136 
April, while in the more stable conditions of the preceding year they 
were found in only one third of the collections. In 1897 most of the 
31 collections examined were made in stable conditions, and nema- 
todes were found in but 5 of these, and 4 of these 5 were made in 
rising flood waters. In 1898, a year of greater hydrographic dis- 
turbance, nematodes occurred in 31 of the 52 collections, averaging 
318 per m.? to 82 in 1897.. Of the 31 occurrences in 1898 all but 6 
were in recent flood waters. The hydrographic conditions attending 
the presence of nematodes in the plankton thus indicate that they are 
adventitious in the plankton. Further evidence of this is to be found 
in their erratic numbers. Thus, on February 20, 1896, none was re- 
corded, and on the 25th their numbers rose in flood waters to the 
maximum record for all of our collections—18,422 per m.? 
No effort was made to determine the species of these nematodes. 
A considerable variety of forms awaits the labors of some courageous 
systematist. 
ACANTHOCEPHALA. 
These worms are found abundantly in the Catostomde and other 
limophagous fishes of the Illinois River, and in many of the water- 
fowl which feed in its waters. A chance occurrence of a single 
specimen in the plankton on August 3, 1896, is probably to be ac- 
counted for as in the case of other intestinal parasites. 
ANNULATA. 
OLIGOCHATA. 
The representatives of this order belong to the smaller aquatic 
species—generally littoral or limicolous forms found especially in 
decaying vegetation or among Lemnacee, and belonging principally 
to the family Naidide—and usually occur in the plankton in 
mutilated condition, since autotomy occurs when the preservative 
is added to the plankton. Specific identification of the fragments 1s 
therefore often impossible and usually of questionable certainty. I 
am indebted to Professor Frank Smith for assistance in such identi- 
fications as have been made. The following list (see Smith, ’00) 
gives the relative frequency of the species from which accessions to 
the plankton are made, with my notes on identified forms in the 
plankton. 
