116 
limnetic ciliates are very few. As such we may include Codonella 
cratera, Tintinnidium fluviatile, and possibly Stentor niger. Car- 
chestum lachmannt and Epistylis enter the plankton only in the 
form of detached and often moribund zoédids, and thus are not 
typical planktonts, though of quantitative importance in our plank- 
ton in the colder months. A large number of species not here 
reported occur in our collections made elsewhere than in the 
river channel, especially in places where the decay of large quan- 
tities of organic matter is in progress. This is not a condition 
normally found in the open water of lakes, though it may occur 
along their shores, where vegetation is found, or in regions of 
sewage contamination. In the waters of the [llinois, on the 
other hand, the current, combined with sewage and industrial 
wastes and the organic detritus from the richest of fertile prairies, 
provides a suitable environment, even in the open water, for 
the support of a ciliate fauna of a magnitude somewhat unusual 
in fresh-water plankton. This fauna is present also in the back- 
waters, but is less abundant there than in the river itself. These 
species occur in greatest numbers of individuals 1n our plankton dur- 
ing the winter months at minimum temperatures, rising in November 
as the temperature falls below 50°, and declining again as 1t rises to 
this point in April. As shown by the bacteriological investigations 
of Jordan (00) and Burrill ('02 and ’04), the bacterial pulse attend- 
ing the decay of the sewage and wastes at Peoria does not reach 
Havana during the warmer months (see table on p. 231, Pt. I.), 
but when temperatures pass below 50° in November the increase in 
bacteria is marked. The decay is less rapid at low temperatures, 
and the process is still going on when the water in the channel 
passes Havana during the prevalence of low temperatures, and the 
ciliates that thrive in such an environment abound in the plankton 
at that time. 
The temperature limits of these ciliates of the period of bacterial 
development thus seem to lie between 50° and 32°. An examination 
of the plankton in the river at several points between Peoria and 
Havana at intervals throughout a year, will reveal how far the 
component species of this ciliate fauna are governed in their seasonal 
distribution in the plankton at Havana, respectively, by conditions 
of temperature and by the state of sewage contamination. The 
work of Roux (’01) upon the Czlzata about Geneva would seem to 
