6o H. G. Krihs 



and then treated with these weak solutions the results are similar 

 to those produced when they are subjected to a raise in tempera- 

 ture, although the threshold varies within much narrower limits. 



The "Normal" Stimuli 



Reactions Under Uniform Conditions 



Mineral Acids. When solutions of this strength are applied to 

 the tip of the prostomium there is first evident a wrinkling of the 

 prostomium and an erection of the setae of the anterior segments. 

 The head is then drawn backwards and quickly turned toward one 

 side. Sometimes the negative reaction may be so marked as to 

 reverse the general direction of its movement prior to stimulation. 

 As a rule the animals move at varying angles aw^ay from the field 

 of stimulation. When applied laterally to the prostomium the 

 head segment is frequentlv turned toward the pipette before the 

 negative reaction is expressed. Stimulation at the posterior 



Fig. 2 



papillae causes the posterior segments to contract, thus pulling the 

 papillae forward. Sometimes the caudal segment is swung away 

 from the stimulus. When the chemical is first applied at the 

 caudal segment there is no change in the attitude or movement of 

 the anterior segments. Repeated stimulation may cause rapid 

 crawling forward. 



WTien the chemical is applied to the middle segments of the 

 body the ventral muscles always contract more than the dorsal. 

 The resultant movements are determined by the attitude of the 

 animal at the moment of stimulation (Fig. 2). If this attitude is 



