1913] Gee: Behavior of Leeches 229 



7. Chemotactic Responses 



In the experiments with chemicals, the tests were all made 

 with a half dozen individuals for each chemical, the leech being 

 placed in a separate glass dish. Before a diii'erent strength of 

 the same chemical was used the leech was placed for a few min- 

 utes in clean water. The substances were allowed to diffuse into 

 the water from a capillary pipette, the rate being so regulated 

 as not to produce a response from the current. This can be done 

 by ganging the size of the opening of the delicate capillary tube. 



Nitric acid, acetic acid, sodium chloride, copper sulphate, and 

 cane sugar were the substances used. All strengths of nitric 

 acid of a greater intensity than 1/80 per cent produced a nega- 

 tive response. To dilutions less than this, an indifferent reaction 

 was observed. Acetic acid revealed a similar condition at 1/30 

 per cent; sodium chloride at 1/5 per cent; copper sulphate at 

 1/320 per cent. Copper sulphate even in small quantities pro- 

 duced a very marked effect on the leech, producing swimming 

 and a long continued activity. Cane sugar in an intensity of 

 5 per cent produced no response that could be interpreted as 

 either negative or accentuation of positive reaction. The response 

 to the injurious chemicals localized in this way is as decided as 

 in the case of localized contact stimulus. 



As would be conjectured, the animal shows a directly positive 

 reaction to diffusing snail juices. The nature of this has already 

 been described in the discussion of the feeding of leeches. Best 

 results are secured when the food juices are fairly strong in 

 concentration, and placed relatively close to the anterior end of 

 the body. 



The significance of these results lies in the fact that the 

 animals respond negatively to chemicals of an injurious nature 

 in just as adaptive a manner as to contact stimuli, turning sharply 

 awa.y, and if the stimulus is a strong one, swimming rapidly 

 out of the unfavorable environment. To chemical stimuli of 

 such a nature as those found in its food material, the Dina re- 

 sponds in a positive manner. 



