﻿Stphonophora 
  of 
  the 
  Canary 
  Islands. 
  217 
  

  

  the 
  inferior 
  open 
  ends 
  of 
  the 
  tracheae, 
  from 
  which 
  it 
  afterwards 
  

   passes 
  into 
  the 
  chambers 
  to 
  be 
  discharged 
  outwards 
  through 
  

   the 
  stigmata. 
  In 
  opposition 
  to 
  this 
  we 
  find 
  that 
  the 
  cushion 
  

   of 
  tissue 
  in 
  question 
  is 
  quite 
  different 
  in 
  its 
  histological 
  struc- 
  

   ture 
  from 
  the 
  secondary 
  ectoderm, 
  which, 
  as 
  I 
  have 
  demon- 
  

   strated 
  *, 
  has 
  the 
  function 
  of 
  a 
  gas-gland, 
  and, 
  further, 
  that 
  

   the 
  trachea? 
  which 
  surround 
  the 
  polypes 
  for 
  the 
  most 
  part 
  

   terminate 
  at 
  spots 
  where 
  the 
  tissue 
  in 
  question 
  is 
  deficient. 
  

   The 
  latter 
  is 
  rather 
  composed, 
  of 
  undeveloped 
  thread-cells, 
  

   and 
  is 
  precisely 
  homologous 
  with 
  that 
  urticating-pad 
  which 
  

   surrounds 
  the 
  anterior 
  stomachal 
  portion 
  of 
  the 
  sucking-tubes. 
  

   ]>y 
  the 
  great 
  development 
  of 
  this 
  urticating-pad 
  the 
  proven- 
  

   triculus 
  is 
  divided 
  into 
  four 
  or 
  five 
  vessel-like 
  canals, 
  which 
  

   subsequently, 
  in 
  Porpita, 
  become 
  increased 
  to 
  eight 
  canals. 
  

   In 
  the 
  absence 
  of 
  a 
  secondary 
  ectoderm 
  after 
  the 
  development 
  

   of 
  the 
  chitinous 
  concentric 
  air-chambers 
  the 
  Velellidae 
  are 
  

   quite 
  unable 
  to 
  secrete 
  any 
  gaseous 
  mixture, 
  but 
  they 
  are 
  

   confined 
  to 
  taking 
  the 
  atmospheric 
  air 
  into 
  the 
  chambers 
  and 
  

   expelling 
  it 
  again 
  through 
  the 
  stigmata. 
  As 
  the 
  colony 
  is 
  

   subjected 
  to 
  a 
  very 
  intense 
  material 
  change, 
  as 
  is 
  shown 
  

   by 
  the 
  abundant 
  separation 
  of 
  guanine 
  crystals 
  in 
  the 
  so- 
  

   called 
  liver, 
  it 
  is 
  clear 
  that 
  sufficient 
  oxygen 
  is 
  conveyed 
  to 
  

   the 
  polyps 
  by 
  means 
  of 
  the 
  air 
  contained 
  in 
  the 
  tracheae. 
  

   Moreover 
  the 
  Velellw 
  and 
  Porpitce 
  perform 
  movements 
  which 
  

   remarkably 
  resemble 
  the 
  actions 
  of 
  the 
  air-breathing 
  Arthro- 
  

   pods 
  for 
  the 
  removal 
  of 
  the 
  air 
  contained 
  in 
  the 
  tracheae. 
  

   Thus 
  if 
  we 
  observe 
  a 
  freshly 
  captured 
  Velella 
  or 
  Porpita, 
  we 
  

   find 
  that 
  twice 
  in 
  a 
  minute 
  (seldom 
  oftener) 
  the 
  surface 
  turned 
  

   towards 
  the 
  water 
  and 
  bearing 
  the 
  sexual 
  polyps 
  and 
  the 
  central 
  

   stomachal 
  sac 
  (siphosoma, 
  polypite) 
  is 
  very 
  energetically 
  pressed 
  

   against 
  the 
  air-chambers. 
  During 
  this 
  respiratory 
  movement, 
  

   caused 
  by 
  the 
  contraction 
  of 
  the 
  circularly 
  arranged 
  muscular 
  

   fibres, 
  the 
  whole 
  of 
  the 
  polyps, 
  with 
  the 
  exception 
  of 
  the 
  ten- 
  

   tacles, 
  are 
  simultaneously 
  contracted. 
  The 
  tentacles 
  are 
  moved 
  

   downwards 
  independently 
  of 
  the 
  above-mentioned 
  movements, 
  

   either 
  all 
  together 
  or 
  in 
  groups. 
  After 
  the 
  contraction 
  the 
  

   lower 
  surface 
  of 
  the 
  disk 
  returns 
  slowly 
  into 
  its 
  position 
  of 
  

   rest, 
  and 
  the 
  polyps 
  begin 
  again 
  to 
  extend 
  themselves, 
  to 
  be 
  

   again, 
  after 
  the 
  lapse 
  of 
  half 
  a 
  minute, 
  rapidly 
  contracted. 
  

   For 
  hours 
  this 
  attractive 
  spectacle, 
  which 
  has 
  hitherto 
  been 
  

   mentioned 
  by 
  no 
  naturalist, 
  may 
  be 
  observed 
  on 
  freshly 
  

   captured 
  Velellce 
  and 
  Porpitoz. 
  The 
  effect 
  of 
  such 
  regularly 
  

   repeated 
  respiratory 
  movements 
  is 
  perfectly 
  clear 
  ; 
  by 
  the 
  

   energetic 
  contraction 
  of 
  the 
  disk 
  and 
  individual 
  polyps 
  the 
  

   richly 
  ramified 
  tracheal 
  tufts 
  are 
  contracted, 
  their 
  air 
  passes 
  

   * 
  Zoolog. 
  Anzeiger, 
  1887, 
  nos.261 
  and 
  202. 
  

  

  