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, Oligocheta, Hexap- 
oda, Hydrachnida, Gastrotricha, 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.* of water 1n the weekly collections for the year 1898. 
This was a year of no marked departure from the normal regimen 
of hydrographic conditions (Part I., Pl. XII.). The summer and 
autumn flushes tend to lower the population somewhat below that 
of more stable seasons, but beyond this feature there 1s 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 alge 
and Protozoa include many species enumerated in filter-paper col- 
lections, which accounts for the large numbers 1n 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 
