﻿112 
  E. 
  J. 
  ALLEN. 
  

  

  headj 
  and 
  of 
  three 
  long, 
  teutacle-like 
  processes 
  extending 
  

   backwards 
  from 
  it 
  (fig. 
  7, 
  nuch.). 
  Of 
  these 
  three 
  processes, 
  

   the 
  middle 
  one 
  is 
  the 
  longest, 
  and 
  may 
  run 
  at 
  least 
  as 
  far 
  

   backwards 
  as 
  segment 
  6, 
  the 
  lateral 
  ones 
  ending 
  about 
  seg- 
  

   ment 
  4 
  (fig. 
  1). 
  The 
  lateral 
  processes 
  have 
  occasionally 
  been 
  

   observed 
  with 
  a 
  secondary 
  branch. 
  The 
  whole 
  organ 
  is 
  

   covered 
  with 
  a 
  number 
  of 
  sensory 
  hairs, 
  and 
  each 
  process 
  

   possesses 
  two 
  lateral 
  ciliated 
  grooves, 
  which 
  run 
  along 
  the 
  

   whole 
  of 
  its 
  length 
  and 
  extend 
  on 
  to 
  the 
  basal 
  portion 
  (PL 
  11, 
  

   figs. 
  42 
  and 
  47, 
  nuch.). 
  

  

  Claparede 
  (in 
  Ehlers, 
  1874) 
  and 
  Levinsen 
  (1883) 
  have 
  both 
  

   described 
  the 
  three 
  processes, 
  but 
  have 
  failed 
  to 
  recognise 
  

   their 
  true 
  nature 
  as 
  nuchal 
  organs. 
  These 
  two 
  authors 
  have, 
  

   however, 
  shown 
  cleai-ly 
  that 
  the 
  organ 
  in 
  question 
  develops 
  

   as 
  an 
  outgrowth 
  from 
  the 
  posterior 
  cephalic 
  region. 
  Such 
  

   enlarged 
  nuchal 
  organs 
  are 
  by 
  no 
  means 
  unknown 
  amongst 
  

   Polycha3tes, 
  though 
  none 
  have 
  yet 
  been 
  described 
  having 
  

   dimensions 
  comparable 
  with 
  those 
  of 
  Poecilochaetus. 
  The 
  

   nuchal 
  organs 
  of 
  Virchowia 
  clavata 
  figured 
  by 
  Viguier 
  

   (1886) 
  may 
  be 
  referred 
  to, 
  as 
  well 
  as 
  those 
  of 
  Amblyo- 
  

   syllis 
  spectabilis 
  and 
  Autolytus 
  longiferiens, 
  figured 
  

   by 
  Malaquin 
  (1893). 
  Gravier 
  (1896) 
  describes 
  the 
  nuchal 
  

   organ 
  of 
  Notophyllu 
  ni, 
  which 
  takes 
  the 
  form 
  of 
  two 
  lappets 
  

   extending 
  from 
  the 
  posterior 
  end 
  of 
  the 
  prostomium 
  to 
  the 
  

   middle 
  of 
  the 
  third 
  segment. 
  Racovitza 
  (1896) 
  shows 
  that 
  the 
  

   caruncle 
  of 
  the 
  Amphinomidaj 
  is 
  an 
  enlarged 
  nuchal 
  organ. 
  

  

  In 
  the 
  living 
  Poecilocha?tus 
  the 
  nuchal 
  organ 
  has 
  a 
  

   brown 
  or 
  brownish-green 
  colour. 
  Sections 
  show 
  that 
  this 
  

   colour 
  is 
  due 
  to 
  granules 
  deposited 
  in 
  the 
  epidermic 
  cells, 
  

   and 
  also 
  to 
  a 
  number 
  of 
  spherical 
  bodies 
  scattered 
  through 
  

   the 
  tissue, 
  which 
  possess 
  a 
  single, 
  deeply 
  staining 
  nucleus, 
  

   and 
  are 
  filled 
  with 
  dark 
  granules 
  (PI. 
  11, 
  fig. 
  40). 
  

  

  The 
  base 
  of 
  the 
  nuchal 
  organ 
  is 
  occupied 
  by 
  the 
  nuchal 
  

   ganglion 
  (fig. 
  42, 
  nuch. 
  yany.), 
  which 
  has 
  already 
  been 
  

   described 
  (p. 
  104). 
  The 
  central 
  axis 
  of 
  each 
  of 
  the 
  pro- 
  

   cesses 
  of 
  the 
  organ 
  is 
  formed 
  by 
  a 
  tube 
  lined 
  with 
  mesoderm- 
  

   cells, 
  the 
  tube 
  being 
  in 
  direct 
  communication 
  with 
  the 
  general 
  

  

  