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 112. 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 K/izopoda 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. discotdes 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) 
