LIGHT REACTIONS AND PHOTOTAXIS 611 



10. Magnesium chloride or sulphate solution acts as an antago- 

 nist to sodium. Added in the proportion prevaihng in natural 

 seawater to pure NaCl solution, MgCl2 brings about the negative 

 reaction which is suspended in pure NaCl solution. There is no 

 difference in response to photic stimuli between larvae in the 

 sodium magnesium mixture and in pure seawater. For a normal 

 production of light reactions it is necessary to have the correct 

 proportion of sodium on one side and magnesium on the other. 



IV. Other chemical effects 



11. Sodium hydrate or amn^^nia above a certain concentra- 

 tion had a strong negativating effect. 



12. The mineral acids HCl, H2SO4 and HNO3 in a certain 

 concentration had the opposite effect. Acetic acid and car- 

 bonic acid had no effect. 



13. Lack of oxygen (brought about by evacuating the sea- 

 water) had a very strong positivating effect. 



14. Traces of copper had a negativating effect. 



15. Alcohol, chloroform and ether caused the animals to lose 

 their reactions to light. 



V. Effects of changes in concentration 



16. Hypertonic solutions of NaCl or MgCl2 had a strong posi- 

 tivating effect, hypotonic solution of NaCl had an equally obvious 

 negativating effect. 



VI. Effect of stains 



17. Staining nauplii with Bismarck brown or methylene blue 

 had a weak negativating influence, due, however, not to a spe- 

 cific color effect ('sensitisation') but to certain chemical con- 

 tents of the stain. Other stains tried had no effect whatever. 



