92 



1895 and 1896. It occurred from the first of May till the end of 

 August, but always in small numbers. It is occasionally abundant 

 in backwaters where there is much vegetation. 



In addition to the Mastigophora above listed there were many 

 individuals belonging to unidentified species . They were as a rule the 

 smaller forms, which are not readily identified in preserved material 

 and under the conditions of plankton enumeration. They consti- 

 tute about twenty-six per cent, of the total Mastigophora enu- 

 merated. In silty planktons their number is relatively somewhat 

 larger on account of the difficulties attending the determination of 

 species in such material. These unidentified flagellates occur in 

 every collection, and are somewhat more abundant in the summer 

 months. 



RHIZOPODA. 



Average number, 55,364, including filter-paper collections; 

 23,826 without them. This group of Protozoa is numerically of less 

 importance than the ciliates or flagellates, but its quantitative 

 significance is greater than the numbers of individuals indicate. 

 This is due to the relatively large size of the Rhizopoda, and also to 

 the fact that plankton collections afford only an irregular and in- 

 complete record of the rhizopodan fauna of any body of water, and 

 give but an imperfect idea of the part which these organisms play 

 in the total economy of the lake or stream. This results from the 

 fact that they are as a rule largely bottom or shore-loving species, 

 and are generally either adventitious or temporary constituents of 

 the plankton. 



The seasonal distribution of the total Rhizopoda in the Illinois 

 River gives evidence of the adventitious or temporary nature of the 

 contributions of the group to the plankton. There are pulses in 

 1898 on January 25 (66,388), February 22 (141,524), August 23 

 (36,800), September 27 (59,200), and November 15 (42,000), all of 

 which appear on rising water and are largely adventitious, their 

 presence in the plankton being due to the disturbances of currents, 

 waves, and the like. There are pulses on May 10 (49,800), June 28 

 (37,000), and July 19 (28,800) which cannot be traced to any 

 general hydrographic condition. These, as will be suggested in the 

 discussion of the seasonal fluctuations of individual species, are 

 probably due to the temporary adoption of a limnetic habit on the 



