﻿THE 
  ANATOMY 
  OF 
  PCECILOGHiETUS, 
  OLAPAREDE. 
  91 
  

  

  smooth, 
  slender 
  cliaetfe 
  of 
  the 
  anterior 
  segments 
  are 
  replaced 
  

   by 
  large; 
  liairy, 
  feather-like 
  bristles 
  (PI. 
  7, 
  fig. 
  d 
  ; 
  PI. 
  9, 
  figs. 
  

   10, 
  14, 
  and 
  16), 
  the 
  most 
  dorsal 
  and 
  most 
  ventral 
  in 
  each 
  

   segment 
  having 
  long, 
  fairly 
  stiff 
  shafts, 
  with 
  lateral 
  hairs 
  of 
  

   moderate 
  length 
  (fig. 
  14), 
  whilst 
  the 
  inner 
  ones 
  (ventral 
  

   bundle 
  of 
  notopodium 
  and 
  dorsal 
  bundle 
  of 
  neuropodium) 
  are 
  

   more 
  slender 
  and 
  flexible, 
  but 
  have 
  very 
  much 
  longer 
  hairs 
  

   (fig. 
  16). 
  These 
  bristles 
  give 
  to 
  the 
  region 
  of 
  the 
  body 
  now 
  

   under 
  consideration 
  a 
  kind 
  of 
  woolly 
  appearance. 
  

  

  The 
  spiny 
  bristles 
  of 
  the 
  anterior 
  segments 
  also 
  undergo 
  a 
  

   special 
  modification 
  in 
  this 
  region. 
  The 
  stoutness 
  of 
  their 
  

   shafts 
  becomes 
  very 
  greatly 
  reduced, 
  the 
  spines 
  themselves 
  

   become 
  much 
  elongated, 
  show 
  a 
  slight 
  thickening 
  near 
  the 
  

   tip, 
  and 
  are 
  connected 
  with 
  the 
  shaft 
  along 
  almost 
  their 
  

   entire 
  length 
  by 
  a 
  thin, 
  transparent 
  membrane, 
  which 
  is 
  

   practically 
  invisible 
  in 
  fresh 
  material, 
  but 
  becomes 
  quite 
  

   obvious 
  after 
  staining 
  (PI. 
  9, 
  fig. 
  17). 
  By 
  this 
  arrangement 
  

   the 
  surface 
  of 
  the 
  bristle 
  becomes 
  very 
  greatly 
  extended. 
  

  

  The 
  hairy, 
  feather-like 
  bristles, 
  together 
  with 
  the 
  modified 
  

   spiny 
  bristles 
  just 
  described 
  spread 
  out 
  in 
  each 
  parapodium 
  

   into 
  a 
  large 
  fan, 
  the 
  movements 
  of 
  which 
  are 
  mainly 
  respon- 
  

   sible 
  for 
  the 
  current 
  of 
  water 
  which 
  the 
  worm 
  constantly 
  

   draws 
  through 
  its 
  U-shaped 
  tube 
  (see 
  p. 
  83). 
  

  

  In 
  this 
  region 
  the 
  lateral 
  sense-organ 
  no 
  longer 
  has 
  the 
  

   form 
  of 
  a 
  papilla 
  protruding- 
  from 
  the 
  face 
  of 
  the 
  parapodium, 
  

   but 
  is 
  seen 
  as 
  a 
  slight 
  depression 
  from 
  the 
  centre 
  of 
  which 
  a 
  

   bundle 
  of 
  sensory 
  hairs 
  arises. 
  The 
  depression 
  is 
  Surrounded 
  

   by 
  a 
  circular 
  rim, 
  which 
  rises 
  slightly 
  above 
  the 
  general 
  face 
  of 
  

   the 
  parapodial 
  surface. 
  

  

  Gills. 
  — 
  The 
  gill 
  filaments 
  commence 
  on 
  segment 
  21, 
  and 
  

   are 
  found 
  on 
  the 
  succeeding 
  segments 
  to 
  quite 
  near 
  the 
  end 
  

   of 
  the 
  body. 
  They 
  are 
  at 
  first 
  short 
  and 
  small 
  in 
  size 
  (PI. 
  7, 
  

   fig. 
  1), 
  but 
  soon 
  become 
  longer 
  and 
  larger. 
  When 
  fully 
  

   developed 
  they 
  consist 
  of 
  long 
  filaments, 
  as 
  long 
  as 
  or 
  longer 
  

   than 
  the 
  cirri 
  of 
  the 
  parapodia 
  (PI. 
  9, 
  fig. 
  11), 
  which 
  appear 
  

   bright 
  red 
  in 
  the 
  living 
  worm 
  from 
  the 
  colour 
  of 
  the 
  blood 
  

   which 
  is 
  in 
  them. 
  Two 
  pairs 
  of 
  such 
  filaments 
  occur 
  upon 
  

  

  