﻿ON 
  SOMK 
  MOVEMENTS 
  AND 
  REACTIONS 
  OP 
  HYDEA. 
  601 
  

  

  As 
  to 
  the 
  mecLaiiiym 
  of 
  this 
  ''escape" 
  movemcut 
  it 
  will 
  

   be 
  readily 
  recognised 
  that 
  it 
  is 
  identical 
  with 
  the 
  mechanism 
  

   ordinarily 
  employed 
  by 
  Hydia 
  to 
  move 
  from 
  place 
  to 
  place, 
  

   as 
  shown 
  in 
  Fig. 
  3. 
  The 
  important 
  point 
  is 
  that 
  here 
  on 
  

   repeated 
  stimulation 
  there 
  is 
  at 
  first 
  a 
  simple 
  contraction, 
  

   then 
  comes 
  a 
  period 
  where 
  apparently 
  there 
  is 
  some 
  sort 
  of 
  

   acclimatisation 
  to 
  tbe 
  stimulus 
  which 
  now 
  has 
  apparently 
  no 
  

   effect. 
  However, 
  this 
  is 
  only 
  apparently 
  so, 
  for 
  there 
  may 
  

   soon 
  follow 
  the 
  third 
  stage 
  in 
  which 
  the 
  Hydra 
  responds 
  to 
  

   the 
  stimulus 
  by 
  a 
  movement 
  entirely 
  different 
  and 
  even 
  

   directly 
  opposed 
  to 
  the 
  first 
  response 
  by 
  contraction. 
  This 
  

   final 
  movement 
  is 
  furthermore 
  one 
  of 
  very 
  considerable 
  com- 
  

   plexity. 
  

  

  Jennings 
  ('02) 
  has 
  found 
  in 
  Stentor 
  and 
  Vorticella 
  a 
  very 
  

   similar 
  modification 
  of 
  reactions 
  due 
  to 
  repetition 
  of 
  stimu- 
  

   lation. 
  In 
  Stentor 
  roeselii, 
  in 
  response 
  to 
  continued 
  

   stimulation 
  by 
  powdered 
  carmine, 
  there 
  is 
  the 
  following 
  

   sequence 
  : 
  bending 
  away 
  from 
  source 
  of 
  stimulation, 
  reversal 
  

   of 
  ciliary 
  current, 
  contraction, 
  and 
  finally 
  abandonment 
  of 
  its 
  

   tube. 
  In 
  Stentor 
  coeruleus 
  the 
  results 
  are 
  similar. 
  When 
  

   continuously 
  stimulated 
  by 
  means 
  of 
  some 
  solid, 
  like 
  powdered 
  

   carmine, 
  this 
  form 
  responds 
  at 
  first 
  by 
  bending 
  into 
  a 
  new 
  

   position; 
  on 
  continuance 
  of 
  stimulation 
  it 
  reverses 
  the 
  ciliary 
  

   current, 
  and 
  thus 
  repeats 
  its 
  first 
  manoeuvre; 
  this 
  is 
  followed 
  

   by 
  contraction, 
  and 
  finally 
  by 
  loosening 
  its 
  hold 
  on 
  the 
  sub- 
  

   stratum 
  and 
  swimming 
  away. 
  

  

  These 
  responses 
  result 
  from 
  stimulation 
  by 
  carmine 
  powder, 
  

   which 
  stimulates 
  in 
  part 
  chemically. 
  If 
  a 
  purely 
  mechanical 
  

   stimulus 
  be 
  used 
  Stentor 
  coeruleus 
  at 
  first 
  contracts 
  after 
  

   a 
  single 
  blow, 
  but 
  subsequently 
  it 
  may 
  require 
  as 
  many 
  as 
  

   forty. 
  But 
  eventually 
  this 
  form 
  always 
  loosens 
  its 
  hold 
  on 
  

   the 
  substratum 
  and 
  swims 
  away. 
  In 
  particular 
  sensitive 
  

   individuals 
  this 
  last 
  movement 
  is 
  resorted 
  to 
  at 
  the 
  first 
  blow 
  

   from 
  a 
  glass 
  rod, 
  but 
  this 
  is 
  the 
  exception. 
  Stentor 
  

   roeselii, 
  however, 
  as 
  well 
  as 
  Vorticella 
  and 
  some 
  other 
  

   forms, 
  show 
  true 
  acclimatisation, 
  and 
  never 
  break 
  away 
  as 
  

   the 
  result 
  of 
  a 
  purely 
  mechanical 
  stimulation. 
  

  

  VOL. 
  48, 
  PART 
  4. 
  — 
  NEW 
  SEKIES. 
  43 
  

  

  