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 Illinois 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 algae, 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 30-45/1 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 Ciliata are of wider occurrence in the plankton 

 than has hitherto been found to be the case. 



