44 Kansas Academy of Science. 



anterior end comes in contact first. But, as Jennings has shown, 

 if the animal is stimulated from behind, from the side or from any 

 localized area the same reflex action takes place, even though it 

 carries the animal into the midst of the chemical. 



Some chemicals in weak solution do not stimulate the motor re- 

 flex. These are mostly those of an acid reaction. I have experi- 

 mented with 0.0005 per cent. HCl and 0.00002 per cent. HCl. Several 

 trials were made. The stronger solution did not cause the reflex, 

 but it caused the animal to hesitate before entering. The weaker 

 solution seemed to attract, from the fact that on contact the animal 

 immediately swam into it. Two small drops were placed side by 

 side, one containing the acid solution and the other paramoecium 

 in water. Now, by the aid of a needle or a fine glass rod, the two 

 drops are connected by a narrow neck. In its random course the 

 little animal comes in contact with the diffusing chemical, but in- 

 stead of backing off immediately it hesitates, perhaps turning 

 toward the aboral side a few times and then entering the drop. 

 This turning toward the aboral side is a normal movement. 



After entering the drop the paramoecium darts about, showing 

 signs of stimulation. If the connection between the two drops is 

 small, so that the diffusion of the chemical is slow, the paramoecium, 

 upon starting to reenter the drop of water, is repelled by having its 

 motor reflex stimulated. The result is that the acid drop acts as a 

 trap. If, on the other hand, the connection between the drops is 

 large enough to permit a rapid diffusion of the chemical, instead of 

 always being repelled by the drop of water, the animal often reenters, 

 • The explanation is easily found. In the first instance the diffu- 

 sion of the chemical is so slight that the two drops remain very 

 different in chemical composition. Now, if an animal attempts to 

 pass back into the drop of water it encounters such a sudden 

 change that its reflex is stimulated, and the animal backs away. In 

 the second instance, where the diffusion of the chemical is rapid, 

 the drops blend into each other, so that the animal encounters no 

 sudden change, and therefore is not repelled. Paramoecium gave 

 a similar reaction to weak acetic acid. 



What seems most peculiar in this reaction to acids is that there 

 is no repulsion in entering the acid, but after having entered it the 

 paramoecium is repelled by the water which it has just left. This 

 shows conclusively that weak acid solutions have a positive attrac- 

 tion for paramoecia. 



It has been found by other investigators that paramoecia collect 

 in CO2 which has been placed in the water. The CO2 renders the 



