101 
half the collections, the river levels for this latter year being subject 
to more than the usual disturbance. 
-Arcella vulgaris is found throughout the whole year, with a 
marked predominance of occurrences during the warmer months, 
June to September inclusive, for during this period,in which a total 
of 68 collections were made, this species was found in the plankton 
34 times. If the month of October be included, the ratio is 44 oc- 
currences in 83 collections, while in the remaining 97 collections, 
from November to June, only 17.occurrences were recorded. Of 
the 10 occurrences in October, 7 were in water at or above 55°. The 
season of frequency in the plankton thus ranges from June 
through October. In both frequency of occurrence and in numbers 
of individuals (see Table I.) there is an apparent maximum in 
August, preceded by an increase in June and July and followed by 
_a decline in September and October. Arcella vulgaris thus seems 
to be a late summer planktont. The continuance into October 
may in part be due to the temperature conditions above cited, and 
perhaps also to constant seining of the river by fishermen in the 
low-water stages at that time, causing repeated disturbances of the 
bottom and shores, where Arcella habitually lives. This maximum 
frequency of Arcella during the warmer months in the plankton 
is, however, probably due to the formation of gas or oil vacuoles in 
the plasma under the conditions of higher temperatures. Their 
flotation is thus facilitated, and they become, in a way, semi-active 
but temporary planktonts. 
That floods are also in part responsible for the presence of Arcella 
in the plankton is evident from the fact that 32 of the 61 occurrences 
come with rapidly rising waters, or shortly after rapid rises, during 
the interval of rapid decline. The larger numbers of individuals 
also appear in flood-waters, occurrences of more than 1000 per cubic 
meter happening 10 times with floods to only 4 in more stable 
conditions. The maximum occurrence, 25,272 per cubic meter, 
came with the flood of February, 1898, indicating the presence of 
this species in large numbers, even under winter conditions, in some 
local environment tributary to the flood plankton. 
The average number per cubic meter in the 61 collections con- 
taining Arcella was 1,260; and the maximum, 25,272,as above noted. 
This species occurred in only 10 collections in stable conditions of 
the river, when the temperature of the water was below 55°. The 
