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 Bohn’s contention, but the first is much more difficult to 
substantiate. 
DIFFERENT KINDS OF RESPONSE TO A GIVEN STIMULUS 
Qualitative variations in 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 
