399 



It is noted both of Tubifcx tubifcx and of the more abundant species 

 of Limnodrihis that, quite contrary to the findings in the river above 

 Chiilicothe in 1923, their numbers in upper and middle Peoria Lake ran 

 about as high if not higher in the cliannel than in either of the outer zones. 

 This is, however, in agreement with such information as we have about 

 velocities, which evidently permit much more sedimentation in the lake 

 channel during flood recessions than can ordinarily take place in average 

 river above the lake. 



The sludge worms as a whole showed an irregular decline southward 

 of the upper lake in 1923, both in the channel and in the outer zones, 

 with exception of some upturn in the figures at Peoria Narrows and the 

 U. S. Slips, which are unexplained. Unworked out specific dift'erences 

 might be concerned here. It is at least known that not all species of 

 Limnodrilus. or even Tubifex, forthat matter, are equally tolerant of or 

 partial to polluted bottom. 



Compared with 1922, both the Tubificidae and the very tolerant 

 sphaeriid snail {M. transvcrsmn) showed conspicuous decline in num- 

 bers in the upper lake in 1923 collections. This was true in all zones 

 in the upper lake and should, it would seem, mean improvement. In the 

 middle lake, however, numbers both o.f the worms arid of the snail showed 

 a tendency to equal or even to exceed the numbers per square yard of 

 bottom area found in 1922. Perhaps efl^ects of seining, and of varying 

 amounts of feeding by bottom-ranging fishes might be concerned in 

 these irregularities. If they are, evidence such as we have on the variety 

 of bottom-dwelling forms present, and their preferences, are of more 

 importance in the definition of degrees of pollution than such numbers 

 are likely to be. 



Peoria Lake, 1923 Compared with 1922. Av. Nos. veb, Sq. Yd. 



• Foot of Horshor Isl. 



