252 LEE RAYMOND DICE 



the surface during the sunny periods, but rose to one-half meter 

 during the cloudy intervals. ; 'The rise immediately followed 

 the obscuring of the sun and the return was as prompt when 

 the sun shone again." 



The same author (1897) was unable to find that wind and 

 waves affect the distribution of the plankton Crustacea. The 

 upper meter was fully occupied by them when the lake was almost 

 too rough to venture out in a row boat. These results do not 

 agree with those of France (1894), who found that in stormy 

 weather the Crustacea were found at a greater depth than during 

 still weather. 



In those lakes where the water in the lower levels becomes 

 stagnant at certain periods in the year the Crustacea are usu- 

 ally found to be entirely absent below the thermocline (Birge 

 1897). In these lakes the daphnids in their seasonal migra- 

 tions move down, closely following the downward movement of 

 the thermocline. 



In the Vierwaldstattersee, Burckhardt (1900) observed a 

 diurnal movement of Daphnia hyalina of about 32 meters during 

 the early part of September. The mean depths of these daph- 

 nids at different times in the day on September 1st and 2nd 

 are given below: 



9 P.M. to 4 A.M. at 5 meters, 

 at 9 A.M. at 30 meters, 

 at 12 M. at 37 meters, 

 at 5 P.M. at 28 meters. 



From the above observations, we find that there is often a 

 diurnal migration in Daphnia, and that in such cases the animals 

 are found nearest the surface at night and deeper in the lake 

 during the day. This diurnal migration varies greatly in extent 

 in different lakes and in different seasons. In Oconomowoc 

 Lake, during October, there is a diurnal movement from a 

 depth of 12 meters to the surface, while in the same lake in June 

 the movement is only from a depth of 1 meter to the surface. 

 In some lakes there is only a slight diurnal movement, while in 

 others no diurnal movement has been found. 



In many lakes there is an annual movement, which may be 

 illustrated by the movement of Daphnia pulex pidicaria in 

 Lake Mendota. Here the animals have a definite vertical range 



