﻿Structure 
  o/Magelona. 
  445 
  

  

  sometimes 
  of 
  a 
  symmetrical 
  appearance, 
  especially 
  if 
  one 
  

   fold 
  of 
  the 
  proboscis 
  is 
  included 
  within 
  another. 
  A 
  base- 
  

   ment 
  layer 
  succeeds 
  the 
  foregoing, 
  and 
  then 
  a 
  coat 
  of 
  circular 
  

   fibres, 
  which 
  attains 
  its 
  greatest 
  development 
  at 
  the 
  superior 
  

   or 
  inner 
  side 
  (i. 
  e., 
  next 
  the 
  snout 
  in 
  extrusion) 
  and 
  especially 
  

   near 
  the 
  base. 
  There 
  does 
  not 
  appear 
  to 
  be 
  any 
  special 
  

   longitudinal 
  coat, 
  other 
  than 
  the 
  expansion 
  of 
  the 
  long 
  

   retractor 
  muscle 
  on 
  the 
  walls 
  near 
  the 
  extremity 
  of 
  the 
  

   extruded 
  organ. 
  At 
  the 
  latter 
  region 
  the 
  thinner 
  parts 
  have 
  

   crossed 
  fibres 
  internally, 
  an 
  appearance 
  probably 
  due 
  to 
  the 
  

   commingling 
  of 
  the 
  circular 
  coat 
  and 
  the 
  expanded 
  fibres 
  of 
  

   the 
  long 
  retractor. 
  The 
  latter 
  forms 
  a 
  great 
  muscular 
  mass 
  

   which 
  occupies 
  the 
  dorsal 
  region 
  of 
  the 
  organ 
  in 
  extrusion, 
  

   and 
  therefore 
  lies 
  under 
  the 
  digestive 
  canal 
  on 
  approaching 
  

   the 
  body-cavity. 
  It 
  then 
  is 
  included 
  between 
  the 
  anterior 
  

   fan-shaped 
  retractors, 
  which 
  occupy 
  the 
  lateral 
  region, 
  and 
  

   thereafter 
  proceeds 
  along 
  the 
  ventral 
  region 
  of 
  the 
  body- 
  

   cavity 
  to 
  the 
  termination 
  of 
  the 
  anterior 
  region. 
  In 
  trans- 
  

   verse 
  section 
  the 
  muscle 
  is 
  divided 
  into 
  many 
  large 
  masses, 
  

   which 
  are 
  for 
  the 
  most 
  part 
  grouped 
  in 
  two 
  divisions 
  

   superiorly, 
  and 
  a 
  flattened 
  band 
  applied 
  to 
  the 
  upper 
  surface 
  

   of 
  the 
  transverse 
  ventral 
  muscle. 
  The 
  latter 
  division 
  — 
  at 
  

   the 
  point 
  of 
  differentiation 
  of 
  the 
  transverse 
  ventral 
  into 
  two 
  

   muscles 
  — 
  has 
  become 
  the 
  most 
  important, 
  forming 
  a 
  compact 
  

   layer 
  of 
  elongated 
  fasciculi 
  (in 
  section) 
  ; 
  while 
  the 
  other 
  

   divisions 
  occupy 
  the 
  lateral 
  regions 
  in 
  irregular 
  masses 
  and 
  

   in 
  diminished 
  bulk. 
  Finally, 
  behind 
  the 
  pharyngeal 
  region 
  

   the 
  lateral 
  divisions 
  disappear 
  in 
  the 
  transverse 
  sections 
  — 
  

   probably 
  by 
  being 
  attached 
  to 
  the 
  superior 
  raphe 
  at 
  each 
  

   side, 
  — 
  and 
  only 
  the 
  ventral 
  portion 
  of 
  the 
  retractors 
  remains 
  

   as 
  a 
  firm 
  double 
  muscle 
  with 
  symmetrically 
  arranged 
  fasci- 
  

   culi, 
  situated 
  in 
  the 
  central 
  line 
  over 
  the 
  transverse 
  ventral. 
  

   As 
  the 
  muscle 
  diminishes 
  in 
  bulk 
  and 
  approaches 
  the 
  central 
  

   line 
  on 
  each 
  side, 
  a 
  firm 
  granular 
  mass 
  is 
  formed, 
  first 
  be- 
  

   neath 
  it, 
  then 
  at 
  each 
  side 
  from 
  the 
  vertical 
  muscle 
  inward, 
  

   and 
  lastly 
  above 
  it 
  — 
  just 
  before 
  the 
  fibres 
  cease 
  at 
  the 
  

   ninth 
  body-segment, 
  and 
  in 
  front 
  of 
  the 
  commencement 
  of 
  

   the 
  ventral 
  blood-vessel. 
  The 
  importance 
  of 
  this 
  muscle 
  is 
  

   shown 
  by 
  its 
  extensive 
  connections, 
  all 
  of 
  which 
  could 
  not 
  

   be 
  readily 
  destroyed 
  at 
  once, 
  so 
  as 
  to 
  interfere 
  with 
  the 
  

   boring. 
  

  

  The 
  foregoing 
  structure 
  enables 
  the 
  functions 
  of 
  the 
  

   organ 
  in 
  the 
  economy 
  of 
  the 
  animal 
  to 
  be 
  understood. 
  By 
  

   its 
  agency 
  the 
  annelid 
  can 
  bore 
  almost 
  continuously 
  forward 
  

   in 
  the 
  sand 
  and 
  fashion 
  a 
  tunnel 
  in 
  which 
  the 
  more 
  delicate 
  

   posterior 
  part 
  reposes 
  in 
  safety. 
  The 
  contraction 
  of 
  the 
  

  

  