JUN 4 1900 



Changes in environment op daphnia magna. 199 



On the Reaction of Daphnia magna (Straus) 

 to certain Changes in its Environment. 



By 



Ernest IVai'i'cii, D.Sc, 



University College, London. 



Foe some years I have been engaged in breeding Daphnia 

 magna (Straus)^ for the purpose of obtaining data Avhich 

 would be of value in testing the current theories oE heredity. 

 The investigation has led me to a series of experiments on 

 the action of common salt, and on the effect of a confined 

 volume of water on this animal. Some of the results seem 

 to present certain features of considerable interest and of 

 wide biological significance ; they illustrate how closely the 

 organism is knit to its external conditions of life. 



It will be best to give here a brief epitome of some of the 

 results. 



In the first place it will be shown that the times of killing 

 with varying strength of solutions of sodium chloride are 

 between certain wide limits ("9 per cent, to 6'0 per cent.), 

 well repi'esented by a rectangular hyperbola. The modifica- 

 tion induced by varying temperature is then discussed. 



The physiological condition of the animal at the time of 

 immersion into the salt solution is shown to have an enor- 

 mous effect on the resisting power. From the results ob- 

 tained an antagonism between excessive vegetation and 

 animal life was surmised. In a foot-note to the description 

 of the experiment it is suggested that the observed injurious 

 effect was due to the green light and the shading caused by 



VOL. 43, PART 2. — NEW SERIES. V 



