LIGHT REACTIONS AND PHOTOTAXIS 605 



When natural seawater is added to the artificial magnesium-free 

 seawater in a proportion of 3:1, the animals would live in this 

 solution for some hours without becoming negative, save in a 

 few exceptional cases. Decrease of magnesium has therefore the 

 same effect as increase of sodium or potassium. In other words, 

 for a normal production of light reactions it is necessary to have the 

 correct proportion of sodium (potassium) on one *side and magne- 

 sium on the other. In my experiments magnesium chloride proved 

 slightly more effective than the sulphate. 



Lastly, among the constituents of seawater I have to men- 

 tion the alkali content. With the concentration prevailing in 

 natural seawater no effect could be detected. When all alkali 

 was left out of artificial seawater or added to pure NaCl solution, 

 the reaction of the nauplii was not changed. A stronger con- 

 centration of OH-ions, however, ^(about 2 cc. t^ NaOH in 100 

 cc. of seawater) had a visible negativating influence; ammonia 

 was still more effective (about 1 cc. t^ NH3 in 100 cc. of sea- 

 water), due according to the experiments on sea-urchin eggs, 

 to its permeating more quickly into the protoplasm. The reac- 

 tion of the seawater had in both cases become strongly alkahne 

 to neutral red. Both alkalies in the concentration mentioned 

 would quickly make positive animals negative and prevent 

 negative animals from becoming positive, even in very weak 

 light. 



I believed it to be of interest, with reference to the papers 

 by Loeb mentioned above, to see whether acids would have 

 the opposite effect. Contrary to the negative results that author 

 obtained in marine forms of America, I found that the mineral 

 acids HCl, H2SO4 and HNO3 had a positivating effect, but that 

 the effective concentration had very narrow limits. Only such 

 concentration would prove successful, as gave neutral red a 

 slight pink hue, natural seawater giving a reddish yellow color. 

 Shghtly higher concentrations killed the animals, lower ones had 

 no effect. In acetic acid and C02 1 saw no positivating influence 

 nor was it very strong in the mineral acids mentioned. 



Lack of oxygen has a very strong effect on phototaxis. If 

 nauplii are put in seawater which has been evacuated for a 



