LIGHT REACTIONS AND PHOTOTAXIS 603 



the action of the ultra-violet rays, sinking down on the posi- 

 tive side of the glass. They had apparently lost the possibility 

 of negative orientation. The same effect could also be obtained 

 by adding to the NaCl solution pure natural seawater in the 

 proportion of 1:1; in this case the animals were less affected 

 by the solution, while in pure NaCl they sometimes died after 

 a quarter or a half-hour. Added to natural seawater in small 

 doses, NaCl solution accelerates the positive and retards the 

 negative reaction at a degree depending on the value of the quo- 

 tient - (Lichtstimmung) . Correspondingly, the threshold con- 

 n 



centration required to bring about positive reaction in negative 

 animals is different. Leaving NaCl out of artificial seawater 

 has an effect contrary to that obtained by adding it to pure 

 seawater. It increases the negative and diminishes the positive 

 reaction. The best solution proved to be one containing about 

 two-thirds of the ordinary amount of NaCl. Lower concen- 

 trations were harmful. The result obtained pointed to the prob- 

 ability, that there were other salts in the solution antagonistic 

 to NaCl, whose effect predominated when NaCl was diminished. 



Potassium chloride had an effect similar to, but considerably 

 weaker than that exhibited by sodium chloride. Pure isotonic 

 KCl solution killed the animals instantly and if added to natural 

 seawater in proportions above 1:150 proved to be poisonous 

 also. If the concentration is slightly weaker than 1:150, posi- 

 tive reaction is increased, negative reaction diminished, but I 

 never saw large quantities of animals become instantly positive, 

 as they did after a sufficient addition of isotonic NaCl. KCl 

 does not change the effect of pure NaCl solution if added in the 

 proportion prevailing in natural seawater, nor does its omission 

 from artificial seawater cause any alteration of the reaction. 



Calcium chloride belongs to the same group as the two first 

 mentioned salts. Pure 0.65 molecular solution has a toxic effect. 

 If added to natural seawater in the proportion of 1-15 it inhibits 

 negative reaction in a very short time, both in negative and in 

 positive animals about to become negative. But it has the singu- 

 lar secondary effect of apparently paralyzing the larvae. The 



