128 



2. Annri'ii nciih'iitd and TfiiirtJint occupied the region below a depli 

 of ten nu'tei-s. They Avere usually most abundant -within two or three 

 meters of the bottom. 



.S. A II urea cocJiUaris and Xnthnlca Jmuiisjiina were found tlirougli the 

 entire depth of the lake. 



MAXIMUM NUMBER AT SURFACE. 



This diurnal migration nmst not he taken to mean that the individuals 

 of the various species concerned, congregate at the surface at night in 

 such numbers as to form what might be called a "swarm," for no such 

 aggregation was noted. This is shown by the fact that Dlaptomus, Cyclops 

 and Daphnia hj/dHiia. in a majority of the night ol)servations. were more 

 numerous at a depth of one or two meters or even deeper, than at the 

 surface. It simply means that the upper stratum, one or two meters in 

 thickness, is sparsely populated on briglit, sunny days, liut that this r(^gion 

 is more or less densely populated at. night. 



Blanc (1898) and Fordyce (lOOO) found the greatest number of Crus- 

 tacea at the surface at 4 a. m. IXIy observations do not agree with their 

 results. Figs. 1 to 4 do not show a morning maximum with the exception 

 of young Daphnia hyalina in the second series. On the. other hand, Diap- 

 tomiis reached a maximum at 7::)0 jx m., adnlt 7). hiialina at 7:.')0 and 8 

 p. m., and Cyclops at 7:no p. m. and midnight. Also, each of the other 

 forms showing a diurnal nioveinent, i'c;ulied its maxinnim aliout S p. ra. 



CAUSES OF DIURNAL MOVEMENT. 



Various theories have been advanced to account for this pheimmenon. 

 It has been ascribed to various factors such as food, temperature, light, 

 gravity, and in some cases chemical stimuli. Experiments on several of 

 the Crustacea whicli show diurn;il movement, seem to show that light is 

 the primary factor. But generally, there are other factors involved which 

 may modify the effect of light to a very considerable degree, thus making 

 the phenomenon complex instead of apparently very simple. 



The migrating forms of Winona Lake may be separated into two 

 groups. The first groiip includes those whose day position bears a more 

 or less direct relation to the intensity of the sunlight. DiijJiuid }u/aliiia 

 and rclrocitrrd, l>i(ij)tii)iiii.f, and ('i/rlojtK belong to this group. The Daphnias 



