262 LEE RAYMOND DICE 



tioned. A numl er of individuals of Daphnia pulex collected 

 from a small pond on the morning of February 5th showed very 

 strong positive phototaxis in sunlight. As a further abnormality 

 these individuals showed the peculiarity of changing temporarily 

 from positive to negative phototaxis when the light intensity 

 was reduced by shading the dish. 



Very likely other factors besides those considered will be 

 found to be operative in affecting the vertical movements of 

 Daphnia, but it seems likely that the more important factors 

 have been mentioned. 



REVERSAL OF GEOTAXIS IN OTHER PLANKTON ANIMALS 



Reversal of geotaxis by changes in light and temperature 

 similar to that found in Daphnia pulex has been observed by 

 previous authors in several different groups of plankton animals. 

 A brief reference will be made to some of these observations. 



Esterly (1907) found that the females of Cyclops albidus 

 were positively geotactic in daylight, but were negatively geo- 

 tactic in darkness. 



It was observed by McGinnis (1911) that Branchippus serratus 

 is positively geotactic in light and negatively geotactic in darkness. 



An increase of light makes Balanus larvae positively geotactic, 

 while a decrease of light makes them negatively geotactic (Ewald, 

 1912). After a certain exposure to light of any intensity they 

 assume a general distribution in respect to gravity. 



The phantom larvae of Corethra become positively geotactic 

 in sunlight or strong electric light (Harper, 1907). 



Macropsis, sl plankton schizopod of the family Mysididae, is 

 always positively phototacitc, but in light from above the ani- 

 mals remain at the bottom of the vessel (Bauer, 1908). Per- 

 haps it will be possible to show that this behavior is due to light 

 causing a positive geotaxis which is more potent than the posi- 

 tive phototaxis. 



Loeb (1893) states that Polygordius larvae may be made 

 negatively phototactic by heating or by intense light, and that 

 the geotaxis is always positive when the phototaxis is negative 

 and negative when the phototaxis is positive. No such correla- 

 tion as this is found in Daphnia pulex. 



Massart (1891) found that the geotaxis of Chromulina woro- 

 ::iniana, a spirillum, is positive at temperatures of from 5° to 



