374 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. " 



