In addition to these groups, the Turbellaria, Oligoch&ta, Hexap- 

 oda, Hydrachnida, Gastrotr-icha, and Bryozoa furnish a few species 

 and individuals of a semi-limnetic or adventitious character to the 

 plankton. 



In the table which follows, these various groups are listed, and 

 the number of forms occurring in each is noted. In order to give 

 some idea of the proportionate representation of these groups in 

 our plankton, the table includes the sum of the number of indi- 

 viduals per m. 3 of water in the weekly collections for the year 1898. 

 This was a year of no marked departure from the normal regimen 

 of hydrographic conditions (Part I., PI. XII.). The summer and 

 autumn flushes tend to lower the population somewhat below that 

 of more stable seasons, but beyond this feature there is nothing to 

 suggest that the plankton of this year may not represent a fair 

 average of that recurring each year in the Illinois River. The fig- 

 ures given, in all cases refer to the number of individuals per cubic 

 meter (excepting only such cases as Synura and Uroglena, where 

 the colony rather than the individual becomes the unit). The algae 

 and Protozoa include many species enumerated in filter-paper col- 

 lections, which accounts for the large numbers in some of the totals. 

 The "number of forms" listed refers to the total number found in 

 the waters of the river during the period of our operations. Some 

 species not noted in 1898 are therefore included. Unidentified 

 forms are not included in the list of number of species, though the 

 groups here listed to which they belong were known. Some forms 

 referred to genera but not determined as to species are, however, 

 included. 



This table throws some light upon the ecological relations of 

 the groups composing the plankton, since it gives some clue to their 

 relative numbers, and these condition in a general way the food 



