﻿606 
  GEORGE 
  WAGNER. 
  

  

  stool. 
  The 
  foot 
  of 
  the 
  animal 
  extends 
  straig-ht 
  up 
  into 
  the 
  

   water. 
  Almost 
  immediately 
  the 
  body 
  begins 
  to 
  extend. 
  

   When 
  about 
  half 
  extended 
  it 
  bends 
  to 
  one 
  side 
  until 
  the 
  foot 
  

   touches 
  the 
  glass 
  and 
  attaches 
  itself. 
  Sometimes 
  the 
  tentacles 
  

   loosen 
  before 
  the 
  foot 
  is 
  attached 
  and 
  the 
  Hydra 
  simply 
  

   tumbles 
  over. 
  But 
  more 
  commonly 
  the 
  entire 
  action 
  is 
  an 
  

   active 
  one 
  of 
  orientation, 
  not 
  a 
  passive 
  tumbling 
  over. 
  I 
  

   have 
  tried 
  to 
  represent 
  the 
  process 
  in 
  Fig. 
  4. 
  

  

  We 
  have 
  here, 
  then, 
  a 
  process 
  comparable 
  to 
  that 
  observed 
  

   by 
  Loeb 
  in 
  Cerianthus, 
  but 
  not 
  quite 
  so 
  complicated. 
  Ceri- 
  

   anthus 
  seeks 
  not 
  only 
  contact 
  for 
  its 
  foot 
  but 
  a 
  vertical 
  posi- 
  

   tion. 
  In 
  Hydra 
  thigmotaxis 
  alone 
  is 
  involved, 
  for 
  it 
  can 
  be 
  

   easily 
  observed 
  that 
  Hydra, 
  normally, 
  have 
  their 
  long 
  axes 
  

   disposed 
  at 
  all 
  possible 
  angles 
  to 
  the 
  force 
  of 
  gravity, 
  

  

  Reactions 
  to 
  Chemical 
  Stimuli. 
  

  

  Non-localised. 
  — 
  To 
  any 
  solution 
  in 
  which 
  Hydra 
  is 
  im- 
  

   mersed 
  it 
  responds, 
  if 
  the 
  solution 
  be 
  strong 
  enough 
  to 
  affect 
  

   it 
  at 
  all, 
  by 
  a 
  general 
  contraction. 
  Only 
  in 
  rare 
  cases 
  is 
  

   another 
  reaction 
  produced 
  ; 
  these 
  will 
  be 
  discussed 
  in 
  the 
  

   account 
  of 
  the 
  food 
  reactions. 
  When 
  contraction 
  does 
  occur 
  

   the 
  body 
  remains 
  contracted 
  for 
  a 
  considerable 
  time, 
  then 
  

   slowly 
  begins 
  to 
  extend. 
  But 
  this 
  extension 
  never 
  goes 
  

   very 
  far 
  before 
  contraction 
  again 
  occurs. 
  This 
  continues 
  

   until 
  the 
  animal 
  perishes, 
  usually 
  in 
  a 
  comparatively 
  short 
  

   time. 
  

  

  Localised. 
  — 
  The 
  method 
  of 
  applying 
  localised 
  chemical 
  

   stimuli 
  was 
  as 
  follows 
  : 
  — 
  The 
  tip 
  of 
  a 
  capillary 
  glass 
  tube 
  was 
  

   pressed 
  into 
  the 
  finely 
  powdered 
  chemical. 
  In 
  this 
  way 
  a 
  

   considerable 
  amount 
  of 
  the 
  chemical 
  was 
  forced 
  up 
  into 
  the 
  

   tube. 
  If 
  the 
  end 
  of 
  the 
  tube 
  was 
  then 
  placed 
  under 
  water 
  

   the 
  water 
  dissolved 
  the 
  chemical 
  slowly, 
  and 
  the 
  solution 
  

   gradually 
  diffused 
  from 
  the 
  month 
  of 
  the 
  tube. 
  If 
  this 
  mouth 
  

   was 
  placed 
  very 
  close 
  to 
  a 
  Hydra 
  the 
  diffusion 
  cloud 
  would 
  

  

  