﻿594 
  GEORGI'] 
  VVAGNEIt. 
  

  

  pletely 
  contract; 
  instead, 
  it 
  partly 
  contracts, 
  remains 
  at 
  rest 
  

   a 
  few 
  seconds, 
  then, 
  witliont 
  additional 
  stimulation, 
  contracts 
  

   further, 
  and 
  so 
  on, 
  repeating 
  the 
  process 
  three 
  or 
  four 
  times 
  

   before 
  contraction 
  is 
  complete. 
  

  

  Of 
  variation 
  in 
  sensitivity 
  between 
  individual 
  Hydras 
  I 
  

   have 
  already 
  spoken. 
  I 
  attempted 
  to 
  discover 
  whether 
  there 
  

   was 
  any 
  such 
  variation 
  between 
  various 
  parts 
  of 
  an 
  individual. 
  

   There 
  is 
  one 
  difficulty 
  in 
  the 
  way 
  of 
  such 
  an 
  attempt. 
  Even 
  

   after 
  considerable 
  practice 
  it 
  is 
  almost 
  impossible 
  to 
  give 
  two 
  

   successive 
  stimuli 
  of 
  exactly 
  the 
  same 
  magnitude. 
  Perhaps 
  

   slight 
  differences 
  in 
  sensitivity 
  would 
  be 
  thus 
  obscured; 
  but 
  

   I 
  am 
  convinced 
  that 
  so 
  large 
  a 
  number 
  of 
  experiments 
  as 
  were 
  

   made 
  would 
  eliminate 
  this 
  soui'ce 
  of 
  error. 
  I 
  therefore 
  feel 
  

   justified 
  in 
  saying 
  that 
  all 
  parts 
  of 
  Hydra 
  are 
  about 
  equally 
  

   sensitive. 
  An 
  individual 
  Hydra 
  Avill 
  give 
  practically 
  the 
  

   same 
  response 
  after 
  each 
  of 
  many 
  stimulations 
  of 
  approxi- 
  

   mately 
  equal 
  strength. 
  This 
  is 
  true 
  whether 
  the 
  successive 
  

   stimuli 
  are 
  applied 
  to 
  the 
  same 
  region 
  of 
  the 
  body 
  or 
  to 
  

   different 
  regions 
  of 
  the 
  body, 
  foot, 
  hypostome, 
  or 
  tentacles. 
  

  

  Next 
  the 
  effect 
  of 
  stimuli 
  of 
  different 
  intensities 
  was 
  tested 
  

   when 
  applied 
  to 
  the 
  same 
  part 
  of 
  the 
  organism. 
  Of 
  course, 
  

   very 
  sensitive 
  specimens 
  completely 
  contract 
  after 
  even 
  an 
  

   excessively 
  weak 
  stimulus. 
  But 
  in 
  most 
  cases 
  there 
  is 
  a 
  

   variation 
  in 
  the 
  reaction 
  parallel 
  to 
  the 
  variation 
  in 
  the 
  

   stimulus. 
  This 
  is 
  not 
  in 
  accord 
  with 
  Marshall, 
  who 
  states 
  

   (loc. 
  cit.) 
  that 
  in 
  response 
  to 
  an 
  external 
  stimulus 
  both 
  body 
  

   and 
  tentacles 
  always 
  contract. 
  

  

  Remembering 
  that 
  "weak" 
  and 
  '^strong" 
  are 
  relative 
  terms 
  

   only, 
  the 
  reactions 
  may 
  be 
  perhaps 
  classified 
  as 
  follows 
  : 
  

  

  A. 
  Stimulation 
  of 
  body 
  : 
  

  

  1. 
  Weak: 
  body 
  partly 
  contracts. 
  

  

  2. 
  Medium 
  : 
  body 
  completely 
  contracts. 
  

  

  3. 
  Strong 
  : 
  body 
  and 
  tentacles 
  contract. 
  

  

  B. 
  Stimulation 
  of 
  a 
  tentacle 
  : 
  

  

  1. 
  Weak 
  : 
  tentacle 
  stimulated 
  contracts. 
  

  

  2. 
  Medium 
  : 
  all 
  tentacles 
  contract. 
  

  

  3. 
  Strong 
  : 
  tentacles 
  and 
  body 
  contract. 
  

  

  