﻿598 
  GEORGE 
  WAGNER. 
  

  

  It 
  seems 
  to 
  me 
  tLat 
  Richet, 
  as 
  quoted 
  by 
  Baldwin 
  (1. 
  c, 
  p. 
  178), 
  

   has 
  expressed 
  the 
  probable 
  object 
  of 
  coBtractious 
  much 
  better 
  

   Avheu 
  he 
  says, 
  " 
  There 
  takes 
  place 
  a 
  series 
  of 
  general 
  move- 
  

   ments 
  of 
  flexion, 
  as 
  if 
  the 
  animal 
  wished 
  to 
  make 
  itself 
  

  

  smaller, 
  and 
  to 
  offer 
  less 
  surface 
  to 
  the 
  pain 
  '^ 
  This 
  

  

  is 
  exactly 
  the 
  lesult 
  we 
  get 
  in 
  Hydra. 
  As 
  it 
  contracts 
  it 
  

   becomes 
  more 
  nearly 
  spherical, 
  and 
  so 
  reduces 
  the 
  size 
  of 
  its 
  

   exposed 
  surface. 
  It 
  is 
  in 
  this 
  reduction 
  of 
  exposed 
  surface, 
  

   and 
  the 
  consequent 
  reduction 
  of 
  the 
  chances 
  of 
  being 
  hit, 
  that 
  

   the 
  adaptive 
  value 
  of 
  contraction 
  in 
  Hydra 
  really 
  lies. 
  Perhaps 
  

   the 
  closer 
  crowding 
  of 
  cnidoblasts, 
  consequent 
  on 
  such 
  reduc- 
  

   tion 
  of 
  surface, 
  also 
  plays 
  a 
  part. 
  

  

  What 
  will 
  happen 
  if 
  a 
  localised 
  mechanical 
  stimulus 
  is 
  

   repeated 
  at 
  regular 
  intervals 
  ? 
  A 
  Hydra 
  viridis 
  was 
  stimu- 
  

   lated 
  so 
  as 
  to 
  contract. 
  It 
  was 
  then 
  allowed 
  to 
  expand 
  again, 
  but 
  

   the 
  moment 
  expansion 
  was 
  complete 
  the 
  stimulus 
  was 
  repeated, 
  

   and 
  so 
  on 
  for 
  several 
  hours. 
  Two 
  questions 
  are 
  here 
  of 
  special 
  

   interest 
  : 
  first, 
  does 
  repetition 
  of 
  the 
  stimulus 
  cause 
  Hydra 
  to 
  

   contract 
  less 
  readily 
  ; 
  and 
  second, 
  has 
  such 
  repetition 
  any 
  

   effect 
  on 
  the 
  subsequent 
  re-expansion, 
  either 
  as 
  to 
  rapidity 
  or 
  

   the 
  occurrence 
  of 
  any 
  orientation 
  movement 
  ? 
  Both 
  questions 
  

   are 
  answered 
  decidedly 
  in 
  the 
  negative 
  as 
  far 
  as 
  stimulation 
  

   at 
  longer 
  intervals 
  is 
  concerned. 
  Here, 
  as 
  with 
  non-localised 
  

   stimulation, 
  the 
  response 
  after 
  many 
  stimulations 
  did 
  not 
  

   change. 
  The 
  contraction 
  was 
  as 
  rapid 
  and 
  as 
  complete 
  after 
  

   the 
  fiftieth 
  stimulation 
  as 
  after 
  the 
  first. 
  

  

  Re-expansion 
  also 
  was 
  not 
  changed 
  in 
  character. 
  As 
  to 
  

   the 
  direction 
  of 
  such 
  expansion 
  one 
  might 
  perhaps 
  expect 
  

   that 
  in 
  the 
  course 
  of 
  time 
  it 
  would 
  be 
  in 
  a 
  direction 
  away 
  

   from 
  the 
  side 
  from 
  which 
  the 
  stimulus 
  was 
  applied. 
  But 
  no 
  

   such 
  thing 
  occurred. 
  As 
  after 
  spontaneous 
  contraction, 
  so 
  

   here 
  re-expansion 
  was 
  toward 
  a 
  different 
  direction 
  after 
  each 
  

   contraction. 
  But 
  this 
  change 
  in 
  direction 
  could 
  not 
  be 
  

   referred 
  in 
  any 
  way 
  to 
  the 
  direction 
  from 
  which 
  stimulation 
  

   came. 
  The 
  re-expansion 
  was 
  as 
  often 
  toward 
  the 
  stimulus 
  as 
  

   away 
  from 
  it, 
  and 
  equally 
  often 
  it 
  was 
  at 
  right 
  or 
  oblique 
  

   angles 
  to 
  the 
  direction 
  of 
  stimulation. 
  Such 
  repeated 
  stimu- 
  

  

  