394 



Chirononms dccorus, has a previous definite record of pollutional habit ; 

 and three species of sphaeriid snails, of which one, Musculhim transvcr- 

 siim, has previously been found in company with tubificid worms where 

 their numbers exceeded 40,000 per square yard. Both in the channel 

 and the outer zones of the river between La Salle and Henry, in the first 

 38.5 miles of the section dredged in 1923, nothing at all but tubificid 

 worms were taken. Miisculium transz'ersum was taken at most stations 

 between the Henry dam and Chillicothe (15.5 miles) ; the few kinds of 

 midge larvae and leeches not until Lacon or below : and the two less 

 tolerant Sphaeriidae only at or within two or three miles of Chillicothe. 



The bulk of the tubificid worms taken in the more polluted reaches. 

 as in upper Peoria Lake in 19-?2 and 1923, were not identified as Tttbifc.v 

 tubifcx, the common pollutional form recorded in Europe, but as species 

 of the genus Limnodrilus. 



The largest numbers of all tubificid worms combined, and of Tubifcx 

 tubifcx and the two principal species of Limnodrilus taken singly, per 

 unit of bottom area, did not occur in the river channel, but in the shal- 

 lower water close to or outside the 3-foot line, where conditions favor 

 the greatest sedimentation. 



Peoria Lake, 

 ChiUicothe— P. P. U. Bridge*, 20 Miles 



In Peoria Lake in 1923 rather rapid increase is visible in the variety 

 of the small bottom fauna as we proceed southward, as well as increase 

 of the less tolerant forms in identical areas as compared with 1920 and 

 1922. But it is equally clear that the increases in kinds and in abun- 

 dance of the less tolerant groups are greatest, and that they in fact show 

 a strong tendency to be localized, either (1) quite close to shore; (2) in 

 the faster current of Peoria Narrows or other essentially river situations 

 included in the comparisons; or (3) in the rather limited area represented 

 l)y the southwest third or less of the lower lake that lies between Long 

 Shore Beach and Al Fresco Park. Except at a few stations in swifter 

 current*, also, numbers and bulk of the species with greatest preference 

 for clean bottom are uniformly nuich smaller per unit of bottom area 

 than are the numbers and volume of the tubificid worms and others of 

 the pollutional or more tolerant groups taken alongside of them. 



* Where bulky growths of sponge or Bryozoa occurred. 



