﻿ANIMAL 
  BEHAVIOUR 
  187 
  

  

  In 
  typical 
  and 
  simple 
  cases, 
  a 
  reflex 
  action 
  involves 
  (1) 
  the 
  

   receptor 
  of 
  a 
  stimulus 
  the 
  sensory 
  or 
  perceptory 
  nerve-cell 
  

   from 
  which 
  impulses 
  pass 
  in 
  to 
  the 
  central 
  nervous 
  system 
  ; 
  

   (2) 
  a 
  ' 
  motor 
  ' 
  nerve-cell 
  which 
  connects 
  the 
  central 
  nervous 
  

   system 
  with 
  a 
  muscle 
  or 
  a 
  gland; 
  and 
  (3) 
  between 
  these 
  

   two 
  a 
  * 
  communicating 
  ', 
  or 
  internuncial, 
  or 
  ' 
  associative 
  ' 
  

   nerve-cell 
  connecting 
  them 
  within 
  the 
  nervous 
  system. 
  The 
  

   receptor 
  neurone 
  has 
  its 
  cell-body 
  outside 
  of 
  the 
  nerve-centre 
  ; 
  

   the 
  motor 
  neurone, 
  with 
  its 
  cell-body 
  within 
  the 
  nerve-centre, 
  

   sends 
  a 
  nerve-fibre 
  to 
  some 
  peripheral 
  effector 
  organ 
  ; 
  the 
  

   associative 
  neurone 
  connects 
  the 
  two 
  others. 
  Thus 
  is 
  formed 
  

   a 
  ' 
  reflex 
  arc 
  ', 
  the 
  functional 
  unit 
  of 
  the 
  nervous 
  system. 
  In 
  

   most 
  cases 
  the 
  arrangements 
  are 
  more 
  complex 
  and 
  several 
  

   i 
  reflex 
  arcs 
  ' 
  become 
  interlinked. 
  But 
  the 
  point 
  is 
  that 
  

   reflex 
  actions 
  do 
  not 
  require 
  individual 
  correlation; 
  that 
  is 
  

   pre-established. 
  Yet 
  it 
  is 
  important 
  not 
  to 
  think 
  of 
  reflexes 
  

   too 
  simply. 
  

  

  Combination 
  of 
  Reflexes 
  in 
  Unified 
  Behaviour. 
  The 
  per- 
  

   fection 
  of 
  reflexes 
  is 
  well 
  illustrated 
  in 
  the 
  behaviour 
  of 
  

   a 
  sea-urchin, 
  which 
  has 
  no 
  nerve-ganglia. 
  Its 
  test 
  is 
  cov- 
  

   ered 
  with 
  mobile 
  spines 
  and 
  snapping 
  blades 
  (pedicellaria?) 
  

   which 
  react 
  in 
  definite 
  ways 
  to 
  definite 
  stimuli 
  and 
  have 
  an 
  

   astonishing 
  independence. 
  For 
  a 
  single 
  spine 
  or 
  pedicellaria 
  

   on 
  an 
  isolated 
  fragment 
  of 
  shell 
  reacts 
  very 
  much 
  as 
  usual. 
  

   In 
  the 
  uninjured 
  creature 
  the 
  spines 
  and 
  other 
  structures 
  are 
  

   all 
  connected 
  by 
  a 
  nervous 
  network 
  on 
  the 
  surface 
  of 
  the 
  

   shell, 
  and 
  they 
  act 
  harmoniously, 
  working 
  into 
  one 
  another's 
  

   hands, 
  securing 
  effective 
  defence 
  and 
  locomotion. 
  According 
  

   to 
  von 
  Uexkiill 
  the 
  sea-urchin 
  is 
  a 
  " 
  republic 
  of 
  reflexes 
  ". 
  

   " 
  The 
  separate 
  reflex 
  arcs 
  are 
  so 
  constituted 
  and 
  so 
  put 
  to- 
  

   gether 
  that 
  the 
  simultaneous 
  but 
  independent 
  course 
  of 
  the 
  

   reflexes 
  in 
  response 
  to 
  an 
  outer 
  stimulus 
  produces 
  a 
  definite 
  

  

  