﻿THE 
  ANATOMY 
  OF 
  PcEOILOCH.ETUS, 
  CLAPAUEDK. 
  105 
  

  

  all 
  spring- 
  from 
  the 
  same 
  point 
  in 
  the 
  centre 
  of 
  the 
  bi'ain 
  

   (m. 
  h.) 
  would 
  seem 
  to 
  point 
  to 
  this 
  region 
  as 
  the 
  mid-brain 
  of 
  

   that 
  author. 
  With 
  regard 
  to 
  the 
  hind 
  brain, 
  there 
  is 
  more 
  

   difficulty. 
  Judging 
  from 
  Racovitza's 
  figures 
  of 
  Eurythoe 
  

   borealis 
  and 
  Euphrosyne 
  Audonini, 
  it 
  would 
  seem 
  that 
  

   what 
  I 
  have 
  termed 
  the 
  nuchal 
  ganglion 
  ofPoecilochsetus 
  is 
  

   homologous 
  with 
  what 
  he 
  calls 
  the 
  hind 
  brain 
  in 
  those 
  species. 
  

   But 
  in 
  Poecilochtetus 
  this 
  structure 
  is 
  separated 
  sharply 
  

   from 
  the 
  brain 
  itself, 
  being 
  only 
  connected 
  with 
  it 
  by 
  nerve- 
  

   fibres. 
  These 
  fibres 
  leave 
  the 
  brain 
  at 
  its 
  posterior 
  end, 
  but 
  

   there 
  is 
  no 
  region 
  in 
  the 
  brain 
  itself 
  which 
  can 
  be 
  clearly 
  

   marked 
  oil 
  as 
  a 
  hind 
  brain. 
  The 
  large 
  posterior 
  ganglion- 
  

   cells 
  to 
  which 
  reference 
  has 
  been 
  made 
  (Text-fig. 
  1, 
  post, 
  

   gang, 
  cells) 
  might 
  at 
  first 
  sight 
  be 
  regarded 
  as 
  an 
  indication 
  

   of 
  such 
  a 
  structure, 
  but 
  against 
  this 
  view 
  it 
  can 
  be 
  urged 
  that 
  

   they 
  give 
  off 
  their 
  processes 
  to 
  the 
  region 
  of 
  the 
  mid-brain, 
  

   from 
  which 
  the 
  commissures 
  and 
  tentacle 
  nerves 
  take 
  origin. 
  

  

  The 
  relations 
  found 
  in 
  PoocilocliEetus 
  seem 
  to 
  indicate 
  

   that 
  it 
  would 
  be 
  more 
  correct 
  to 
  term 
  what 
  Racovitza 
  calls 
  

   the 
  hind 
  brain 
  in 
  Eurythoe 
  and 
  Euprosyne 
  the 
  nuchal 
  

   ganglia. 
  The 
  nuchal 
  ganglion 
  of 
  P 
  cecilocheetus 
  is 
  clearly 
  

   comparable 
  to 
  the 
  palp-gaglion, 
  and 
  not 
  to 
  any 
  division 
  

   of 
  the 
  brain 
  itself. 
  

  

  Ventral 
  Nerve 
  Cord 
  — 
  The 
  ventral 
  nerve-cord 
  lies 
  en- 
  

   tirely 
  in 
  the 
  epidermis 
  (PI. 
  10, 
  fig. 
  32). 
  The 
  ganglia 
  of 
  the 
  

   different 
  segments 
  are 
  not 
  very 
  sharply 
  marked 
  off 
  from 
  each 
  

   other, 
  ganglion-cells 
  being 
  scattered 
  somewhat 
  irregularly 
  

   along 
  the 
  whole 
  length 
  of 
  the 
  cord. 
  Definite 
  ganglia 
  are, 
  how- 
  

   ever, 
  indicated 
  in 
  each 
  segment 
  by 
  a 
  considerable 
  increase 
  in 
  

   the 
  number 
  of 
  ganglion-cells, 
  by 
  the 
  presence 
  of 
  masses 
  of 
  

   nervous 
  felt-work 
  (punctated 
  substance) 
  as 
  well 
  as 
  by 
  the 
  

   roots 
  of 
  the 
  lateral 
  nerves. 
  

  

  Two 
  giant 
  fibres 
  (fig. 
  32 
  g. 
  f.) 
  run 
  along 
  the 
  cord. 
  In 
  pre- 
  

   served 
  specimens 
  these 
  fibres 
  vary 
  in 
  diameter 
  in 
  different 
  

   regions, 
  but 
  are 
  generally 
  of 
  very 
  large 
  size. 
  The 
  connection 
  

   of 
  these 
  fibres 
  with 
  ganglion- 
  cells 
  has 
  not 
  been 
  traced. 
  

  

  Stomatogastric 
  System 
  — 
  What 
  seems 
  to 
  be 
  a 
  well- 
  

  

  VOL. 
  48, 
  PART 
  1. 
  — 
  NEW 
  SERIES. 
  8 
  

  

  