G6^ RAYMOND PEARL. 



SO defiued. It" they came in contact with the strong solution 

 again the}' repeated the reaction. In no case did they turn 

 squarely around with their heads directly away from x and 

 the long axis parallel to the lines of diffusion. 



It would be unprofitable to further multiply accounts of 

 these experiments, since all led to the same result. No 

 definite orientation occurred, but only the positive and nega- 

 tive motor reflexes coupled with random movements. 

 Whether, as some maintain, we have in these positive and 

 negative reactions the " Diuge an sich " of orientations is a 

 question for the metaphysician rather than the physiologist 

 to decide. The objective reality of the matter is that in the 

 behaviour of planariaus towards chemicals there is no orien- 

 tation in the lines of diffusing ions, i. e. no phenomenon like 

 the orientation of Paramecium to the electric current. 



Another problem o£ importance in connection with the re- 

 actions of the organisms to chemicals has to do with the 

 formation of collections of individuals. Are collections 

 formed in certain chemicals, as is the case with certain of the 

 Infusoria as described by Jennings ? As this author has set 

 forth, Paramecia will form dense aggregations in drops of 

 various chemicals, particularly weak acids, introduced into 

 the culture water. The method by which this is done is as 

 follows: — Individuals swimming about at random strike the 

 drop of acid by chance and pass into it without giving 

 any reaction; when, however, they come to the opposite 

 side of the drop, and start to pass from it to the water again, 

 they are stimulated and give their characteristic motor re- 

 action (jerk back and turn towards the aboral side). This 

 reaction turns them back into the drop, which forms, as it 

 were, a trap for all that enter it. In a short time a dense 

 aggregation is formed. This is almost the only method of 

 active reaction, known aside from orientation, which will 

 produce collections of organisms in chemicals. Its essential 

 feature is not the getting of the organisms into the chemical, 

 this being purely a matter of chance, but the holding of 

 them in the chemical after they have entered it, by what 



