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in great numbers (see Kelly, '99) in the Unionida which are found 

 in the bottom of the channel. Occurrences in the plankton were 

 limited to the months of May-August, and may be due in part, 

 especially in the warmer months, to the release of the parasites by 

 the death and flotation of their hosts. Flood waters in warm 

 months were often disastrous to the Unionidce because of the load 

 of silt, sewage, and industrial wastes which they carry in channel 

 confines at the lower river stages often prevailing in these months. 

 Other small aquatic Acarina were also present, probably adven- 

 titious from the littoral or bottom ooze. With two exceptions their 

 occurrences in the plankton were all in warmer months, April- 

 September, though not in flood waters. During the period of the 

 migration of waterfowl, parasitic Acarina were noted in plankton 

 collections in a few instances. 



TARDIGRADA. 



Macrobiotus macronyx Duj. Average number, 11. This species 

 is found principally in the colder part of the year, from October to 

 May. The earliest autumnal record was October 30, 1895, at 45, 

 and the latest vernal one, May 1, 1896, at 68.8, and the maximum 

 number (2,980 per m. 3 ) was recorded on April 10, 1&96, at 46.2. Of 

 this number, one sixth were females with eggs. Females with eggs 

 were also found in November, February, and March. Because of 

 its seasonal distribution it is found principally, though not solely, in 

 disturbed hydrographic conditions, and its occurrence in the plank- 

 ton is largely adventitious. 



HEXAPODA. 



Owing to the shoal waters, relatively narrow confines, and the 

 hydrographic fluctuations in our fluviatile environment, the aquatic 

 insects, both larval and adult, have many points of contact with 

 the plankton. They constitute a large element in the total volume 

 of the animal population of shore and bottom, and are all connected 

 by chains of food relations, more or less complex and remote, to the 

 plankton organisms or their sources of food. With the single 

 exception of the larvae of Corethra they are all in the main adventi- 

 tious members of the plankton assemblage, and are much more 

 abundant in the vegetation-rich backwaters than in the channel. 



