﻿ON 
  SOME 
  IIOVEJIENTS 
  AND 
  EEACTIdNS 
  OF 
  HYDEA. 
  587 
  

  

  After 
  each 
  contraction 
  Hydra 
  soon 
  expands 
  again^ 
  but 
  

   toward 
  a 
  different 
  direction 
  from 
  what 
  it 
  previously 
  occupied 
  

   — 
  a 
  fact 
  to 
  which 
  my 
  attention 
  was 
  called 
  by 
  Professor 
  

   Jennings. 
  Let 
  us 
  suppose 
  that 
  the 
  Hydra 
  was 
  previously 
  

   standing 
  with 
  its 
  long 
  axis 
  oblique 
  to 
  the 
  substratum. 
  If 
  

   now, 
  for 
  any 
  reason, 
  it 
  contracts, 
  it 
  soon 
  begins 
  to 
  expand 
  

   again, 
  at 
  first 
  perpendicularly 
  to 
  the 
  substratum, 
  then 
  the 
  

   body 
  flexes 
  more 
  or 
  less 
  in 
  a 
  new 
  direction, 
  so 
  that 
  when 
  

   expansion 
  is 
  completed 
  the 
  head 
  and 
  tentacles 
  are 
  directed 
  

   into 
  a 
  region 
  different 
  from 
  that 
  which 
  they 
  occupied 
  before. 
  

   As 
  illustrated 
  in 
  Fig. 
  la, 
  when 
  a 
  represents 
  the 
  contracted 
  

   condition 
  the 
  Hydra 
  may 
  occupy, 
  successively, 
  positions 
  b, 
  a, 
  

  

  C, 
  A, 
  D, 
  A, 
  E, 
  etc. 
  

  

  It 
  may 
  be 
  useful 
  to 
  give 
  here 
  a 
  case 
  from 
  actual 
  observation. 
  

  

  A 
  Hydra 
  viridis 
  was 
  placed 
  in 
  a 
  small 
  dish 
  under 
  a 
  dis- 
  

   secting 
  microscope 
  and 
  left 
  undisturbed 
  for 
  half 
  an 
  hour. 
  

   Then 
  its 
  " 
  spontaneous 
  " 
  movements 
  were 
  recorded. 
  Pre- 
  

   cautions 
  were 
  taken 
  to 
  prevent 
  disturbance 
  of 
  the 
  dish 
  in 
  any 
  

   manner. 
  For 
  the 
  sake 
  of 
  brevity 
  in 
  writing 
  the 
  record 
  the 
  

   plane 
  of 
  the 
  microscope 
  table 
  was 
  looked 
  upon 
  as 
  a 
  map. 
  

   Thus 
  ''north" 
  means 
  the 
  side 
  away 
  from 
  the 
  observer, 
  

   " 
  east'' 
  lies 
  to 
  his 
  right, 
  etc. 
  A 
  movement 
  "upwards" 
  

   means 
  a 
  movement 
  toward 
  the 
  surface 
  of 
  the 
  water. 
  

  

  Here 
  is 
  a 
  portion 
  of 
  a 
  record 
  made 
  in 
  this 
  manner. 
  

  

  A.M. 
  

  

  9.23.- 
  Hydra 
  contracted. 
  Expansion 
  to 
  south 
  by 
  west, 
  then 
  

  

  west. 
  

   9.26.* 
  Contraction. 
  Slight 
  expansion 
  to 
  west. 
  Contraction. 
  

  

  9.27. 
  Expansion 
  to 
  north. 
  Swaying 
  to 
  west. 
  

  

  9.28. 
  Swaying 
  to 
  north-west. 
  

  

  9.31. 
  Contraction. 
  Expansion 
  to 
  south-east. 
  

  

  9.35. 
  Partial 
  contraction. 
  Rest. 
  Total 
  contraction. 
  Expan- 
  

   sion 
  to 
  west, 
  slightly 
  north. 
  

   >9.3G. 
  Contraction. 
  Expansion 
  to 
  east, 
  slightly 
  south, 
  and 
  

   strongly 
  upward. 
  

  

  9.38. 
  Contraction. 
  Expansion 
  to 
  north-east. 
  

  

  And 
  so 
  forth 
  for 
  three 
  hours. 
  

  

  