﻿588 
  GEOUGH 
  WAGNER. 
  

  

  The 
  extended 
  Hydra 
  may 
  also 
  change 
  the 
  direction 
  of 
  its 
  

   long 
  axis 
  without 
  a 
  general 
  contraction, 
  by 
  mere 
  flexion 
  of 
  

   the 
  expanded 
  body. 
  Sometimes 
  the 
  change 
  from 
  one 
  oblique 
  

   position 
  to 
  another 
  is 
  brought 
  about 
  by 
  first 
  swaying 
  to 
  the 
  

   vertical^ 
  and 
  then 
  to 
  the 
  new 
  oblique 
  position. 
  Quite 
  as 
  often, 
  

   however, 
  it 
  occurs 
  through 
  circumnutation 
  around 
  the 
  

   attached 
  foot. 
  In 
  this 
  case 
  there 
  appears 
  first 
  a 
  contraction 
  

   of 
  the 
  ectoderm 
  on 
  one 
  side 
  near 
  the 
  foot. 
  This 
  contraction 
  

   then 
  travels 
  towards 
  the 
  hypostome 
  in 
  slightly 
  spiral 
  form. 
  

   The 
  Hydra, 
  in 
  this 
  manner, 
  slowly 
  swings 
  around, 
  the 
  body 
  

   curved 
  into 
  a 
  complete 
  loop 
  or 
  even 
  beyond 
  (Fig. 
  1). 
  

  

  It 
  can 
  be 
  seen 
  that 
  by 
  either 
  of 
  these 
  methods 
  Hydra 
  

   extends 
  its 
  body 
  successively 
  in 
  many 
  different 
  directions 
  in 
  

  

  Till. 
  1. 
  — 
  Hydra 
  viridis 
  changing 
  position 
  of 
  body 
  by 
  a 
  spiral 
  

   contraction 
  near 
  the 
  foot. 
  

  

  a 
  comparatively 
  short 
  space 
  of 
  time. 
  It 
  is 
  thus 
  enabled 
  to 
  

   explore 
  a 
  relatively 
  large 
  space, 
  and 
  so 
  greatly 
  increase 
  the 
  

   probability 
  of 
  its 
  chances 
  of 
  capturing 
  prey. 
  Herein 
  lies, 
  

   undoubtedly, 
  the 
  biological 
  significance 
  of 
  the 
  behaviour 
  

   described. 
  

  

  These 
  intermittent 
  spontaneous 
  contractions 
  and 
  expansions 
  

   are 
  much 
  more 
  frequent 
  in 
  Hydra 
  viridis 
  than 
  in 
  the 
  other 
  

   species. 
  Marshall 
  (1. 
  c.) 
  is 
  probably 
  correct 
  when 
  he 
  corre- 
  

   lates 
  this 
  fact 
  with 
  the 
  shorter 
  tentacles 
  of 
  Hydra 
  viridis. 
  

   The 
  other 
  species 
  have 
  long 
  thread-like 
  tentacles 
  often 
  ex- 
  

   tended 
  to 
  a 
  length 
  several 
  times 
  that 
  of 
  the 
  body. 
  By 
  swaying 
  

   these 
  to 
  and 
  fro 
  they 
  can 
  explore 
  a 
  large 
  territory 
  without 
  any 
  

   movement 
  of 
  the 
  body, 
  while 
  in 
  a 
  quiet 
  Hydra 
  viridis 
  the 
  

   tentacles 
  have 
  very 
  little 
  spread. 
  

  

  