﻿Structure 
  of 
  Magelona. 
  437 
  

  

  longitudinal 
  dorsal 
  and 
  the 
  region 
  external 
  to 
  it, 
  and 
  pass 
  

   obliquely 
  downward 
  to 
  be 
  inserted 
  at 
  the 
  inner 
  border 
  of 
  the 
  

   ventral. 
  The 
  oblique 
  arise 
  from 
  the 
  wide 
  region 
  between 
  

   the 
  upper 
  (external) 
  bolder 
  of 
  the 
  longitudinal 
  ventral 
  

   upward 
  to 
  the 
  superior 
  lateral 
  region, 
  and 
  the 
  fibres 
  extend 
  

   downward 
  to 
  the 
  outer 
  part 
  of 
  the 
  common 
  insertion 
  — 
  close 
  

   to 
  the 
  inner 
  border 
  of 
  the 
  longitudinal 
  muscle. 
  Shortly- 
  

   after 
  their 
  attachment, 
  as 
  described, 
  to 
  the 
  basement-tissue, 
  

   the 
  arrangement 
  of 
  both 
  is 
  somewhat 
  interfered 
  with 
  by 
  the 
  

   very 
  large 
  fasciculi 
  which 
  pass 
  transversely 
  outward 
  from 
  

   the 
  sides 
  of 
  the 
  alimentary 
  canal 
  to 
  the 
  body-wall 
  (in 
  the 
  

   extruded 
  state 
  of 
  the 
  proboscis). 
  The 
  fibres 
  of 
  the 
  oblique, 
  

   again, 
  at 
  this 
  part 
  seem 
  to 
  run 
  superiorly 
  into 
  the 
  largely 
  

   increased 
  lateral 
  vertical. 
  Immediately 
  behind, 
  the 
  vertical 
  

   appears 
  as 
  a 
  nearly 
  cylindrical 
  muscle 
  (in 
  transverse 
  

   vertical 
  section), 
  and 
  the 
  vascular 
  channel 
  lies 
  between 
  it 
  

   and 
  the 
  oblique. 
  Masses 
  of 
  the 
  great 
  fasciculi 
  (from 
  the 
  

   alimentary 
  canal) 
  still 
  occur 
  at 
  the 
  superior 
  and 
  inferior 
  

   attachments 
  of 
  the 
  vertical 
  muscles, 
  and 
  probably 
  represent 
  

   the 
  retractor 
  muscles 
  of 
  that 
  part 
  of 
  the 
  pharyngeal 
  region 
  

   which 
  is 
  thrust 
  into 
  the 
  proboscis. 
  By 
  the 
  gradual 
  passage 
  

   of 
  the 
  longitudinal 
  ventral 
  muscle 
  downward, 
  the 
  inferior 
  

   fibres 
  of 
  the 
  oblique 
  become 
  more 
  horizontal, 
  and 
  both 
  

   vertical 
  and 
  oblique 
  muscles 
  attain 
  great 
  development 
  

   throughout 
  the 
  entire 
  anterior 
  region 
  of 
  the 
  body, 
  so 
  

   as 
  to 
  act 
  as 
  powerful 
  compressors, 
  while 
  their 
  elongation 
  

   and 
  contraction 
  directly 
  affect 
  the 
  blood-channels 
  between 
  

   them. 
  On 
  approaching 
  the 
  ninth 
  body-segment 
  the 
  lower 
  

   fibres 
  of 
  the 
  oblique 
  are 
  nearly 
  horizontal, 
  and 
  both 
  this 
  

   muscle 
  and 
  the 
  vertical 
  are 
  large 
  ; 
  indeed, 
  as 
  the 
  dorsal 
  and 
  

   ventral 
  longitudinal 
  muscles 
  diminish 
  in 
  bulk, 
  the 
  former 
  

   increase. 
  Immediately 
  in 
  front 
  of 
  the 
  last-mentioned 
  seg- 
  

   ment 
  the 
  relations 
  of 
  the 
  muscles 
  are 
  unchanged. 
  It 
  is 
  then 
  

   noticed 
  that 
  a 
  series 
  of 
  muscular 
  fibres 
  arise 
  from 
  the 
  

   superior 
  insertion 
  of 
  the 
  vertical, 
  outward 
  along 
  the 
  whole 
  

   upper 
  arch 
  of 
  the 
  body, 
  and, 
  indeed, 
  to 
  a 
  point 
  considerably 
  

   below 
  the 
  upper 
  fibres 
  of 
  the 
  oblique. 
  These 
  by-and-by 
  

   form 
  a 
  powerful 
  fan-shaped 
  mass 
  sweeping 
  continuously 
  

   from 
  the 
  raphe 
  at 
  the 
  dorsal 
  longitudinal 
  muscle, 
  and 
  at 
  

   first 
  even 
  extending 
  over 
  the 
  latter, 
  to 
  the 
  edge 
  of 
  the 
  

   longitudinal 
  ventral 
  inferiorly, 
  and 
  converging 
  to 
  the 
  raphe 
  

   at 
  its 
  inner 
  border. 
  In 
  some 
  preparations 
  fibres 
  are 
  observed 
  

   to 
  pass 
  into 
  the 
  transverse 
  ventral 
  muscle 
  so 
  as 
  to 
  form 
  a 
  

   continuous 
  band. 
  The 
  direction 
  of 
  the 
  fibres 
  of 
  this 
  great 
  

   muscular 
  expansion 
  is 
  not 
  straight 
  or 
  vertical, 
  but 
  they 
  

   curve 
  from 
  before 
  backward, 
  as 
  well 
  as 
  from 
  above 
  down- 
  

  

  