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. 



