116 



limnetic ciliates are very few. As such we may include Codonella 

 cratera, Tintinnidium fluviatile, and possibly Stentor niger. Car- 

 chesium lachmanni and Epistylis enter the plankton only in the 

 form of detached and often moribund zooids, 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 Illinois, 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 in our plankton dur- 

 ing the winter months at minimum temperatures, rising in November 

 as the temperature falls below 50, and declining again as it 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 Ciliata about Geneva would seem to 



