﻿ON 
  SOME 
  MOVEMENTS 
  AND 
  REACTIONS 
  OF 
  HYDRA. 
  603 
  

  

  tised 
  by 
  chloroform 
  or 
  etlier^ 
  always 
  assumed 
  this 
  position 
  of 
  

   semi-extension. 
  

  

  There 
  still 
  remaius 
  the 
  very 
  involved 
  "escape" 
  movement. 
  

   It 
  will 
  be 
  useful 
  to 
  compare 
  this 
  with 
  the 
  reactions 
  of 
  Para- 
  

   mecium 
  toward 
  stiuiuli 
  of 
  various 
  kinds 
  as 
  studied 
  by 
  

   Jennings. 
  Paramecium 
  responds 
  by 
  stopping 
  its 
  spiral 
  

   movement, 
  jerking 
  backwards, 
  swiuging 
  toward 
  the 
  aboral 
  

   side, 
  and 
  finally 
  moving 
  forward 
  again. 
  If 
  it 
  then 
  comes 
  in 
  

   contact 
  with 
  the 
  source 
  of 
  stimulation 
  again 
  it 
  goes 
  through 
  

   the 
  whole 
  manoeuvre 
  again, 
  until 
  finally 
  its 
  forward 
  movement 
  

   carries 
  it 
  out 
  of 
  the 
  region 
  of 
  stimulation. 
  As 
  far 
  as 
  is 
  now 
  

   known 
  this 
  method 
  of 
  reaction 
  is 
  never 
  varied 
  as 
  a 
  result 
  of 
  

   experience. 
  That 
  escape 
  is 
  finally 
  effected 
  depends 
  entirely 
  

   upon 
  the 
  element 
  of 
  chance 
  involved. 
  The 
  movement, 
  as 
  

   such, 
  bears 
  no 
  relation 
  to 
  the 
  source 
  of 
  stimulation 
  ; 
  it 
  bears 
  

   a 
  very 
  definite 
  relation 
  to 
  the 
  structure 
  of 
  the 
  Para- 
  

   mecium. 
  

  

  The 
  mere 
  contraction 
  of 
  Hydra 
  in 
  response 
  to 
  a 
  mechanical 
  

   stimulus 
  is 
  a 
  reaction 
  quite 
  parallel 
  to 
  this. 
  It 
  also 
  has 
  no 
  

   relation 
  to 
  the 
  direction 
  from 
  which 
  stimulation 
  comes, 
  while 
  

   it 
  has 
  a 
  fixed 
  relation 
  to 
  the 
  structure 
  of 
  the 
  Hydra. 
  There 
  

   is, 
  however, 
  a 
  difference. 
  If 
  the 
  Paramecium 
  does 
  not 
  suc- 
  

   ceed 
  in 
  escaping 
  from 
  the 
  stimulus 
  at 
  the 
  first 
  trial, 
  it 
  may 
  do 
  

   so 
  at 
  a 
  second 
  or 
  any 
  subsequent 
  attempt. 
  If 
  the 
  Hydra 
  

   does 
  not 
  escape 
  by 
  the 
  first 
  contraction 
  it 
  will 
  not 
  escape 
  by 
  

   a 
  subsequent 
  one, 
  for 
  its 
  sessile 
  mode 
  of 
  life 
  precludes 
  the 
  

   element 
  of 
  chance 
  involved 
  in 
  the 
  movements 
  of 
  a 
  free 
  

   swimming 
  form 
  like 
  Paramecium. 
  Hydra, 
  however, 
  does 
  

   not 
  continue 
  to 
  respond 
  indefinitely 
  by 
  contraction, 
  but 
  

   resorts, 
  after 
  a 
  short 
  time, 
  to 
  another 
  method 
  not 
  a 
  necessary 
  

   consequence 
  of 
  the 
  first, 
  and 
  by 
  this 
  second 
  method 
  accom- 
  

   plishes 
  its 
  purpose. 
  This 
  second 
  movement 
  also 
  has 
  no 
  strict 
  

   relation 
  to 
  the 
  direction 
  from 
  which 
  the 
  stimulation 
  proceeds. 
  

   Nevertheless, 
  it 
  always 
  succeeds 
  in 
  removing 
  the 
  Hydra 
  from 
  

   the 
  influence 
  of 
  the 
  stimulus. 
  Having 
  no 
  fixed 
  relation 
  to 
  

   the 
  stimulus 
  the 
  idea 
  arises 
  that 
  it 
  may 
  have 
  a 
  definite 
  rela- 
  

   tion 
  to 
  some 
  structural 
  feature 
  of 
  the 
  Hydra. 
  But 
  evidence 
  

  

  