120 



October 1 that is, above 60 are irregular and the numbers few 

 (Table I.). It is thus predominantly a cold-water planktont. 

 Winter collections in 1894-95 and 1896-97 were too few to trace 

 its seasonal movements. In 1896-97 it appeared November 5, rose 

 to a maximum of 964,600 on November 27, and declined in the 

 December- January flood (Pt. I., PI. IX.) almost to extinction, but 

 recovered during its decline to a minor pulse of 16,160 on January 

 30. It again fell off in numbers during the floods of February 

 (Pt. I., PI. X.), but rose during the decline of March to a maximum 

 of 104,535 on March 17. Numbers become smaller and occurrences 

 irregular after May 1. 



In 1897, Carchesium increased rapidly in late October to a small 

 pulse of 13,200 on November 2, with a decline in the following fort- 

 night, and a pulse culminating December 7 at 283,800, with subse- 

 quent decline. The fluctuations during 1898 may be followed in 

 Table I. The numbers increase during the slowly rising flood of 

 January to a maximum of 197,600 on February 8 at 32, and decline 

 again during the more rapid rise (Pt. I., PI. XII.) of the next three 

 weeks. Stable conditions in early March bring about a pulse of 

 89,600 on March 15, and numbers decline again to 2,400 as the flood 

 passes its maximum in the early part of April. As the levels fall 

 another pulse of 99,200 appears April 26, from which a descent to 

 minimum numbers which prevail during the summer takes place 

 within a fortnight. The floods, especially sudden ones, seem thus 

 to interfere with the appearance of Carchesium in the plankton, 

 while gradual rises, as that of November, 1898, are not so detri- 

 mental. 



The table of bacterial occurrences (Jordan, '00) in the Illinois at 

 Havana and Pekin given on p. 231, Part I., indicates that the bac- 

 terial development consequent upon the sewage and industrial 

 wastes of Peoria extends down the river to Havana during the 

 colder months of the year. The occurrence of Carchesium in the 

 plankton is thus coincident with that of greatest sewage pollution 

 and bacterial development at Havana. Carchesium is much more 

 abundant in the channel of the river, where sewage pollution is 

 greatest, than it is in the adjacent backwaters. It seems probable 

 that the bacteria either directly or indirectly contribute towards its 

 development, constituting, it may be, an important element in its food. 

 Flood waters, which dilute the sewage (cf . hydrograph and chlorine 



