thousands per square yard. None of the reaches of Illinois River from 

 LaSalle to Beards town had an average count of tubificids as high as 

 Station 250 (Richardson, 1928). However, LaSalle is about 100 miles 

 below the principal source of pollution (Chicago), and it is probable 

 that parts of the river above LaSalle would show much higher counts 

 than those below (see Forbes and Richardson, 1913) . 



At Station 252 there was marked improvement over the station 

 nearer the mouth of the river. All of the groups of organisms listed in 

 the table were found in at least one of the two years, while at 

 Station 2^0, four groups were absent in both years. More striking 

 evidence of improvement is seen in the great reduction in numbers of 

 tubificid worms and Musculiiun, and the appearance of Hexagenia. 

 Comparing the two years at Station 2^2 we find that the number of 

 tubificids remafaed almost unchanged, but Hexagenia and Musculium in- 

 creased. These and other differences, such as the increase in numbers 

 of Hirudinea, Chironomidae, and Amnicola, are considered to be of no 

 significance as indicators of changes in the degree of pollution. 

 According to the number of tubificid worms, this station belongs to 

 the zone of moderate pollution. The single sample taken in 1928 showed 

 6,320 tubificids per square meter, but no Hexagenia. 



The data for Station 2% are rather contradictory in that 

 both tubificids and Hexagenia increased in 1930 over 1929, while most of 

 the associated forms decreased. In the season of 1928, 10 samples were 

 taken at this station on four dates. The mean number of Hexagenia was 

 38 per square meter, and of Tubificidae, l63 per square meter. These 

 results fall between those of 1929 and 1930. Differences in the data 

 of the three years are probably best explained by the existence of marked 

 inequalities in distribution. That the distribution is not uniform is 

 shown by the fact that, in the various samples taken in 1930 the number 

 of Tubificidae ranged from 7 to 1,930 per square meter, and Hexagenia 

 from 27 to l68 per square meter. It is probable that the channel was the 

 most important factor in determining such large differences in samples 

 taken within a small area. It will be shown later that organic debris 

 tends to be confined within the channel, and we should expect a marked 

 falling off in pollution with increased distance from the channel. It 

 is, of course, impossible to take successive samples at exactly the same 

 point, and differences in position relative to the channel would be 

 reflected in differences in the character of the faima. Averages of the 

 index organisms for the three years place this station in the zone of 

 light pollution. 



272 



