57 
While sludge-worms showed some increase in this section since 1915, 
their gain was slight and of very small moment compared with the in- 
creases shown in five years either in the Peoria section or in the reach 
just north of Havana. All of the small oligochaetes taken, however, be- 
longed to genera of the family Tubificidae, about half of them being 
species of the genus Tubifex. 
Though, as already mentioned, a marked increase in variety as well 
as an actual average increase in poundage since 1915, is plainly shown 
by the group of “other insects, etc.”, there were a good many eliminations 
from the lists even in this group in the five years preceding 1920. Non- 
insect groups or insects other than Chironomidae that were readily 
taken in this part of the river in 1913—1915 but that did not ap- 
pear in collections in 1920, include the little crustaceans Asellus and 
Hyalella ; two or three species of Ephemeridae; two species of Corixidae; 
adult bottom-beetles of the genus Stenelmis ; several fresh-water sponges ; 
-and at least one genus and species of Bryozoa, namely, Paludicella sp., 
which was formerly common on almost all the dead shells of Unionidae. 
Total average-valuations of all small bottom-animals for this sec- 
tion in 1920—30 pounds per acre in the channel, 89 pounds in the 4—7- 
foot zone, and 526 pounds in the 1—3-foot zone—exceed 1915 figures in 
the first and last instances; while in the 4—7-foot zone they were well 
under the older valuations, as shown in the table following. Both in the 
channel and in the 1—3-foot zone the decreases in snails were more than 
made up for by increases in midge larvae and “other insects” and mis- 
cellaneous species. In the 4—7-foot zone the increase in snails was great- 
er and the increase in Chironomidae and minor insects, etc., less, either 
relatively or absolutely or both,-than in the 1—3-foot zone. To what 
extent these zonal differences in distribution both of the total bottom 
fauna and its subdivisions may be connected with differences in whole- 
someness of conditions on different sections of the river floor, or to what 
extent they may reflect mere casual migration of surviving species fol- 
lowing a general mortality, our present data give us no certain basis for 
an opinion. 
