264 LEE RAYMOND DICE 



Strong wave action may cause a temporary inhibition of 

 locomotor activity and this allows the animals to sink to lower 

 levels. 



No definite vertical movement due to changes in the chem- 

 ical content of the water, or to the action of a diurnal rhythm, 

 independent of the effect of recurrent external physical changes, 

 could be found. 



Young individuals are more strongly positively phototactic 

 and negatively geotactic than adults. They also sink less 

 rapidly in water. For these reasons they are usually found 

 nearer the surface than the adults. There is a downward move- 

 ment due to the aging of the individuals. 



Much of the behavior of Daphnia is determined by orienta- 

 tion to various stimuli, but some regulatory behavior is obtained 

 by means of random movements. 



Change in geotaxis by changes in light intensity and in tem- 

 perature has been observed in other plankton organisms, and is 

 probably of widespread importance in determining the vertical 

 movements of the plankton. 



LITERATURE 



Bauer, Victor. Uber die reflektorische Regulierung der Schwimmbewegungen 



1908. bei den Mysiden mit besonderer Beriicksichtitung der doppelsinmgen 



Reizbarkeit der Augen. Zeitschr. f. allgem. Physiol., 8: 343-369. 



Birge, E. A. Plankton Studies on Lake Mendota. I. The Vertical Distribution 



1895. of the Pelagic Crustacea during July, 1894. Trans. Wis. Acad., 



10: 421-484. 

 1897. Plankton Studies on Lake Mendota. II. The Crustacea of the Plank- 

 ton from July, 1894 to December, 1896. Trans. Wis. Acad., 11: 

 274-448. 

 1897a. The Vertical Distribution of the Limnetic Crustacea of Lake Mendota. 

 Biol. Cent., 17: 371-374. 

 Burckhardt, G. Quantitative Studien fiber das Zooplankton des Vierwald- 

 1900. stattersees. Mitteilungen der Naturforschender Gesellschafl Luzern, 

 3 Heft, 1-309. 

 Davenport, C. B., and Cannon, W. B. On the Determination of the Direction 

 1897. and Rate of Movement of Organisms by Light. Jour. Physiol., 

 21: 22-32. 

 Davenport, C. B., and Lewis, F. T. The Phototaxis of Daphnia. Sci. N. S., 



1899. 9: 368. 

 Esterly, C. O. The Reactions of Cyclops to Light and to Gravity. Amer. Jour. 



1907. Physiol., 18: 47-57. 

 Ewald, W. F. f'Tber Orientierung, Lokomotion, und Lichtreaktionem einiger 

 1910. Cladoceran. Biol. Cent., 30: 1-16, 49-63, 379-384, 385-399; 4 figs.; 



8 curves. 

 1912. On Artificial Modification of Light Reactions and the Influence of 

 Electrolytes on Phototaxis. Jour. Exp. Zoo!., 13: 591-612. 

 Fitzgerald, Desmond. The Temperatures of Lakes. Trans. Amer. Soc. Civ. 

 1895. Eng., 34: 67-109; 12 tables; 11 curves. 



