6(38 RAYMOND PEARL. 



food aud chemical i-eactions is tlie relation of the condition 

 of the organism as regards hunger to its reactions to stimuli. 

 It might be supposed that an individual which had not had food 

 for some time would be more apt to give the positive reaction 

 to a given stimulus than one which had just fed. 



To test this })oint parallel experiments were instituted with 

 specimens allowed to feed till they left the food spontaneously 

 about three hours before the experiments, and specimens 

 which had been kept for three weeks in a dish of clear water. 

 NaBi- was used as the stimulating solution, and Avas applied 

 by the capillary method. The specimens chosen were of the 

 same species, P. dorotocephala, and as nearly as possible 

 of the same size. The only difference which could be detected 

 between the fed and the unfed animals in their behaviour 

 towards a -| per cent, solution of NaBr was that the unfed 

 animals gave the whole food reaction on the end of the 

 capillary tube, while the recently fed specimens only went so 

 far as to give the positive reaction, and touched the end of 

 the tube with the anterior end of the head. They did not 

 "grip" it and pass up on to it, as did the others. In the 

 main point at which I was working, namely, the giving of 

 the definite positive reaction, there was no discoverable 

 difference between the fed and unfed specimens. One set 

 gave the reaction just as promptly and decidedly as did the 

 other. Next a weaker solution, yL per cent., was tried. 

 With this solution about 50 per cent, of the specimens in 

 ordinary condition give a v/eak positive reaction, and 50 per 

 cent, are indifferent. This concentration, being about on the 

 boi-dcr line between that which affords no stimulus, at all and 

 that which is a definite stimulus for the positive reaction, 

 ought to bring out any differences which may exist between 

 fed and unfed individuals in the sensitivity to stinmli for the 

 positive reaction. As a matter of fact, no difference in the 

 behaviour of the two sets was to be observed. One gave a 

 well-marked positive reaction in as many cases as did the 

 other. In some instances the reaction time oi' the fed 

 specimens socmod to be slightly greater than that of llic 



