568 RAYMOND PEARL. 



will frequently be seen to give a well-defined positive 

 reaction, and turn towards the group. The reaction is of 

 precisely the same character as that given by the organism 

 to weak chemicals (to be described later), and the behaviour 

 convinces one observing it that the specimen is stimulated 

 by some chemical diffusing out from the group. After 

 turning towards the group the specimen will glide into it 

 and usually coiiie to rest, in the manner which has been 

 described above. 



What the nature of the chemical substance present in the 

 region about the groups is, I have not been able to discover. 

 Neither rosolic acid nor methyl orange is discoloured by it. 

 Whatever its nature, it must be in an extremely diluted 

 state. This seems evident for two reasons : first, because it 

 does not affect delicate indicators ; and second, because it 

 does not have any effect on active specimens of Plan aria. 

 A large number of experiments have been performed to test 

 this latter point, but always with the same result. Unless 

 the individuals were in the proper predisposing condition of 

 lowered tonus, they would pass by or through groups of 

 other individuals without giving any reaction. 



Attempts to produce, artificially, collections of planarians 

 in chemicals have been unsuccessful. I have tried various 

 solutions (such as sugar, weak alkalies, etc.) to which the 

 organisms showed a well marked positive chemotaxis when 

 tested by other methods, but have not been able to get any 

 formation of collections in them. The animals would give 

 the positive reaction on coming to the edge of the diffusing 

 chemical and pass into it, but would not come to rest. This 

 failure to produce collections artificially is not surprising 

 when one considers the number of conditions necessary for 

 the production of the desired result. The organism must be 

 in just the right physiological condition, the chemical must 

 be of a certain concentration, and finally, it must be located 

 in an area of a certain light intensity. It is practically 

 almost or quite impossible to fulfil all these conditions at 

 the same time in an experiment. 



