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 1 1 , and the latest 

 on November 3 1 . 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 Anabozna spir aides 

 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. similis 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 



