99 



June-September, in water at or above 70, while in the remaining 

 eight months there were but 60 occurrences. This contrast is 

 heightened by the ratio of occurrences to the total number of collec- 

 tions, which in the period from June to September inclusive is 55 

 to 68 and in the remainder of the year only 60 to It2. - The num- 

 ber per cubic meter is also higher during this warm period, averag- 

 ing for a single occurrence 1,376 to 1,028 for one in the remainder of 

 the year. The average for the colder months falls to 850 if the 

 large accessions attending the floods of February and November are 

 omitted in the totals. The same causes efficient in determining the 

 summer maximum in other Rhizopoda of the plankton are doubtless 

 operative here, and as in A. vulgaris the impetus of the summer in- 

 crease is carried over into the autumn, causing a slight increase in 

 numbers as compared with the numbers at corresponding temper- 

 atures in the spring months. It seems probable that high temper- 

 atures favor its occurrence in the plankton, not, however, directly, 

 but because of greater abundance of food under those conditions, 

 greater metabolism, and the storage of the products as oil or gas 

 vacuoles which tend to lower the specific gravity and thus to bring 

 the animal into the plankton. 



The adventitious occurrence of A. discoides in the plankton is 

 shown by the fact that 45 of the 115 occurrences are with rising 

 flood waters. The greater part of them lie in the colder months; 

 in fact, nine tenths of the occurrences between October and May are 

 correlated with flood movements. For reasons above given, how- 

 ever, A. discoides may be regarded as temporarily adopting a lim- 

 netic habit during warm months as a result of its physiological 

 condition ; at least many individuals of the species exhibit this habit 

 during the warmer months. The data do not indicate that the 

 open water is at any time the center of distribution of the species. 



There are no indications of recurrent pulses in the species and, 

 as might be expected in case of adventitious planktonts, but little 

 evidence of a characteristic seasonal distribution. There is some 

 evidence that the summer is the period of most active multiplica- 

 tion, and that an exceedingly transparent and hyaline form other- 

 wise resembling A. discoides is the young of this species. In 1898 . 

 separate records were kept of the two types with the result that they 

 were about equally abundant 24,159 and 26,387 for the brown 

 and hyaline types respectively. 



(8) 



