10 University of California Publications in Zoology [Vol.19 



(p. 123) : "Females rise to the surface with the setting of the sun, 

 because they are positively phototropic to faint light and negatively 

 geotropic ; they descend into deep water with the rising of the sun, 

 because they are negatively phototropic to strong light (their negative 

 geotropism being overcome by their negative phototropism) ; the males 

 follow the females in migration, because they are probably positively 

 cbemotropic toward the females." Parker notes (p. 122) that such 

 an explanation need not apply to other plankton organisms, although 

 their depth migrations may be made at the same time as those of 

 Labidocera. 



The important paper by Dice (1914) deals with the reactions of 

 Daphnia and their bearing on the vertical movements. These care- 

 ful and thorough experiments led Dice to conclude (p. 263) that "the 

 diurnal movements of Daphnia pulex are caused chiefly by variations 

 in geotaxis induced by changes in light intensity." Increase of in- 

 tensity leads toward positive geotropism, decrease toward negative 

 geotropism. Temperature also has an effect, since high temperatures 

 tend to make the animals positively geotactic, while low temperatures 

 have the opposite tendency. Dice gives (p. 262) the results of the 

 experiments of others that may be taken to show that reversal of 

 geotropism by changes in light and temperature occurs in other forms. 



Another point of view, as regards the causes of the diurnal move- 

 ment, is that of Ewald (1910, 1912). His laboratory observations 

 on the behavior of Cladocera and of Balanus larvae led him to the 

 conclusion that while light is an important factor it is not necessary 

 to hold that there is continually recurring alternation between positive 

 anil negative heliotropism. The forms that Ewald used show what 

 he called locomotion periods ; these are, objectively, alternate ascents 

 and descents. If the light is too intense, the animals cease their loco- 

 motion movements and therefore sink, and when they reach a region 

 of lower intensity the swimming begins again. There are, therefore, 

 successive periods of inhibition and stimulation (Ewald, 1912, p. 609) 

 which have the effect of maintaining the animals in regions of about 

 equal illumination all day long; they ascend in the evening and 

 descend in the morning without giving positive and negative helio- 

 tropic reactions. 



There is still another possible factor that may determine the 

 diurnal movement. This is the effect of a physiological or metabolic 

 rhythm. Such a factor has not received general consideration, but 

 Menke (1911) makes much of it from his point of view. His experi- 



