REGULATION OF REACTIONS 281 



spiders soon became more active and began to swim 

 toward the light side of the aquarium, where in the course 

 of a very few moments they formed a dense aggregation. 

 Those which had been negative as w^ell as those which were 

 neutral had become positive. They remained at the more 

 highly illuminated side of the aquarium only four to five 

 minutes, then gradually scattered about again. A little 

 more CO2 was then added to the water and the organisms 

 became positive again. This process was repeated several 

 times. When air was forced through the water they 

 scattered almost immediately, and became indifferent to 

 light, or sometimes slightly negative. It is consequently 

 not the agitation which makes them positive when CO2 is 

 allowed to bubble through the water. These results seem 

 to indicate that the change in reaction to light is dependent 

 upon the change in amount of CO2 as well as upon the 

 absolute amount. 



In these experiments the water spiders became quiet 

 when the CO2 was added and sank to the bottom, but in 

 some later experiments they also became strongly positive. 

 In Stentor, Chlamydomonas, Vol vox and Scapholeberis no 

 change in sense of reaction could be induced by means of 

 adding CO2. They become quiet and sink to the bottom 

 after the CO2 reaches a certain concentration. I was 

 unable to change the sense of reaction to light in several 

 different zoeae and in the larvae of Hydroides dianthus 

 by means of hydrochloric-acid solutions. The hydroides 

 larvae were exposed in sea-water solutions of HCl varying 

 in strength from n/250, in which they were immediately 

 killed, to w/7250, in which their response was normal in 

 every respect, both in light intensities so low that they were 

 positive and so high that they were negative. 



In Arenicola larvae however the sense of reaction to 

 light can be reversed by means of various chemical solu- 

 tions. These larvae are strongly positive during the first 

 few days, even in very intense light. They swim freely 

 through the water near the surface. Later they settle to 



