341 



Small Bottom Animals 



Nineteen species of small bottom invertebrates were recorded from 

 twenty-three dredging stations between Chillicothe and a mile and a 

 half above Spring Bay, near the lower end of upper Peoria Lake, in 

 the summer of 1922 (Table, p. 342). This increased number compare; 

 with eleven or twelve kinds taken at eight stations in very nearly the 

 same area in the corresponding months of 1920. That the increase 

 may to some extent be due to the greater number of stations visited, 

 particularly in the wide waters, in 1922, has not been allowed to escape 

 our notice. IMore probable explanations, however, include the increase 

 in the worm list (+2) due to a more thorough study of that group 

 in 1922, and the evident increase, mostly in the wide waters, of the 

 less tolerant snails (+5), that seem to have escaped total destruction 

 between 1915 and 1920, but that had not had time by 1920 to extend 

 again over their old range sufficiently to make the chance good of 

 getting them in collections. It will be noted, also, in ensuing pages, 

 that other equally important changes were apparent at the end of the 

 two year interval that could have no possible connection with an in- 

 crease in the number of stations. 



Both in 1922 and 1920 the majority of the species taken in the 

 upper lake, whether in channel or wide waters, belonged to the more 

 distinctly pollutional or tolerant groups of small bottom animals; that 

 is, small annelid worms (Tubificidae), leeches, midge larvae, and toler- 

 ant snails. The more sensitive varieties of the old snail, insect, crus- 

 tacean, and other small bottom population of 191.j and before were 

 in 1922 as in 1920 wholly absent from all hauls taken in the upper 

 lake more than 50 feet from shore. There was, however, to be set 

 down fairly definitely on the side of improvement during the two years 

 following 1920 the noticeable increase in the number of kinds and 

 abundance of the tolerant snails, already mentioned ; and, more quali- 

 fiedly, because they may have merely been washed out by the violent 

 flood of the spring of 1922, the decrease in both numbers and kinds of 

 more pollutional midge larvae. Suggesting caution, on the other hand, 

 in accepting at face value these apparent evidences of improvement, 

 was the remarkable increase in abundance at all stations in the upper 

 lake in 1922 of the small oligochaete worms, which were present in 

 numbers exceeding any that we have before recorded from anywhere 

 in the Illinois River. 



