128 



2. Aiiiiriii (ii-iih'niii and Tr/ar/hrx occupied the region below ;i deph 

 of ten meters. They were nsnally most al)nndant within two or three 

 meters of the l)ottom. 



8. Aiiiircd cnchh-aris and XdlJidlfd IdiKjisitina were found tlirough tlie 

 entire deptli of tlie hike. 



MAXIMUM NUMBER AT SURFACE. 



This diurnal migi-ation must not be taken to mean that the individuals 

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

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

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

 and Daplniia Jii/alhia. in a majority of the night observations, were more 

 numeroiis 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 l)right, sunny days, but that this region 

 is more or less densely populated at night. 



Blanc (1898) and Fordyce (190O) found the greatest number of Crus- 

 tacea at the surface at 4 a. m. My oljservations do not agree with their 

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

 of young Daplniia hyaliiia in the second series. On the other hand, Dlap- 

 tooiKS reached a maximum at 7:.>0 p. m., adult D. hyalina at 7:30 and 8 

 p. m., and Cyclops at 7:30 p. m. and midnight. Also, each of the other 

 forms showing a diurnal movement, reached its maximum about S p. m. 



CAUSES OF DIURNAL MOVEMENT. 



Various theories have been advanced to account for this phenomenon. 

 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 which show diurnal movement, seem to show that light is 

 the primary factor. Hut 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 group includes those whose day position bears a more 

 or less direct relation to the intensity of the sunlight. Lkijiliaia hyalina 

 and irtrocarra, Diaptdimix, and <'yrtdj,x belong to this group. The Daphnias 



