﻿ON 
  SOME 
  MOVEMENTS 
  AND 
  EEACTIONS 
  OE 
  HYDRA. 
  597 
  

  

  body 
  or 
  of 
  tentacles 
  definitely 
  toward 
  or 
  away 
  from 
  the 
  source 
  

   of 
  mechanical 
  stimulation. 
  

  

  Considerable 
  importance 
  attached 
  to 
  this 
  comparatively 
  

   simple 
  point. 
  It 
  is 
  at 
  present 
  quite 
  frequently 
  assumed 
  that 
  

   all 
  organisms 
  respond 
  to 
  stimuli 
  having 
  hedonic 
  value^ 
  by 
  

   either 
  moving 
  toward 
  or 
  away 
  from 
  the 
  source 
  of 
  stimulation. 
  

   Of 
  sessile 
  organisms 
  it 
  is 
  similarly 
  stated 
  that 
  they 
  expand 
  

   toward 
  or 
  contract 
  away 
  from 
  the 
  stimulus. 
  So 
  Professor 
  

   Baldwin 
  ('97^ 
  pp. 
  198-9) 
  says 
  : 
  "All 
  organisms 
  behave 
  in 
  

   two 
  great 
  and 
  opposite 
  ways 
  towards 
  stimulations; 
  they 
  

   approach 
  them, 
  or 
  they 
  recede 
  from 
  them. 
  Creatures 
  which 
  

   move 
  as 
  a 
  whole 
  move 
  towards 
  some 
  kinds 
  of 
  stimulations, 
  

   and 
  recede 
  from 
  others. 
  Creatures 
  which 
  are 
  fixed 
  in 
  their 
  

   habitat 
  expand 
  towards 
  certain 
  stimulations 
  and 
  contract 
  

   away 
  from 
  others." 
  . 
  . 
  . 
  . 
  " 
  The 
  stimulations 
  which 
  the 
  

   organism 
  tends 
  toward 
  are 
  those 
  which 
  heighten 
  its 
  vitality, 
  

   which 
  give 
  it 
  pleasure 
  ; 
  and 
  those 
  from 
  which 
  it 
  draws 
  back 
  

   are 
  those 
  whose 
  effect 
  upon 
  it 
  is 
  the 
  contrary 
  — 
  the 
  damaging, 
  

   the 
  painful 
  ones." 
  

  

  If 
  we 
  try 
  to 
  apply 
  this 
  statement, 
  as 
  far 
  as 
  it 
  concerns 
  

   contraction, 
  to 
  the 
  response 
  of 
  Hydra 
  to 
  a 
  mechanical 
  stimulus, 
  

   we 
  find 
  that 
  it 
  goes 
  beyond 
  the 
  facts. 
  Hydra 
  does 
  indeed 
  

   contract 
  after 
  such 
  stimulation. 
  But 
  this 
  contraction 
  is 
  not 
  

   necessarily 
  a 
  movement 
  away 
  from 
  the 
  stimulus. 
  It 
  is 
  such 
  

   if 
  the 
  stimulus 
  is 
  applied 
  to 
  hypostome 
  or 
  tentacles. 
  It 
  is 
  

   not 
  such 
  if 
  the 
  stimulus 
  acts 
  near 
  the 
  foot. 
  In 
  this 
  case, 
  in 
  

   fact, 
  the 
  body 
  comes 
  as 
  a 
  whole 
  nearer 
  to 
  the 
  source 
  of 
  

   stimulation 
  than 
  it 
  was 
  when 
  expanded. 
  This 
  is 
  true 
  also 
  of 
  

   a 
  tentacle 
  which 
  contracts 
  when 
  stimulated 
  near 
  its 
  base. 
  

   As 
  a 
  whole 
  the 
  tentacle 
  approaches 
  the 
  source 
  of 
  stimulation 
  

   rather 
  than 
  moving 
  away 
  from 
  it. 
  It 
  may 
  be 
  objected 
  that 
  

   the 
  stimuli 
  given 
  in 
  the 
  laboratory 
  represent 
  artificial 
  con- 
  

   ditions, 
  and 
  that 
  in 
  nature 
  the 
  contraction 
  of 
  Hydra 
  probably 
  

   draws 
  it 
  away 
  from 
  the 
  painful 
  stimulus. 
  But 
  to 
  make 
  this 
  

   objection 
  alloNvable 
  it 
  must 
  first 
  be 
  shown 
  that 
  harmful 
  

   stimuli 
  are 
  more 
  apt 
  to 
  reach 
  the 
  hypostome 
  than 
  the 
  foot. 
  

   At 
  present 
  I 
  see 
  no 
  reason 
  for 
  believing 
  that 
  such 
  is 
  the 
  case. 
  

  

  