222 ERNEST WARUKN. 



Daphnia, like all living orgaiiisuiis, must continually be 

 throwing off into the water excretory matter, using this term 

 in its widest sense. This matter, either in itself or thi-ough 

 its favouring the presence of certain bacteria, may feasibly 

 be supposed to be particularly injurious to Daphnia; for when 

 the Daphnia are fast disappearing, there may be a swarm of 

 Ostracods or Copepods. Thus the first swarm of Daphnia 

 may be supposed to render the Avater unfit for Daphnia life. 

 A few individuals, however, are strong enough to sui'vive, 

 the rest die. Now, by some natural process, the water 

 gradually becomes purified (possibly by the action of plant 

 life), and at last the few Daphnia that are still alive are able 

 to reproduce. 



Summary. 



From the foregoing experiments certain conclusions of 

 some interest may be drawn. The curve of the time of 

 killing with salt is quite clearly not a logarithmic curve, as 

 the usual method of stating Feclmer's supposed relation of 

 stimulus to effect might perhaps lead us to expect. Between 

 the limits of "8 per cent, and 6'0 per cent, solution, the rate 

 of killiug appears to directly depend on the number of 

 molecules (above the number contained in "8 per cent, solu- 

 tion) of salt which- beat on the Daphnia per unit of time. 

 With an increase of temperature the molecules are moving 

 with greater speed, and consequently strike the Daphnia 

 more frequently and with greater momentum, and the 

 chemical or physical reactions which take place are per- 

 formed with greater rapidity. Thus a Daphnia in 1"6 per 

 cent, at 3° C. dies in one hundred and five minutes, but at 

 29° C. in sixteen minutes. 



The physiological condition of the animal at the time of 

 immersion into the salt solution has an enormous effect on 

 the resisting power. Thus in the case of a number of 

 Daphnia plunged into "5 per cent. NaCl, whether they all die 

 in about twenty-two hours or live indefinitely depends on 



