﻿Structure 
  of 
  Magelona. 
  447 
  

  

  externally 
  ; 
  wliile 
  certain 
  longitudinal 
  fibres 
  which 
  appear 
  

   in 
  the 
  latter 
  become 
  by-and-by 
  incorporated 
  with 
  its 
  wall, 
  

   which 
  externally 
  shows 
  traces 
  of 
  longitudinal 
  fibres, 
  outside 
  

   the 
  circular, 
  though 
  they 
  are 
  difficult 
  to 
  discern. 
  The 
  

   relations 
  of 
  the 
  organ 
  with 
  the 
  vascular 
  system 
  at 
  this 
  part 
  

   are, 
  indeed, 
  complex. 
  The 
  form 
  of 
  the 
  alimentary 
  canal 
  in 
  

   section 
  near 
  the 
  ninth 
  body-segment 
  is 
  now 
  rounded 
  or 
  oval, 
  

   with 
  a 
  smooth 
  outline, 
  and 
  instead 
  of 
  the 
  few 
  bold 
  folds 
  of 
  

   glandular 
  tissue 
  covered 
  by 
  the 
  chitinous 
  layer 
  internally, 
  

   the 
  glandular 
  substance 
  is 
  granular 
  and 
  projects 
  in 
  conical 
  

   elevations 
  or 
  papilla: 
  having 
  a 
  dendritic 
  or 
  radiated 
  appear- 
  

   ance. 
  As 
  the 
  canal 
  diminishes 
  the 
  latter 
  become 
  more 
  

   pointed 
  and 
  translucent, 
  the 
  basement-tissue 
  more 
  pro- 
  

   minent, 
  and 
  within 
  the 
  circular 
  coat 
  are 
  distinct 
  traces 
  of 
  

   a 
  layer 
  of 
  longitudinal 
  muscular 
  fibres. 
  At 
  the 
  ninth 
  body- 
  

   segment 
  the 
  circular 
  coat 
  is 
  much 
  thickened, 
  the 
  basement- 
  

   tissue 
  increased, 
  and 
  the 
  glandular 
  layer 
  more 
  consistent, 
  so 
  

   that 
  it 
  projects 
  inward 
  very 
  distinctly. 
  The 
  canal 
  reaches 
  its 
  

   minimum 
  diameter 
  about 
  the 
  termination 
  of 
  the 
  transverse 
  

   ventral 
  muscles, 
  near 
  the 
  commencement 
  of 
  the 
  ventral 
  

   blood-vessel. 
  Just 
  at 
  the 
  point 
  where 
  the 
  dorsal 
  longitudinal 
  

   muscles 
  begin 
  to 
  expand, 
  the 
  tube 
  is 
  surrounded 
  by 
  a 
  

   powerful 
  ring 
  of 
  muscular 
  fibres, 
  which 
  in 
  the 
  mid-ventral 
  

   line 
  join 
  a 
  transverse 
  chitinous 
  raphe, 
  from 
  the 
  under 
  

   surface 
  of 
  which 
  spring 
  the 
  strong 
  muscular 
  walls 
  of 
  the 
  

   ventral 
  vessel 
  of 
  the 
  region. 
  The 
  canal 
  has 
  now 
  increased 
  

   a 
  little 
  in 
  size, 
  but 
  is 
  often 
  squeezed 
  (in 
  the 
  preparations) 
  by 
  

   the 
  pressure 
  of 
  the 
  superincumbent 
  blood-vessels, 
  so 
  that 
  it 
  

   forms 
  a 
  transverse 
  slit. 
  It 
  then 
  passes 
  the 
  boundary-line 
  

   into 
  the 
  succeeding 
  region, 
  and 
  will 
  fall 
  under 
  notice 
  

   shortly. 
  

  

  The 
  anterior 
  region 
  of 
  the 
  alimentary 
  canal 
  is 
  distinguished 
  

   as 
  a 
  whole 
  by 
  its 
  greater 
  muscularity 
  and 
  firmness, 
  by 
  the 
  

   massive 
  nature 
  and 
  chitinous 
  covering 
  of 
  its 
  glandular 
  coat, 
  

   and 
  by 
  the 
  absence 
  of 
  evident 
  capillaries 
  from 
  its 
  walls. 
  It 
  

   presents 
  a 
  close 
  homology 
  with 
  the 
  Nemertean 
  oesophageal 
  

   region, 
  especially 
  in 
  its 
  relation 
  to 
  the 
  circulatory 
  fluid, 
  for 
  

   in 
  the 
  latter 
  a 
  rete 
  mirabile 
  likewise 
  occurs. 
  The 
  minute 
  

   structure 
  of 
  the 
  wall 
  is 
  similar, 
  but, 
  as 
  on 
  the 
  skin, 
  the 
  

   chitinous 
  tunic 
  internally 
  in 
  Magelona 
  takes 
  the 
  place 
  of 
  the 
  

   cilia 
  in 
  the 
  Nemertean. 
  Further, 
  like 
  the 
  Nemertean 
  organ, 
  

   it 
  retains 
  irritability 
  long 
  after 
  the 
  death 
  of 
  the 
  animal, 
  and 
  

   in 
  dissection 
  is 
  often 
  torn 
  from 
  the 
  more 
  fragile 
  tissues 
  of 
  

   the 
  anterior 
  region 
  as 
  a 
  tough 
  thread. 
  The 
  functions 
  of 
  the 
  

   parts, 
  in 
  regard 
  to 
  digestion, 
  in 
  the 
  respective 
  groups 
  are 
  

   probably 
  similar, 
  and 
  may 
  represent 
  all 
  the 
  complex 
  divisions 
  

  

  