66 
The increase in sludge-worms in Liverpool, Dogfish, and Thompson 
lakes since 1915, though comparing with decreases in midge larvae, as 
already mentioned, was greater even than the increase in these worms in 
the Illinois River channel opposite, apparently proving that this group has 
been able to accompany the pollution inward from the river, even if the 
midges have not. The sludge-worms in the first-named lake amounted 
in figures per square yard to 389 in September, 1920, as compared with 
none at all in the collections of July—October, 1915; and in pounds per 
acre to 14 in 1920 as against none in the former period. In Quiver Lake 
in 1920, both in the muddy portion and in the sandier areas, while the in- 
crease in small oligochaetes since 1915 is evident enough, it is very much 
less than in the other three lakes. The small oligochaetes taken in Liver- 
pool, Dogfish, and Thompson in 1920 were nearly all Tubificidae and a 
great majority of them belonged to the genus Tubifex. In the Quiver 
Lake samples, on the other hand, small non-tubificid Oligochaeta were 
visibly more common than Tubificidae. 
SLUDGE-WORMS, (PRINCIPALLY TUBIFICIDAE), 
LAKES NoRTH OF Havana, 1914—15 anp 1920 
I. Mud bottom, 2 to 6 feet 
Number per square yard Pounds per acre 
1914—1915 1920 | 1914—1915 1920 
Liverpool Lake* 0 389 0 14 
Thompson Lake* 0 128 0 4 
| ! 
Dogfish Lake; 0 128 |) 0 4 
Quiver Lakey | 12 48 | 0.4 2 
Ill. River, Liverpool to Ha- Channel, 0 89 -| 0 3 
vana, 1915, for comparison; 4—7ft., 0 4 0 0.1 
II. Sandy or sand and mud bottom 
Quiver Lake ‘‘channel” * 0 | 0 0 0 
| 
Quiver Lake, sandy shore | Not 
east side Not separately counted 30 separately 1 
. | valued 
* Figures in 1914—1915 columns are for year 1915. 
+ Figures in 1914—1915 columns are for year 1914. 
