130 
amphibians, and on young fish. It appears in the plankton during the 
summer months in every year except 1898, a single record only being 
made in that year. The earliest record was on June 11, and the latest 
on November 31. The whole temperature range is practically included 
in these occurrences, though the species disappears within a few weeks 
after the temperature falls below 50°. It usually appears in small 
numbers and irregularly, and no pulses like those of typical plank- 
tonts can be traced. A free life in the plankton is apparently not its 
usual habit. Zacharias (00) has recently called attention to its 
appearance in the plankton in German waters. 
Vorticella rhabdostyloides Kell.—Average number, 61. This little 
Vorticella is found attached in small clusters to Anabena spiroides 
and occasionally to other members.of the phytoplankton. It is some- 
what common in the waters of Lake Michigan, but is rare in spring 
months in the Illinois River. 
Vorticella spp.—Average number, 7,843. At irregular intervals 
from April to November isolated individuals and small clusters at- 
tached to bits of debris in the silt were taken in the plankton. They 
were most abundant at temperatures above 50°. The irregularity 
in their occurrences indicates that they are adventitious in the plank- 
ton. Identifications of plankton material are impracticable except in 
strongly marked species. Hempel (99) has found V. campanula 
Ehrbg., V. microstoma Ehrbg., and V. sumilis Stokes in the river and 
its adjacent waters. ; 
Zoothamnium arbuscula Ehrbg.—A few colonies were taken in 
August and September in 1896 in the plankton, probably adventitious 
during the disturbed hydrograph of that year (Pt. I., Pl. X.). 
The preceding list of 45 species does not complete the catalog of 
the ciliate constituents of the plankton, though it includes all of the 
species of quantitative importance during the years of our operations. 
The residium of unidentified ciliates, which, excluding the partial 
identifications in the above list, does not often exceed two per cent. 
of the total individual ciliates, includes principally isolated individ- 
uals of species difficult of identification or others whose preservation 
did not permit it, and a considerable number of small ciliates and of 
forms ectoparasitic upon Entomostraca and other planktonts. Most 
of these organisms are either adventitious or passive members of the 
plankton, and further study of the littoral region, of stagnating 
