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., Pl. 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., Pl. 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., Pl. X11) of the next thnee 
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 Carchestum 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, itmay be, an important element inits food. 
Flood waters, which dilute the sewage (cf. hydrograph and chlorine 
