﻿ON 
  SOUE 
  MOVEMENTS 
  AND 
  REACTIONS 
  OF 
  HYDEA. 
  617 
  

  

  a 
  very 
  profuse 
  discharge 
  of 
  iiematocysts. 
  The 
  first 
  Hydra 
  

   reciprocated, 
  and 
  in 
  a 
  very 
  short 
  time 
  both 
  Hydra 
  were 
  

   apparently 
  dead. 
  

  

  As 
  far 
  as 
  my 
  observations 
  go 
  the 
  nematocj'sts 
  are 
  not 
  

   always 
  necessary 
  for 
  paralysing 
  prey. 
  Appaiently 
  this 
  is 
  

   sometimes 
  brought 
  about 
  by 
  some 
  purely 
  fluid 
  discharge 
  from 
  

   the 
  tentacles. 
  In 
  larger 
  specimens 
  of 
  Hydra 
  grisea 
  the 
  

   discharged 
  uematocysts 
  can 
  easily 
  be 
  seen 
  under 
  the 
  Braus- 
  

   Driiner 
  microscope. 
  Yet 
  I 
  have 
  often 
  observed, 
  with 
  such 
  a 
  

   microscope, 
  that 
  small 
  Crustacea, 
  such 
  as 
  Cyclops, 
  were 
  pai*a- 
  

   lysed 
  when 
  approaching 
  too 
  near 
  the 
  tentacles, 
  without 
  any 
  

   discharge 
  of 
  nematocysts. 
  At 
  least, 
  very 
  close 
  search, 
  even 
  

   with 
  a 
  microscope 
  of 
  higher 
  power, 
  disclosed 
  none. 
  It 
  may 
  

   be 
  said 
  that 
  what 
  occurred 
  here 
  is 
  simply 
  the 
  death-feigning 
  

   so 
  common 
  in 
  the 
  lower 
  Crustacea, 
  but 
  that 
  is 
  not 
  the 
  case. 
  

   Death-feigning 
  lasts 
  only 
  a 
  very 
  short 
  time. 
  In 
  the 
  pheno- 
  

   menon 
  under 
  consideration 
  the 
  animal 
  remains 
  motionless 
  for 
  

   many 
  minutes, 
  and 
  then, 
  provided 
  the 
  Hydra 
  has 
  been 
  

   removed, 
  or, 
  for 
  some 
  reason, 
  does 
  not 
  attempt 
  to 
  swallow 
  its 
  

   prey, 
  motion 
  slowly 
  and 
  gradually 
  returns. 
  

  

  The 
  structure 
  of 
  the 
  cnidoblast, 
  and 
  especially 
  the 
  pro- 
  

   truding 
  cnidocil, 
  suggest 
  a 
  direct 
  mechanical 
  arrangement 
  for 
  

   its 
  discharge. 
  Schulze 
  ("71) 
  suggests 
  such 
  an 
  explanation 
  

   for 
  these 
  discharges. 
  He 
  believed 
  that 
  a 
  pressure 
  from 
  

   without 
  on 
  the 
  cnidocil 
  would 
  directly 
  disturb 
  the 
  mechanical 
  

   equilibrium 
  in 
  the 
  cnidoblast, 
  and 
  so 
  cause 
  a 
  discharge. 
  

   After 
  the 
  muscular 
  and 
  nervous 
  nature 
  of 
  certain 
  cells 
  iu 
  

   Hydra 
  was 
  recognised 
  the 
  explanations 
  offered 
  took 
  on 
  a 
  

   more 
  physiological 
  character 
  ; 
  according 
  to 
  Chun 
  ('93) 
  there 
  

   was 
  involved 
  a 
  long 
  passage 
  of 
  the 
  stimulus 
  through 
  distinct 
  

   ganglia 
  and 
  nerve 
  fibres; 
  but 
  all 
  explanation, 
  as 
  far 
  as 
  I 
  know, 
  

   hold 
  to 
  the 
  idea 
  that 
  the 
  cnidocil 
  serves 
  the 
  mechanical 
  

   function 
  of 
  a 
  trigger. 
  

  

  Zoja 
  (1. 
  c.) 
  found 
  that 
  he 
  could 
  touch 
  the 
  cnidocils 
  repeatedly 
  

   and 
  rudely 
  without 
  getting 
  a 
  discharge 
  of 
  nematocysts, 
  but 
  

   he 
  did 
  not 
  draw 
  any 
  conclusions 
  from 
  this 
  fact. 
  R, 
  v. 
  

   Lendenfeld 
  ('83), 
  working 
  with 
  actinians, 
  found 
  that 
  the 
  ten- 
  

  

  VOL. 
  48, 
  PART 
  4. 
  — 
  NEW 
  SERIES. 
  44 
  

  

  