﻿Structure 
  of 
  Magelona. 
  443 
  

  

  special 
  muscular 
  arrangements 
  well 
  adapt 
  it 
  for 
  such 
  a 
  

   function. 
  The 
  exsertile 
  portion, 
  forming 
  a 
  distinct 
  fold, 
  

   occupies 
  the 
  dorsal 
  region, 
  and 
  is 
  characterized, 
  in 
  addition 
  

   to 
  its 
  muscular 
  development, 
  by 
  having 
  the 
  inner 
  surface 
  of 
  

   its 
  floor 
  thrown 
  into 
  very 
  prominent 
  transverse 
  rugae, 
  which 
  

   in 
  the 
  uninjured 
  animal 
  appear 
  externally 
  as 
  well-marked. 
  

   Lars. 
  In 
  extrusion 
  a 
  great 
  curve 
  takes 
  place 
  behind 
  the 
  

   buccal 
  region, 
  downward 
  and 
  forward 
  into 
  the 
  base 
  of 
  the 
  

   exserted 
  proboscis 
  ; 
  yet, 
  though 
  the 
  folds 
  are 
  boldly 
  defined, 
  

   it 
  is 
  difficult 
  to 
  follow 
  their 
  windings, 
  or 
  to 
  estimate 
  their 
  

   exact 
  relations 
  with 
  the 
  parts 
  in 
  the 
  retracted 
  condition. 
  

   Further, 
  it 
  occasionally 
  happens 
  that 
  the 
  great 
  muscular 
  

   plexus 
  of 
  the 
  protrusible 
  part 
  is 
  inverted, 
  i. 
  e., 
  is 
  situated 
  

   dorsally 
  instead 
  of 
  ventrally. 
  The 
  region 
  supplied 
  with 
  the 
  

   well-marked 
  muscular 
  coat 
  is 
  much 
  dilated 
  in 
  the 
  interior 
  of 
  

   the 
  proboscis, 
  and 
  this 
  special 
  disposition 
  will 
  materially 
  aid 
  

   in 
  the 
  withdrawal 
  of 
  the 
  organ 
  to 
  its 
  position 
  in 
  the 
  body. 
  

   In 
  longitudinal 
  section 
  the 
  circular 
  muscles 
  of 
  the 
  pharyngeal 
  

   coat 
  are 
  observed 
  to 
  be 
  clasped 
  between 
  the 
  radiating 
  series 
  

   in 
  linear 
  spaces, 
  while 
  the 
  diverging 
  form 
  a 
  very 
  regularly 
  

   looped 
  system 
  of 
  crossed 
  fibres, 
  which 
  probably 
  have 
  a 
  some- 
  

   what 
  spiral 
  arrangement 
  (and 
  the 
  indistinctness 
  of 
  the 
  

   longitudinal 
  fibres 
  in 
  transverse 
  section 
  would 
  thus 
  be 
  

   accounted 
  for). 
  The 
  chief 
  muscular 
  retractor, 
  attached 
  

   posteriorly 
  to 
  the 
  superior 
  raphe 
  on 
  each 
  side, 
  is 
  well 
  shown, 
  

   as 
  is 
  also 
  a 
  band 
  of 
  fibres 
  from 
  the 
  latter 
  raphe, 
  which 
  passes 
  

   backward 
  for 
  insertion 
  at 
  the 
  anterior 
  part 
  of 
  the 
  extruded 
  

   organ, 
  and 
  which 
  would 
  pull 
  it 
  inward 
  and 
  forward 
  in 
  

   retraction. 
  Anteriorly, 
  again, 
  fibres 
  proceed 
  from 
  the 
  

   anterior 
  fold 
  of 
  the 
  organ 
  (ventrally) 
  forward 
  to 
  the 
  fold 
  of 
  

   the 
  buccal 
  glandular 
  tunic, 
  where 
  it 
  joins 
  the 
  proboscis 
  ; 
  and 
  

   in 
  the 
  extrusion 
  of 
  that 
  tissue 
  the 
  anterior 
  region 
  of 
  the 
  

   pharynx 
  would 
  be 
  pulled 
  on, 
  thus 
  explaining, 
  in 
  all 
  probability, 
  

   the 
  inversion 
  of 
  the 
  latter, 
  which 
  sometimes, 
  as 
  above 
  

   mentioned, 
  shows 
  its 
  muscular 
  coat 
  superior 
  in 
  the 
  extruded 
  

   region. 
  

  

  The 
  expulsion 
  of 
  the 
  proboscis 
  is 
  doubtless 
  the 
  primary 
  

   agent 
  in 
  the 
  movement 
  of 
  this 
  pharyngeal 
  region, 
  then 
  

   follow 
  the 
  dragging 
  on 
  the 
  anterior 
  part 
  by 
  the 
  ventral 
  

   fibres 
  and 
  the 
  action 
  of 
  the 
  body-walls 
  of 
  the 
  animal. 
  In 
  

   withdrawal, 
  again, 
  its 
  great 
  retractors, 
  and 
  the 
  contraction 
  

   of 
  the 
  expanded 
  special 
  coat, 
  with 
  its 
  longitudinal, 
  looped, 
  

   and 
  other 
  fibres, 
  would 
  suffice 
  to 
  bring 
  it 
  into 
  position. 
  

  

  The 
  Proboscis. 
  

   This 
  organ 
  in 
  life 
  is 
  thrust 
  out 
  at 
  somewhat 
  regular 
  

  

  