284 
in great numbers (see Kelly,’99) in the Unionideé 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 Umionide 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.*) was recorded on April 10, 1896, 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. 
H ESxCAP OD AS 
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 larvee 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. 
