MOViaiENTS, ETC., OF FRESH-AVATRR PLANARFANS. 583 



account of it-^ fineness of adjustment witli refiM-ence to tlie 

 strength of the stinuilus and tlie general pli^'siological 

 condition of the animal, it is a response which might be 

 very easily overlooked in a superficial examination of the 

 behaviour. As the worm gives this positive reaction in 

 response to a gentle stimulus, turning the head towards the 

 source of stimulation, and at the same time raising it, it 

 gives one the impression that it is seeking something, and 

 such the behaviour would doubtless be called by some 

 animal psychologists. This impression is enhanced by the 

 fact that if the head does not come in contact with the 

 stimulating object at the first reaction, the animal advances 

 in the direction from which the stimulus came, with the 

 anterior part of the body raised and waving from side to 

 side in the water. 



As has been mentioned, the reaction is very delicately 

 adjusted physiologically. In the majority of cases the 

 animal must be in a comparatively quiet condition, — that is, 

 not " stirred up ^' or excited, and gliding smoothly at the 

 ordinary rate, in order that the reaction may appear at all. 

 The stimulus must ordinarily be very weak, and given so as 

 not to disturb the animal by abruptly changing the sur- 

 rounding conditions. It is possible to produce the reaction 

 by the use of a needle or scalpel point if sufficient care is 

 taken, but better results are obtained by the use of a hair 

 as the stimulating point. The point should be lightly touched 

 to the edge or dorsal surface of the head, and then quickly 

 drawn a short distance away. Even when all these precau- 

 tions are taken one may fail to produce the characteristic 

 response. I have frequently found that the same specimen 

 which at one time would give the positive reaction in a very 

 definite and characteristic way to every light touch on the 

 head could not be made to show it a few hours later. This 

 shows how closely it depends on general physiological 

 conditions. On the other hand, specimens will frequently 

 be found that for short periods of time (two or three hours) 

 can hardly be induced to give any other response to median- 



