300 LIGHT AND THE BEHAVIOR OF ORGANISMS 



(6) Decrease in temperature causes swarm-spores, Eu- 

 glena, Chlamydomonas, Vol vox and other similar organ- 

 isms in the positive state to become negative. Increase 

 of intensity causes the opposite change. Decrease in heat 

 energy, therefore, causes the same change in reaction in 

 these forms as increase in light energy. In other forms 

 however this is not true. Polygordius larvae, for example, 

 become negative when the temperature is increased. In 

 many organisms changes in temperature do not cause re- 

 versal in the sense of reaction. 



(7) In Gammarus pulex, Cyclops, Daphnia, Cypris, a 

 small water spider, and various insect larvae, addition of 

 CO2 causes the reactions to light to become strongly 

 positive. In Gammarus various acids and narcotics and 

 all the ammonium salts also cause strong positive reactions. 

 In Cyclops sodium hydrate causes positive specimens to 

 become negative. In Stentor, Chlamydomonas, Volvox, 

 and Scapholeberis carbon dioxid does not cause a change 

 in the reaction. In Arenicola larvae various narcotics, 

 acids, alkalis and neutral salts — in general, apparently 

 any substance which acts as a depressant — cause a change 

 from positive to negative reactions. In Ranatra any con- 

 dition which tends to make the animal quiet produces 

 negative reactions, while any condition which excites it 

 tends to produce positive reactions. 



(8) The sense of reaction in most organisms is only 

 temporarily affected by concentration of the medium or 

 mechanical stimulation. Arenicola larvae become nega- 

 tive in both concentrated and diluted sea water, while Poly- 

 gordius larvae become positive in the former and negative in 

 the latter. 



Mechanical stimulation appears to cause Temora and 

 Cypris to become positive, while it causes Ranatra, Or- 

 chestia and Labidocera to become negative. 



(9) The fact that change in sense of reaction can be 

 produced by such a variety of different means seems to 

 show very clearly that this change is not due to a specific 



