MOVEMENTS OF DAPHNIA 249 



LOCOMOTOR ACTIVITY AT DIFFERENT TIMES OF DAY 

 Experiments performed to determine whether a periodicity 

 in vertical distribution occurred gave only negative results (see 

 geotaxis in relation to time of day). The possibility may be 

 considered that both geotaxis and activity have diurnal rhythms 

 and that the activity decreases in the same degree that the 

 negative geotaxis increases and the reverse. Such a condition 

 is not believable. We may then feel confident that there is no 

 diurnal rhythm in either, except such as is due to daily changes 

 in the external physical factors. 



THERMOTAXIS 

 Yerkes (1903) has shown that the radiant heat from a 16 

 candle-power incandescent lamp does not have an appreciable 

 effect on the movements of Daphnia pulex. In darkness there 

 is an irregular wandering away from a region heated to 29° C. 

 This will evidently be of some importance in determining the 

 vertical positions of the animals in lakes where the surface 

 layers grow very warm in summer. 



CHEMOTAXIS 



No observations have been made as to the possibility of 

 chemotaxis being a factor in the vertical movements of Daphnia. 

 It can hardly be a very important factor, although it might have 

 a modifying influence on behavior. 



PRESSURE 



The pressure acting at any given depth in a body of water 

 will be very nearly constant at all times so that pressure changes 

 cannot be a primary factor determining the movements of the 

 plankton Crustacea. As the animals move upward or down- 

 ward in response to various other tendencies the changes in 

 pressure experienced might be supposed to modify to some 

 extent the action of these tendencies. However, the only 

 experiment performed to test this supposition gave negative 

 results. In this experiment an increase in water pressure roughly 

 guessed at 15 pounds did not seem to cause any change in the 

 vertical distribution of the animals. The extensive daily migra- 

 tions performed by some species of Daphnia, 32 meters in Daph- 

 nia hyalina (Burckhardt, 1900), also argues against pressure 

 being of importance in modifying the factors causing the verti- 

 cal movements in this genus. 



