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, irr 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 



