56 H. G. Krihs 



Under threshold stimuh are grouped those solutions which lie 

 at the threshold of physiological discrimination. They represent 

 the weakest solutions which, under the conditions above stated, 

 will interrupt the physiological poise of the animal at the moment 

 of impact, enough to produce a visible reaction. They produce 

 this effect only when introduced to the sensory hairs of the prosto- 

 mium. 



Under normal stimuh are grouped those solutions which will 

 stimulate a characteristic reaction when introduced to any part of 

 the body. They are not strong enough, however, to inflict any 

 permanent injury to the tissues of the body. The movements 

 occasioned by those reactions are seemingly normal. They have 

 that easy flexibility which the animal daily exhibits under condi- 

 tions favorable to its existence. 



Under strong stimuli are grouped those solutions which force a 

 powerful reflex movement on the part of the organism. The 

 reactions are energetic and exhausting. They quickly develop 

 fatigue, and upon repetition may prove fatal. 



The concentration of the above solutions in each respective 

 group is not exclusive. They vary rapidly toward or from each 

 other under changing conditions. In our experiments, however, 

 they were typical stimuli of the reactions now to be described. 



The '"Threshold'^ Stimuli 



The threshold reactions were tested from a point directly in 

 front of the prostomium and then lateral to the same. 



Aiineral Acids. When stimulated from in front there is a slight 

 wrinkling movement of the prostomium, during which the head is 

 advanced toward the pipette, then withdrawn and moved about 

 with the characteristic exploring movements. In the case of 

 HjSOj these movements were repeated more vigorously than with 

 the others, with the result that the head was soon brought into 

 close contact with the pipette. There then followed a quick 

 reflex away from the source of stimulus. With a lateral applica- 

 tion the head is rotated toward the source of stimulation through 



