874 
Birge—The Crustacea of the Plankton. 
The average of D. hyalina in the 0-3m. level for the first half 
of November was 32,200 per cubic meter, of which at least half 
were immature, so that the catch of November 3d was not an 
exceptionally low one. These facts show that the swarm in 
question was a lateral aggregation and not merely a gathering 
at the surface of the individuals ordinarily below it. 
Great numbers of individuals broke through the surface film 
of the water on all of these occasions. 
This aggregation of Daphnia hyalina in swarms is probably 
more frequent than the number of observations would indicate. 
The swarms are found in the surface water, so that they are 
dislodged by the slightest breeze, and it is impossible to see 
them unless the water is entirely smooth. This condition is not 
often reached, and I have felt myself exceedingly fortunate in 
being able to observe this phenomenon on so many as three oc¬ 
casions. I may say, however, that during the autumn of 1896, 
I looked for these swarms on every calm day when it was possi¬ 
ble for me to go out on the lake, but found them only twice. 
The significance of these aggregations is difficult to state. 
The habits of the animal are completely reversed in one respect. 
The adults are strongly negative in their relation to light, and 
under the conditions of all these occasions should have been 
found at a depth of one-half to one meter below the surface. It 
is possible that these aggregations represent the remains of 
a former sexual period. This may be indicated by the presence 
of the ephippial female. I have no doubt that Daphnia hyalina 
had at one time two sexual periods, in spring and fall, of which 
these swarms may be a remainder, but since the few males 
which appeared in the fall came at a time decidedly later than 
the earlier of these aggregations, I do not feel warranted in 
positively interpreting the swarms in this sense. 
These swarms of Daphnia seem to be phenomena of the same 
order as those described by France (’94, p. 37). In one case the 
swarm was near the littoral region, as were those described by 
him. In the other cases they were well out in the limnetic re¬ 
gion. The swarm was confined within vertical limits even nar¬ 
rower than the one meter named by him and in all three cases 
the swarm was “von weitem erkennbar. ” 
