﻿124 
  F.. 
  .r. 
  ALT-EN. 
  

  

  also 
  with 
  the 
  extension 
  of 
  the 
  palps. 
  I 
  have 
  only 
  once 
  seen 
  

   a 
  pi'otrnsion 
  of 
  the 
  proboscis, 
  and 
  it 
  did 
  not 
  then 
  extend 
  

   mncli 
  beyond 
  the 
  front 
  of 
  the 
  head. 
  The 
  structnres 
  just 
  

   described, 
  however, 
  appear 
  to 
  snggest 
  the 
  possibility 
  of 
  a 
  

   much 
  greater 
  protrusion. 
  

  

  In 
  living 
  specimens 
  of 
  Poecilochastus 
  an 
  indication 
  of 
  

   the 
  backward 
  extension 
  of 
  the 
  septa 
  of 
  segments 
  5 
  to 
  12 
  

   can 
  be 
  seen 
  in 
  the 
  backward 
  course 
  of 
  the 
  lateral 
  blood- 
  

   vessels, 
  which 
  run 
  in 
  the 
  septa 
  (fig. 
  1 
  and 
  p. 
  12(3). 
  

  

  In 
  the 
  segments 
  from 
  the 
  thirteenth 
  backwards, 
  the 
  iiiternal 
  

   and 
  external 
  segmentation 
  con-espond. 
  

  

  Each 
  of 
  the 
  septa 
  dividing 
  the 
  body-cavity 
  consists 
  of 
  a 
  

   double 
  layer 
  of 
  coelomic 
  epithelial 
  cells 
  with 
  a 
  layer, 
  more 
  or 
  

   less 
  strongly 
  developed, 
  of 
  muscle-fibres 
  between. 
  The 
  

   epithelium 
  is 
  extended 
  over 
  the 
  main 
  body 
  muscles, 
  and 
  over 
  

   the 
  other 
  organs 
  of 
  the 
  body. 
  On 
  most 
  organs, 
  however, 
  

   the 
  cells 
  are 
  seldom 
  much 
  developed, 
  their 
  presence 
  being 
  

   often 
  only 
  indicated 
  by 
  occasional 
  nuclei. 
  

  

  Extensions 
  of 
  the 
  body-cavity 
  into 
  the 
  interior 
  of 
  the 
  

   nuchal 
  oi'gan 
  and 
  of 
  the 
  palps 
  are 
  mentione<l 
  in 
  the 
  jiara- 
  

   graphs 
  dealing 
  with 
  those 
  structures. 
  

  

  Musculature. 
  

  

  The 
  muscles 
  of 
  the 
  body-wall, 
  as 
  is 
  usual 
  in 
  the 
  

   Polychaetes, 
  are 
  arranged 
  in 
  two 
  layers, 
  a 
  layer 
  of 
  annular 
  

   muscles 
  and 
  a 
  layer 
  of 
  longitudinal. 
  Of 
  these 
  two 
  layers, 
  

   however, 
  the 
  annular 
  is 
  very 
  feebly 
  developed 
  in 
  Poecilo- 
  

   chaetus, 
  whilst 
  the 
  longitudinal 
  is 
  well 
  developed. 
  The 
  

   principal 
  muscles 
  in 
  each 
  segment 
  are 
  massed 
  into 
  four 
  

   bundles, 
  two 
  doisal 
  lying 
  on 
  either 
  side 
  of 
  the 
  dorsal 
  blood- 
  

   vessel 
  and 
  two 
  ventral 
  on 
  either 
  side 
  of 
  the 
  nerve-cord. 
  The 
  

   slight 
  development 
  of 
  the 
  annular 
  muscles 
  would 
  a])pear 
  to 
  

   be 
  connected 
  with 
  the 
  more 
  or 
  less 
  sedentary 
  habits 
  of 
  the 
  

   worm. 
  The 
  annular 
  muscles 
  attain 
  their 
  greatest 
  develop- 
  

   ment 
  in 
  worms 
  which 
  buriow 
  constantly 
  and 
  rapidly 
  in 
  the 
  

   soil 
  (e. 
  g. 
  Nephthys, 
  Aricia). 
  

  

  