﻿100 
  K. 
  J, 
  ALLEN. 
  

  

  Palps. 
  

  

  The 
  external 
  appearance 
  of 
  the 
  palps 
  (PI. 
  7, 
  fig. 
  1) 
  has 
  

   already 
  been 
  described 
  (p. 
  8Q). 
  A 
  section 
  of 
  the 
  palp 
  shows 
  

   it 
  to 
  be 
  a 
  hollow 
  tube 
  having 
  a 
  large 
  central 
  cavity, 
  through 
  

   which 
  the 
  blood-vessel 
  of 
  the 
  palp 
  runs 
  (PL 
  10, 
  fig. 
  30, 
  ^3. 
  bv.). 
  

   The 
  walls 
  of 
  the 
  tube 
  are 
  composed 
  of 
  two 
  la^^ers 
  of 
  cells, 
  a 
  

   thick 
  outer 
  layer 
  of 
  large 
  epithelial 
  cells 
  (ep.) 
  lined 
  internally 
  

   by 
  a 
  thin 
  sheet 
  of 
  mesoderm 
  cells. 
  The 
  nuclei 
  of 
  both 
  the 
  

   epidermic 
  and 
  mesodermic 
  cells 
  have 
  a 
  very 
  characteristic 
  

   appearance, 
  since 
  each 
  possesses 
  a 
  single, 
  large, 
  deeply 
  staining 
  

   nucleolus. 
  Occasionally 
  a 
  nucleus 
  is 
  seen 
  with 
  two 
  such 
  

   nucleoli 
  of 
  equal 
  size, 
  which 
  may 
  indicate 
  division. 
  The 
  

   walls 
  of 
  the 
  blood-vessel 
  which 
  runs 
  along 
  the 
  length 
  of 
  the 
  

   palp 
  are 
  thick, 
  and 
  contain 
  nuclei 
  similar 
  to 
  those 
  of 
  the 
  

   mesodermic 
  cells. 
  From 
  the 
  pulsation 
  of 
  the 
  vessel 
  in 
  the 
  

   living 
  worm 
  these 
  walls 
  are 
  known 
  to 
  be 
  muscular. 
  

  

  The 
  cavity 
  of 
  the 
  palp 
  communicates 
  with 
  the 
  general 
  body- 
  

   cavity 
  of 
  the 
  first 
  segment 
  of 
  the 
  worm 
  by 
  means 
  of 
  a 
  tube 
  

   formed 
  by 
  a 
  continuation 
  into 
  the 
  body-cavity 
  of 
  the 
  meso- 
  

   derm-cells 
  lining 
  the 
  cavity 
  of 
  the 
  palp 
  (fig. 
  30, 
  j)/^^ 
  f.). 
  This 
  

   tube, 
  just 
  after 
  it 
  leaves 
  the 
  palp, 
  is 
  surrounded 
  by 
  a 
  strong- 
  

   band 
  of 
  annular 
  muscle-fibres, 
  by 
  the 
  contraction 
  of 
  which 
  

   the 
  cavity 
  of 
  the 
  palp 
  can 
  be 
  completely 
  cut 
  off 
  from 
  the 
  body- 
  

   cavity 
  of 
  the 
  first 
  segment. 
  It 
  is 
  clear 
  that 
  the 
  palps, 
  which 
  

   are 
  capable 
  of 
  very 
  great 
  extension, 
  are 
  elongated 
  by 
  tlie 
  

   pressure 
  of 
  fluid 
  from 
  the 
  body-cavity 
  into 
  their 
  cavity. 
  

   When 
  once 
  the 
  palps 
  are 
  filled, 
  the 
  contraction 
  of 
  the 
  annular 
  

   muscle-fibres 
  just 
  described 
  will 
  euable 
  them 
  to 
  continue 
  

   extended 
  Avithout 
  the 
  necessity 
  of 
  the 
  body 
  pressure 
  being 
  

   maintained. 
  (For 
  an 
  account 
  of 
  the 
  muscular 
  septa 
  which 
  

   come 
  into 
  play 
  when 
  the 
  fluid 
  is 
  pressed 
  forward 
  see 
  p. 
  123.) 
  

  

  At 
  the 
  outer 
  side 
  of 
  the 
  palp, 
  between 
  the 
  base 
  of 
  the 
  palp 
  

   and 
  the 
  palp-ganglion, 
  lies 
  a 
  small 
  diverticle 
  (i^lp. 
  du\) 
  of 
  the 
  

   palp 
  cavity, 
  which 
  appears 
  to 
  run 
  forwards 
  and 
  then 
  end 
  

   blindly. 
  The 
  meaning 
  of 
  this 
  structure 
  I 
  have 
  not 
  fully 
  

   understood. 
  

  

  