117 
indicate that many species of the fauna of stagnant water are more 
abundant in that region during the winter months. Owing to the 
difference in food conditions attendant upon the increase of sewage 
and bacteria during the colder months in the Illinois River, it is 
impossible to determine from the data at hand the relative efficiency 
of the two elements of temperature and food in regulating the 
seasonal occurrences of our ciliates. 
Here, as elsewhere, the disastrous effect of sudden floods can be 
traced. The number of ciliates (Table I.) drops as floods rise, and 
recovers as the waters fall again. For this reason the winter occur- 
rences of the total ciliates are subject to considerable disturbances 
in the winter floods of the several years. The combination of the 
two methods of collection and of the two groups of ciliates, typical 
and adventitious, causes further irregularities (Table I.) in the sea- 
sonal distribution of totals. 
In the Ilinois River, for reasons given above, the Ciliata occupy a 
place in the economy of the plankton of more than the usual im- 
portance. They feed principally upon bacteria, decaying organic 
matter, and the smaller alge, and are themselves eaten by the 
rotifers. I have found no evidence that they are utilized by the 
Entomostraca. They thus become active agents in the reduction 
of sewage and in the destruction of the bacteria of decay, in the 
purification of sewage-laden waters, and in the transfer of the matter 
in sewage to higher forms of animal life. 
The ciliates found in the Illinois include all the important species 
reported in the plankton of fresh water, and the list is somewhat 
larger than hitherto recorded in quantitative plankton collections 
in river or lake waters. These organisms escape readily through the 
silk net by reason of their small size, and in some instances the 
larger species, by reason of their mobility and flexibility, escape 
through the silk where less motile organisms of equal size are re- 
tained. By experiment I have found that well-shrunken silk 
bolting-cloth whose meshes average about 30-45 » will not retain 
Paramecium whose diameter is 40-70. It may be that supple- 
mentary methods of collection which will correct the error of leakage 
will show that the Cilzata are of wider occurrence in the plankton 
than has hitherto been found to be the case. 
