372 Birge—The Crustacea of the Plankton. 
the species. The most conspicuous case occurred in August, 
1895. On the 21st of the month the catch of Daphnia pulicaria 
was somewhat under 500; on the 22d it was nearly 2,600, and 
on the 27th it was only 85. This aggregation of the species 
was due to the wind carrying a current of warm water through 
the deeper levels at the point of dredging and so driving into 
deep water the individuals near shore, and the decline in number 
was due to the removal of the large numbers by currents rather 
than to the final scattering of the swarm. 
When a species has once aggregated in this manner, the aggre¬ 
gation may last for a cod siderable length of time; and Daphnia 
pulicaria always showed a greater range of variation in its 
numbers than did any other species, apparently due to these tem¬ 
perature aggregations in summer. For example, on April 18th, 
1896, at one point in the lake, 3,060 of this species was caught; 
while another catch, at a distance of some two kilometers, 
showed only 230. On December 23, 1895, two catches were 
made of 260, and 3,440 respectively. See also the lateral dis¬ 
tribution in Table XXVI, above, which discloses a similar want 
of uniformity. A distribution so irregular as this, it seems to 
me, fairly warrants the title of “swarm. ” I may add that late 
in the spring the species become more uniformly distributed, 
and when at its maximum showed a variation of less than three¬ 
fold in 10 catches, distributed over 21 days. 
(2) Apstein has found no case where a swarm has been seen. 
I have observed true swarms of Daphnia hyalina on at least three 
occasions. On October 17th, 1895, about 9 a. m. a large swarm 
of this species was seen at the surface near the dredging sta¬ 
tion about 800 meters from the shore. The water was perfectly 
calm, and the sun was bright. The Daphnias were aggregated 
at the surface to a depth of about 5 cm. or less and within that 
depth the water was completely filled with them. The swarm 
was about 50 meters in width, and its edges were perfectly dis¬ 
tinct, as the boat passed slowly in and out of it. The length of 
the swarm was probably three times the width. All of these 
animals were adult, so that they were easily seen with the naked 
eye. The occurrence was the more unusual as the bright sun 
should have kept this species well below the surface. 
