﻿184 
  ANIMAL 
  BEHAVIOUR 
  

  

  

  

  A 
  fact 
  of 
  great 
  interest 
  has 
  been 
  established 
  by 
  Professor 
  

   Jennings, 
  that 
  the 
  behaviour 
  of 
  unicellular 
  organisms 
  is 
  

   modifiable 
  by 
  experience. 
  He 
  has 
  experimented, 
  for 
  instance, 
  

   with 
  a 
  trumpet-shaped 
  ciliated 
  Infusorian 
  called 
  Stentor 
  

   which 
  abounds 
  in 
  marshy 
  pools, 
  attaching 
  itself 
  by 
  the 
  nar- 
  

   row 
  end 
  to 
  a 
  water-weed, 
  and 
  surrounding 
  the 
  lower 
  half 
  of 
  

   its 
  body 
  with 
  a 
  mucus-like 
  sheath, 
  the 
  so-called 
  tube. 
  A 
  

   cloud 
  of 
  carmine 
  particles 
  is 
  introduced 
  into 
  the 
  water- 
  

   currents 
  passing 
  to 
  the 
  ciliated 
  mouth 
  of 
  the 
  Stentor. 
  It 
  

   bends 
  to 
  the 
  aboral 
  side, 
  twisting 
  on 
  its 
  stalk 
  two 
  or 
  three 
  

   times 
  as 
  it 
  bends, 
  and 
  thus 
  often 
  avoids 
  the 
  cloud 
  of 
  particles. 
  

   That 
  is 
  answer 
  one. 
  But 
  if 
  the 
  particles 
  continue 
  to 
  come, 
  

   the 
  ciliary 
  movement 
  is 
  suddenly 
  reversed 
  and 
  the 
  water 
  is 
  

   driven 
  away 
  from 
  the 
  mouth. 
  This 
  may 
  be 
  repeated 
  two 
  

   or 
  three 
  times, 
  and 
  is 
  answer 
  two. 
  If 
  the 
  Stentor 
  does 
  not 
  

   get 
  rid 
  of 
  the 
  obnoxious 
  stimulation 
  in 
  either 
  of 
  these 
  two 
  

   ways, 
  it 
  contracts 
  into 
  its 
  tube 
  and 
  suspends 
  activity, 
  this 
  

   being 
  answer 
  three. 
  After 
  half 
  a 
  minute 
  or 
  so 
  it 
  re-expands, 
  

   and 
  if 
  the 
  carmine 
  particles 
  still 
  reach 
  it, 
  it 
  contracts 
  again. 
  

   It 
  will 
  do 
  this 
  many 
  times, 
  and 
  after 
  each 
  contraction 
  it 
  

   stays 
  a 
  little 
  longer 
  in 
  its 
  tube 
  than 
  it 
  did 
  before. 
  Finally, 
  

   if 
  no 
  improvement 
  in 
  circumstances 
  rewards 
  its 
  trials, 
  it 
  

   breaks 
  its 
  attachment 
  and 
  swims 
  forwards 
  or 
  backwards 
  

   away 
  from 
  its 
  tube. 
  And 
  this 
  is 
  answer 
  four. 
  " 
  The 
  stim- 
  

   ulus 
  and 
  other 
  external 
  conditions 
  remaining 
  the 
  same, 
  the 
  

   organism 
  responds 
  by 
  a 
  series 
  of 
  reactions 
  becoming 
  of 
  more 
  

   and 
  more 
  pronounced 
  character, 
  until 
  by 
  on 
  of 
  them 
  it 
  rids 
  

   itself 
  of 
  the 
  stimulation' 
  (p. 
  176). 
  " 
  The 
  same 
  individual 
  

   does 
  not 
  always 
  behave 
  in 
  the 
  same 
  way 
  under 
  the 
  same 
  

   external 
  conditions, 
  but 
  the 
  behaviour 
  depends 
  upon 
  the 
  

   physiological 
  condition 
  of 
  the 
  animal. 
  The 
  reaction 
  to 
  any 
  

   given 
  stimulus 
  is 
  modified 
  by 
  the 
  past 
  experience 
  of 
  the 
  ani- 
  

  

  