The Reversal of Tropisms 105 



It is probably owing to the influence of contact 

 that positively phototactic specimens of Orchestia 

 agilis become immediately and strongly negative 

 when they are thrown into the water. I have shown 

 that this interesting reversal is not the result of tem- 

 perature changes or changes in light intensity. It 

 occurs in the same way whether the water is warmer 

 or colder than the air or at the same temperature, 

 and whether there is an accompanying increase or 

 decrease of illumination. 



Reversals of tropisms brought about by chemicals 

 are common. Addition to the salt content of sea 

 water was found by Loeb to make negative speci- 

 mens of Polygordius larvae and certain copepods 

 positive. Positive specimens of the same forms 

 were made negative by diluting the sea water. Loeb 

 rendered the normally negative Gammarus positive 

 by the addition of carbon dioxide and other acids. 

 Cyclops and Daphnia were rendered positive in the 

 same way. Positive specimens of Diaptomus were 

 found by Moore to be made negative by strychnine, 

 but this negative reaction may be reversed by acids, 

 caffeine, and other substances which ordinarily tend 

 to produce or emphasize the positive response. 

 Ewart discovered that Balanus larvae were rendered 

 positive by acids, certain salts, and by hypertonic 

 sea water, while alkalies and hypotonic sea water 

 made them negative. Several of my students, Jack- 

 son, Michener and Dice who have investigated the 

 problem have shown that there is little apparent 



