﻿608 
  GEORGE 
  WAGNER. 
  

  

  the 
  reaction. 
  This 
  never 
  is 
  the 
  case 
  when 
  the 
  body 
  is 
  the 
  

   part 
  stimulated. 
  

  

  If 
  the 
  tube 
  is 
  placed 
  inside 
  the 
  circle 
  of 
  tentacles, 
  so 
  that 
  

   the 
  cloud 
  first 
  reaches 
  the 
  surface 
  of 
  the 
  hypostome, 
  the 
  

   tentacles 
  simultaneously 
  sway 
  inward, 
  then 
  contract. 
  

  

  Here 
  we 
  have 
  then 
  a 
  definite 
  orientation 
  reaction 
  toward 
  a 
  

   chemical 
  stimulus. 
  It 
  is 
  important 
  that 
  we 
  understand 
  just 
  

   how 
  it 
  comes 
  about. 
  If 
  after 
  the 
  application 
  of 
  such 
  a 
  

   localised 
  stimulus 
  we 
  carefully 
  remove 
  the 
  methyl 
  green 
  that 
  

   has 
  gone 
  into 
  solution 
  by 
  means 
  of 
  a 
  pipette, 
  the 
  Hydra 
  will 
  

   soon 
  begin 
  to 
  expand. 
  But 
  now 
  it 
  can 
  be 
  noticed 
  that 
  in 
  the 
  

   region 
  affected 
  by 
  the 
  stain 
  there 
  is 
  no 
  expansion, 
  and 
  the 
  

   ectoderm 
  in 
  this 
  region 
  is 
  seen 
  to 
  be 
  permanently 
  stained. 
  

   Often 
  there 
  is 
  no 
  return 
  of 
  mobility 
  in 
  this 
  region, 
  even 
  after 
  

   some 
  hours, 
  the 
  cells 
  in 
  such 
  cases 
  being 
  probably 
  dead. 
  On 
  

   account 
  of 
  this 
  local 
  absence 
  of 
  expansion 
  the 
  body, 
  as 
  soon 
  as 
  

   it 
  begins 
  to 
  expand, 
  also 
  begins 
  to 
  flex 
  again 
  toward 
  the 
  same 
  

   side 
  as 
  it 
  did 
  before 
  under 
  the 
  influence 
  of 
  the 
  stimulus. 
  

  

  The 
  direct 
  result 
  then 
  of 
  such 
  local 
  chemical 
  stimulation 
  is 
  

   a 
  contraction 
  of 
  those 
  ectoderm 
  cells 
  that 
  come 
  in 
  direct 
  

   contact 
  with 
  the 
  chemical. 
  This 
  local 
  contraction 
  necessarily 
  

   causes 
  the 
  body 
  (or 
  tentacle) 
  to 
  flex 
  at 
  the 
  point 
  affected, 
  and 
  

   to 
  flex 
  toward 
  the 
  side 
  from 
  which 
  the 
  stimulus 
  acted. 
  But 
  

   only 
  injurious 
  chemicals 
  cause 
  this 
  reaction, 
  and 
  by 
  it 
  the 
  

   Hydra 
  is 
  carried 
  into 
  a 
  destructive 
  solution. 
  This 
  bending 
  

   of 
  the 
  body 
  is, 
  therefore, 
  anything 
  rather 
  than 
  adaptive 
  in 
  

   nature. 
  It 
  is 
  probable 
  that 
  such 
  strongly 
  localised 
  chemical 
  

   stimuli 
  of 
  a 
  destructive 
  nature 
  play 
  no 
  part 
  in 
  the 
  normal 
  life 
  

   of 
  Hydra. 
  The 
  reaction, 
  therefore, 
  forms 
  no 
  part 
  of 
  the 
  

   normal 
  behaviour, 
  but 
  is 
  more 
  or 
  less 
  pathological 
  in 
  character. 
  

   It 
  recalls 
  the 
  false 
  tropisms 
  in 
  plants, 
  more 
  especially 
  false 
  

   traumatropism. 
  Ciesielski 
  (^72) 
  found 
  that 
  when 
  a 
  root 
  was 
  

   severely 
  injured 
  on 
  one 
  side 
  at 
  some 
  point 
  lying 
  within 
  the 
  

   growing 
  zone 
  there 
  was 
  produced 
  a 
  curvature 
  at 
  this 
  point, 
  

   the 
  curving 
  being 
  toward 
  the 
  side 
  injured. 
  If, 
  on 
  the 
  other 
  

   hand, 
  the 
  injury 
  was 
  only 
  slight 
  a 
  curvature 
  also 
  resulted, 
  

   but 
  in 
  this 
  case 
  it 
  was 
  away 
  from 
  the 
  side 
  injured. 
  The 
  

  

  