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. 3 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. 3 



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 Catostomida 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. 

 OLIGOCH.<ETA. 



The representatives of this order belong to the smaller aquatic 

 species generally littoral or limicolous forms found especially in 

 decaying vegetation or among Lemnacea, and belonging principally 

 to the family Naidida 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 is 

 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. 



