﻿586 
  GEORGE 
  WAGNER. 
  

  

  grisea, 
  where 
  the 
  tentacles 
  have 
  enormous 
  capacity 
  for 
  

   expansion, 
  they 
  sometimes 
  hang 
  down 
  in 
  gieat 
  garlands 
  into 
  

   the 
  water, 
  the 
  individual 
  tentacles 
  often 
  so 
  thin 
  that 
  even 
  

   under 
  a 
  lens 
  they 
  are 
  barely 
  visible. 
  

  

  Even 
  when 
  undisturbed 
  Hydra 
  contracts 
  at 
  intervals. 
  This 
  

   contraction 
  is 
  very 
  sudden 
  and 
  rapid, 
  while 
  the 
  expansion, 
  

   which 
  almost 
  immediately 
  follows, 
  is 
  gradual 
  and 
  slow. 
  The 
  

   contraction 
  may 
  involve 
  both 
  body 
  and 
  tentacles. 
  It 
  may, 
  on 
  

   the 
  other 
  hand, 
  be 
  restricted 
  to 
  cither 
  body 
  or 
  tentacles, 
  or 
  

   even 
  to 
  a 
  single 
  tentacle. 
  The 
  contraction 
  occurs 
  at 
  much 
  

  

  D 
  

  

  Fig. 
  la. 
  — 
  Positions 
  occupied 
  by 
  Hydra 
  after 
  successive 
  contractions. 
  

   For 
  explanation 
  see 
  adjacent 
  text. 
  

  

  more 
  frequent 
  intervals 
  in 
  Hydra 
  viridis 
  than 
  in 
  the 
  other 
  

   forms; 
  Hydra 
  viridis 
  is 
  also 
  in 
  most 
  other 
  respects 
  the 
  

   most 
  active 
  form. 
  

  

  On 
  closely 
  observing 
  Hydra 
  viridis 
  for 
  a 
  period 
  of 
  from 
  

   three 
  to 
  four 
  hours 
  it 
  was 
  observed 
  that 
  many 
  of 
  these 
  con- 
  

   tractions 
  were 
  really 
  not 
  spontaneous, 
  but 
  due 
  to 
  slight 
  

   tremors 
  produced 
  by 
  occurrences 
  very 
  easily 
  overlooked. 
  

   Such 
  were 
  the 
  slamming 
  of 
  a 
  door 
  in 
  some 
  remote 
  part 
  of 
  the 
  

   laboratory, 
  some 
  person 
  walking 
  across 
  the 
  room 
  on 
  the 
  floor 
  

   above, 
  and 
  so 
  on. 
  Nevertheless, 
  there 
  remain 
  many 
  contrac- 
  

   tions 
  which 
  are 
  evidently, 
  as 
  Marshall 
  (1. 
  c.) 
  suggests, 
  the 
  

   results 
  of 
  internal 
  physiological 
  changes. 
  These 
  may 
  very 
  

   properly 
  be 
  termed 
  spontaneous. 
  

  

  