O2 
1895 and 1896. It occurred from the first of May till the end of 
August, but always in small numbers. It 1s occasionally abundant 
in backwaters where there is much vegetation. : 
In addition to the Mastigophora above listed there were many 
individuals belonging to unidentifiedspecies. 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 Riizopoda, 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 teinporary constituents of 
the plankton. 
The seasonal distribution of the total Riizopoda 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 
