135 



ton collections. They have occurred singly in February and July, 

 but were not identified. 



The free-swimming larval stages or cercaria of unidentified trem- 

 atodes were also found singly in August, September, and October. 



Aspidogaster conchicola v. Baer, which occurs abundantly in the 

 mantle cavity and pericardium of many of the Unionidcz (see Kelly, 

 '99), which form great beds on the river bottom, was taken in an 

 immature condition in the plankton on June 27. 



Cotylaspis insignis Leidy, likewise a parasite of the Unionidce, 

 associated with Aspidogaster but confined principally to the mantle 

 chamber, was taken in the plankton on February 4. 



CESTODA. 



Tetrarhynchus sp. was -adventitious in the plankton on June 27, 

 and doubtless of similar origin to the adult trematodes above noted. 



NEMERTINI. 



Fresh-water nemerteans were definitely identified as such in the 

 plankton on only two occasions, July 23, 1894, and March 22, 1897. 

 They were doubtless adventitious from the shore or bottom, where 

 they are most abundant. 



NEMATELMINTHES. 

 NEMATODA. 



The free-living nematode worms are predominantly shore and 

 bottom forms, living in the midst of the decaying organic matter of 

 the bottom ooze. In a habitat such as ours, where the quantity of 

 this decaying matter is very great, the nematodes are correspondingly 

 abundant, and, owing to the unstable hydrographic conditions, they 

 find many opportunities of joining the plankton temporarily. Ac- 

 cordingly we find that nematodes are met most frequently and in 

 largest numbers in rising flood waters, when the bottom deposits of 

 tributaries and the main stream are carried in channel waters as silt. 

 Thus, in the month of March nematodes occurred in 13 of the 15 

 collections examined, with an average number per m. 3 of 465, while 

 in August they were found in but 8 of 21 collections, and averaged 

 only 186 per m. 3 . So, also, in the winter flood of 1895-96 nematodes 

 were found in the plankton almost continuously till the middle of 



