DIFFERENT KINDS OF RESPONSE 145 



the mouth. Early in the morning repeated stimulation 

 causes the tentacles to respond with increased energy, later, 

 after a short period of increase of responsiveness, the ten- 

 tacles react less vigorously, and toward evening they be- 

 come insensible after a few stimulations. 



Very similar phenomena were found to occur in the annelid 

 Serpula when stimulated by shadows. The responses were 

 variable; some individuals would show an increase of re- 

 sponsiveness followed by a decrease, while others would show 

 a diminution from the beginning. Bohn has attempted to 

 subsume the variations of responsiveness to a given stimulus 

 under one general "law" resting on a physico-chemical 

 basis. All stimulation according to him is followed at first 

 by an increase and then by a decrease of reaction. In some 

 cases the initial increase is so slight as not to attract atten- 

 tion and we apparently get a falling off from the start. In 

 others the continued increase of responsiveness has not 

 been followed long enough to discover the diminution which 

 must come sooner or later. We may grant the latter part 

 of Bonn's contention, but the first is much more difficult to 

 substantiate. 



DIFFERENT KINDS OF RESPONSE TO A GIVEN STIMULUS 



Qualitative variations hi the reactions to repeated stimula- 

 tions are common. We have seen that they occur in an 

 organism as low in the scale as Stentor. Hydra, according 

 to Wagner, when repeatedly stimulated by a capillary glass 

 rod at sufficiently long intervals will, if it does not in time 

 ignore the stimuli entirely, come to respond in a new way; 

 it bends to one side until the tentacles touch the bottom, 

 then loosens the foot and attaches itself in another locality. 

 Striking changes in the response to a given stimulus are 



furnished by the reactions of sea anemones. The anemone 

 10 



