﻿ON 
  SOME 
  jrOVEMENTS 
  AND 
  REACTIONS 
  OF 
  HYDRA. 
  593 
  

  

  the 
  force 
  of 
  the 
  taps 
  becomes 
  very 
  marked^ 
  or 
  if 
  by 
  the 
  

   motion 
  of 
  the 
  water 
  the 
  Hydra 
  is 
  thrown 
  against 
  any 
  solid 
  

   surface, 
  contraction 
  recurs. 
  

  

  When 
  in 
  its 
  natural 
  surroundings, 
  whether 
  in 
  stagnant 
  or 
  

   running 
  watei". 
  Hydra 
  is 
  exposed 
  to 
  just 
  such 
  a 
  rapid 
  suc- 
  

   cession 
  of 
  stimuli, 
  due 
  to 
  the 
  constant 
  motion 
  of 
  the 
  water. 
  

   If 
  it 
  were 
  not 
  for 
  its 
  power 
  of 
  acclimatisation 
  to 
  such 
  a 
  

   succession 
  the 
  Hydra 
  would 
  necessarily 
  be 
  constantly 
  in 
  a 
  

   state 
  of 
  contraction. 
  The 
  response 
  on 
  increasing 
  the 
  force 
  

   of 
  the 
  blows 
  may 
  have 
  its 
  biological 
  significance 
  as 
  a 
  protec- 
  

   tion 
  against 
  being* 
  washed 
  away 
  by 
  any 
  sudden 
  increase 
  in 
  

   the 
  motion 
  of 
  the 
  water. 
  

  

  If 
  the 
  interval 
  between 
  stimuli 
  is 
  considerably 
  increased 
  so 
  

   as 
  to 
  allow 
  the 
  Hydra 
  to 
  expand 
  fully 
  after 
  each 
  contraction, 
  

   the 
  tap 
  being 
  given 
  the 
  moment 
  expansion 
  ceases, 
  the 
  result 
  

   is 
  a 
  different 
  one. 
  Thei'e 
  is, 
  in 
  this 
  case, 
  no 
  change 
  whatever 
  

   in 
  the 
  reaction 
  after 
  repeated 
  stimulation. 
  The 
  course 
  of 
  

   events 
  after 
  the 
  fiftieth 
  tap 
  is 
  no 
  different 
  from 
  that 
  after 
  the 
  

   first; 
  after 
  each 
  stimulation 
  there 
  is 
  a 
  contraction, 
  followed 
  

   shortly 
  by 
  re-expansion. 
  Thus 
  we 
  get 
  a 
  different 
  result 
  so 
  

   far 
  as 
  acclimatisation 
  to 
  the 
  stimulus 
  is 
  concerned, 
  depending 
  

   on 
  whether 
  the 
  stimuli 
  are 
  repeated 
  rapidly 
  or 
  only 
  after 
  a 
  

   considerable 
  interval. 
  In 
  the 
  lateral 
  case 
  one 
  stimulation 
  has 
  

   evidently 
  no 
  effect 
  on 
  the 
  response 
  to 
  a 
  succeeding 
  one. 
  

   Recovery 
  from 
  the 
  acclimatising 
  effect 
  must, 
  therefore, 
  be 
  

   very 
  rapid. 
  

  

  Localised 
  Mechanical 
  Stimuli. 
  — 
  In 
  order 
  to 
  apply 
  

   localised 
  mechanical 
  stimuli 
  I 
  prepared 
  capillary 
  glass 
  rods, 
  

   attached 
  by 
  means 
  of 
  sealing 
  wax 
  to 
  larger 
  glass 
  rods 
  as 
  

   handles. 
  With 
  such 
  a 
  rod 
  it 
  was 
  very 
  easy 
  to 
  touch 
  with 
  

   any 
  desirable 
  force 
  any 
  part 
  of 
  the 
  Hydra 
  without 
  producino* 
  

   any 
  movement 
  in 
  the 
  water 
  such 
  as 
  in 
  itself 
  might 
  cause 
  a 
  

   contraction. 
  

  

  A 
  Hydra 
  touched 
  with 
  such 
  a 
  rod 
  will, 
  of 
  course, 
  contract, 
  

   provided 
  the 
  blow 
  is 
  not 
  too 
  light. 
  This 
  contraction 
  is 
  

   usually 
  so 
  sudden 
  that 
  no 
  details 
  of 
  the 
  process 
  can 
  be 
  

   observed. 
  In 
  some 
  cases 
  the 
  body 
  does 
  not 
  at 
  once 
  com- 
  

  

  