REGULATION OF REACTIONS 283 



were still negative the following morning. Many of those 

 in diluted sea water and in sea water containing CO2 were, 

 however, positive after being in over night. In these 

 aquaria there were two collections, one at the end toward 

 the light and one at the opposite end. In those solutions 

 containing ammonia, sodium hydrate or magnesium sul- 

 fate the aggregation at the negative side of the aquarium 

 was much more pronounced the following morning than it 

 had been the preceding evening. Larvae taken from any 

 of these solutions and put into normal sea water became 

 positive in the same light intensity almost immediately in 

 every instance. I did not succeed in producing reversal 

 in reactions with chloroform or adrenalin, nor did I suc- 

 ceed by changing the temperature. The experiments under 

 these conditions were, however, not very extensive. 



It is at once evident that there is a striking difference 

 between the reversal in reaction in such forms as Chlamydo- 

 monas and iVrenicola larvae. In the former the change is 

 comparatively sudden, sharp and definite, and the nega- 

 tive orientation is as accurate and precise as the positive 

 orientation. In the latter the change is comparatively 

 slow and indefinite, and negative orientation is much less 

 precise than positive orientation. In Arenicola larvae it 

 appears that any condition which acts as a depressant 

 tends to cause the young positive larvae to become nega- 

 tive. These larvae become negative under normal condi- 

 tions as they grow older. Depressants apparently hasten 

 the appearance of this state, and under their influence 

 larvae become negative earlier than they otherwise would. 



d. Effect of concentration of the medium and mechani- 

 cal stimuli. — We have already stated the fact that Areni- 

 cola larvae become negative both in concentrated and in 

 diluted sea water. Loeb (1893, pp. 94, 96) was able to 

 make negative Polygordius larvae positive by adding i to 

 1 .3 per cent. NaCl to the sea water and positive individuals 

 negative by diluting the sea water with 40 to 60 per cent, 

 fresh water. Similar results were obtained with cope- 



