﻿592 
  C4E0RGE 
  WAGNER. 
  

  

  viridis 
  almost 
  exclusively 
  in 
  these 
  experiments, 
  and 
  the 
  

   descriptions 
  apply 
  entirely 
  to 
  that 
  species. 
  I 
  did, 
  however, 
  

   experiment 
  enough 
  with 
  the 
  other 
  forms 
  to 
  find 
  that 
  their 
  

   reactions 
  are 
  practically 
  the 
  same. 
  

  

  Variation 
  in 
  Sensitivity. 
  — 
  A 
  great 
  variation 
  in 
  sensi- 
  

   tivity 
  to 
  stimulation 
  between 
  different 
  individuals 
  became 
  

   immediately 
  apparent 
  in 
  the 
  work 
  on 
  the 
  reactions 
  of 
  H3'dra. 
  

   Many 
  specimens 
  proved 
  useless 
  for 
  such 
  work, 
  because 
  even 
  

   the 
  slight 
  disturbance 
  on 
  the 
  surface 
  of 
  the 
  water 
  caused 
  by 
  

   the 
  breathing 
  of 
  the 
  operator 
  as 
  he 
  bent 
  over 
  the 
  microscope 
  

   produced 
  immediate 
  and 
  complete 
  contraction. 
  On 
  the 
  other 
  

   hand, 
  there 
  were 
  specimens 
  in 
  which 
  an 
  actual 
  wound 
  had 
  

   to 
  be 
  produced 
  in 
  the 
  ectoderm 
  before 
  they 
  responded 
  at 
  all. 
  

   Between 
  these 
  two 
  extremes 
  all 
  degrees 
  of 
  sensitivity 
  occurred. 
  

   The 
  terms 
  " 
  weak" 
  and 
  " 
  strong," 
  as 
  applied 
  in 
  this 
  paper 
  to 
  

   stimuli, 
  have, 
  therefore, 
  only 
  a 
  relative 
  meaning. 
  What 
  is 
  

   'Sveak"to 
  one 
  specimen 
  may 
  be 
  exceedingly 
  '' 
  strong" 
  to 
  

   another. 
  

  

  Non-localised 
  Mechanical 
  stimuli. 
  — 
  If 
  the 
  watch- 
  

   glass 
  containing 
  a 
  Hydra 
  is 
  slightly 
  jarred 
  the 
  Hydra 
  con- 
  

   tracts. 
  The 
  same 
  result 
  occurs 
  after 
  any 
  disturbance 
  of 
  the 
  

   surface 
  of 
  the 
  water. 
  If 
  after 
  one 
  such 
  stimulus 
  the 
  dish 
  

   remains 
  undisturbed 
  the 
  Hydi'a 
  soon 
  expands 
  again. 
  

  

  Now 
  what 
  is 
  the 
  result 
  if 
  the 
  Hydra 
  is 
  subjected 
  to 
  

   rhythmically 
  repeated, 
  uniform 
  mechanical 
  stimuli 
  ? 
  Such 
  a 
  

   succession 
  of 
  stimuli 
  is 
  best 
  applied 
  by 
  tapping 
  the 
  stage 
  of 
  

   the 
  microscope 
  with 
  some 
  metal 
  body 
  at 
  intervals 
  of 
  about 
  

   one 
  second. 
  After 
  the 
  first 
  tap 
  there 
  occurs 
  complete 
  con- 
  

   traction. 
  As 
  the 
  tapping 
  continues 
  this 
  state 
  of 
  contraction 
  

   is 
  maintained 
  for 
  several 
  seconds, 
  sometimes 
  even 
  for 
  from 
  

   one 
  half 
  to 
  one 
  minute 
  ; 
  but 
  sooner 
  or 
  later, 
  in 
  spite 
  of 
  con- 
  

   tinuous 
  stimulation, 
  the 
  Hydra 
  slowly 
  expands. 
  When 
  it 
  has 
  

   reached 
  its 
  normal 
  state 
  of 
  expansion 
  it 
  remains 
  in 
  that 
  

   position 
  as 
  long 
  as 
  the 
  stimulus 
  is 
  not 
  increased, 
  or 
  even 
  when 
  

   it 
  is 
  slightly 
  increased. 
  Thus 
  Hydra 
  soon 
  becomes 
  used 
  to 
  a 
  

   slight 
  non-localised 
  mechanical 
  stimulus 
  recurring 
  at 
  frequent 
  

   interv^als, 
  and 
  no 
  longer 
  responds 
  to 
  it. 
  If 
  the 
  increase 
  lu 
  

  

  