66 H. G. Kribs 



Results with "Threshold Stimuli" (chemicals very weak, see 



p. 56) 



a Chemical stimulations of threshold in tensity- develop the 

 normal exploring reaction in ^^olosoma. 



b If the stimulus impinges laterally to the prostomium, there 

 is a turning of the head segment toward the field of stimulation 

 before the exploring movements are expressed. 



c After a brief exploring reaction the animal moves toward or 

 away from the field of stimulation — gives a "positive" or a "nega- 

 tive" reaction to the stimulus. 



^ The movements expressed in these reactions vary within 

 rather wide limits, and cannot be coordinated with "lines of difi^u- 

 sion." 



e The aggregate of movements exhibited varies with changes 

 in the chemicals used. The nature of the stimulus is an important 

 factor in determining the nature of reaction. 



/ Any interference with the integrity of the nervous svstem 

 raises the threshold of chemical discrimination. 



g The reactions to threshold stimuli may be loosely correlated 

 with the relative toxicity of the chemical involved. 



h The threshold of chemical discrimination varies rapidly 

 with changes in the physical nature of the environment. 



Results with "Normal Stimuli" (chemicals moderately strong, 



see p. 60) 



a .Tvolosoma may exhibit all of the movements comprised in 

 the action system in response to chemical stimulation of this order. 



b There are no characteristic positive reactions to chemicals 

 of this order. 



c Many of the negative reactions are directly away from the 

 field of stimulation. The great majority of the negative reactions, 

 however, are composed of decidedly random movements, which 

 continue until the animal is freed of the stimulus. 



(i There is a certain degree of individuality among the seg- 

 ments. The posterior segments may be made to give a definite 



