﻿ON 
  SOME 
  MOVEMENTS 
  AND 
  EEACTIONJS 
  OF 
  HYDBA. 
  615 
  

  

  starved 
  for 
  some 
  time. 
  A 
  Cyclops 
  presented 
  to 
  the 
  same 
  

   Hydra 
  was 
  captured 
  aud 
  swallowed 
  very 
  quickly, 
  as 
  was 
  a 
  

   piece 
  of 
  raw 
  beef. 
  

  

  Why 
  does 
  Hydra 
  not 
  capture 
  and 
  swallow 
  ostracods 
  just 
  

   as 
  it 
  does 
  others 
  of 
  the 
  smaller 
  Crustacea 
  'i 
  It 
  seems 
  to 
  me 
  

   the 
  answer 
  is 
  not 
  far 
  to 
  seek. 
  Ostracods 
  are 
  enclosed 
  in 
  

   hard 
  chitiuous 
  shells, 
  which, 
  as 
  far 
  as 
  I 
  have 
  observed, 
  are 
  

   never 
  opened 
  more 
  than 
  a 
  little 
  way. 
  The 
  organic 
  fluids 
  

   given 
  off 
  by 
  the 
  animal 
  therefore 
  escape 
  very 
  slowly 
  beyond 
  

   the 
  confines 
  of 
  the 
  shell. 
  Hence 
  the 
  minute 
  chemical 
  stimulus 
  

   given 
  by 
  an 
  ostracod 
  as 
  it 
  comes 
  into 
  contact 
  with 
  the 
  Hydra 
  

   is 
  inadequate 
  ; 
  but 
  adequate 
  chemical 
  stimulation 
  is, 
  in 
  all 
  

   cases, 
  a 
  prerequisite 
  of 
  the 
  food 
  reaction, 
  and 
  so 
  the 
  ostracod, 
  

   giving 
  only 
  a 
  mechanical 
  impulse, 
  is 
  not 
  recognised 
  as 
  food. 
  

   if 
  this 
  theory 
  is 
  correct, 
  then 
  we 
  ought 
  to 
  be 
  able 
  to 
  cause 
  

   the 
  Hydra 
  to 
  swallow 
  the 
  ostracod, 
  by 
  bringing 
  to 
  bear 
  a 
  

   chemical 
  stimulus 
  while 
  simultaneously 
  presenting 
  the 
  ostra- 
  

   cod. 
  This 
  is 
  easily 
  done 
  by 
  crushing 
  the 
  ostracod 
  slightly, 
  

   so 
  that 
  some 
  of 
  the 
  juices 
  of 
  the 
  body 
  freely 
  flow 
  out. 
  if 
  

   such 
  an 
  ostracod 
  is 
  presented 
  to 
  a 
  starved 
  Hydra 
  the 
  rapidity 
  

   with 
  which 
  it 
  disappears 
  into 
  the 
  food 
  cavity 
  is 
  little 
  short 
  

   of 
  marvellous. 
  

  

  Nematocyst 
  Discharge. 
  

  

  As 
  is 
  well 
  known 
  Hydra, 
  like 
  most 
  ccelenterates, 
  carries 
  in 
  

   its 
  ectoderm 
  a 
  great 
  number 
  of 
  cnidoblasts. 
  They 
  are 
  fairly 
  

   plentiful 
  over 
  most 
  of 
  the 
  body, 
  but 
  occur 
  most 
  numerously 
  

   on 
  the 
  tentacles, 
  where 
  they 
  form 
  small 
  tubercle-like 
  masses. 
  

   They 
  seem 
  to 
  serve 
  chieliy 
  for 
  the 
  capture 
  of 
  prey, 
  though 
  

   secondarily 
  they 
  may 
  also 
  form 
  a 
  means 
  of 
  protection 
  against 
  

   enemies. 
  

  

  There 
  has 
  been 
  considerable 
  controversy 
  as 
  to 
  the 
  nature 
  

   of 
  the 
  action 
  of 
  the 
  nematocysts 
  on 
  the 
  organism 
  at 
  which 
  

   they 
  are 
  discharged. 
  Various 
  authors 
  (for 
  instance, 
  Schneider, 
  

   '90) 
  claim 
  that 
  the 
  usual 
  supposition 
  that 
  the 
  nematocysts 
  

  

  