﻿Structure 
  o/Magelona. 
  441 
  

  

  The 
  centre 
  superiorly 
  and 
  the 
  sides 
  near 
  the 
  ventral 
  surface 
  

   inferiorly 
  are 
  marked 
  with 
  bi'ownish 
  pigment. 
  This 
  buccal 
  

   region, 
  indeed, 
  may 
  be 
  defined 
  as 
  that 
  lying 
  between 
  the 
  

   oral 
  aperture 
  in 
  front 
  and 
  the 
  great 
  vertical 
  muscle 
  posteriorly, 
  

   the 
  latter, 
  moreover, 
  corresponding 
  to 
  the 
  anterior 
  attach- 
  

   ment 
  of 
  the 
  dorsal 
  wall 
  of 
  the 
  proboscis. 
  It 
  has 
  its 
  ventral 
  

   margin 
  for 
  the 
  most 
  part 
  open 
  and 
  only 
  completed 
  in 
  the 
  

   living 
  animal 
  by 
  the 
  approximation 
  of 
  the 
  included 
  proboscis, 
  

   or 
  the 
  closure 
  of 
  its 
  own 
  walls. 
  The 
  buccal 
  mucous 
  tissue 
  

   is 
  not 
  extruded 
  to 
  any 
  extent 
  ; 
  that 
  behind 
  is. 
  To 
  the 
  sides 
  

   of 
  the 
  buccal 
  region 
  are 
  attached 
  various 
  muscular 
  bands, 
  

   which 
  probably 
  retract 
  the 
  buccal 
  flaps 
  or 
  lips 
  during 
  the 
  

   extrusion 
  of 
  the 
  proboscis. 
  The 
  anterior 
  buccal 
  tissue, 
  

   further, 
  is 
  pulled 
  upon 
  by 
  a 
  series 
  of 
  horizontal 
  fibres 
  which 
  

   come 
  from 
  the 
  anterior 
  part 
  of 
  the 
  roof 
  of 
  the 
  pre-oral 
  

   chamber 
  in 
  front. 
  At 
  the 
  termination 
  of 
  this 
  region 
  is 
  

   superiorly 
  a 
  narrow 
  arch 
  of 
  the 
  buccal 
  wall, 
  upon 
  which 
  the 
  

   dorsal 
  blood-vessel 
  lies, 
  and 
  laterally 
  two 
  wide 
  folds 
  which 
  

   diminish 
  at 
  their 
  outer 
  and 
  inferior 
  angle, 
  where 
  they 
  are 
  

   joined 
  by 
  the 
  proboscidian 
  wall. 
  

  

  The 
  next 
  or 
  pharyngeal 
  region 
  begins 
  at 
  the 
  junction 
  of 
  

   the 
  proboscidian 
  wall 
  above 
  mentioned, 
  and 
  is 
  further 
  

   characterized 
  by 
  the 
  presence 
  of 
  a 
  series 
  of 
  longitudinal 
  

   muscular 
  fibres, 
  which 
  appear 
  outside 
  the 
  basement-tissue 
  of 
  

   the 
  lateral 
  regions. 
  The 
  outer 
  margin 
  of 
  the 
  glandular 
  

   layer 
  has 
  laterally 
  a 
  considerable 
  breadth 
  of 
  finely 
  granular 
  

   tissue, 
  then 
  the 
  chitinous 
  basement-substance, 
  and 
  externally 
  

   the 
  layer 
  of 
  longitudinal 
  fibres, 
  which 
  are 
  entirely 
  lateral 
  in 
  

   position, 
  i. 
  e. 
  extending 
  from 
  the 
  dorsal 
  to 
  the 
  ventral 
  curve 
  

   on 
  each 
  side. 
  The 
  canal 
  at 
  this 
  point 
  is 
  much 
  enlarged, 
  

   while 
  inferiorly 
  the 
  broad 
  glandular 
  lining 
  becomes 
  thinner, 
  

   and 
  merges 
  into 
  the 
  chitinous 
  coat 
  of 
  the 
  proboscis, 
  which, 
  

   near 
  the 
  junction, 
  shows 
  an 
  incipient 
  glandular 
  layer 
  in- 
  

   ternally. 
  Further 
  backward 
  bands 
  of 
  strong 
  oblique 
  fibres 
  

   are 
  attached 
  to 
  the 
  inferior 
  and 
  outer 
  region 
  of 
  the 
  canal 
  — 
  

   now 
  completed. 
  These 
  muscular 
  bands 
  are 
  evidently 
  the 
  

   retractors 
  of 
  the 
  organ. 
  Superiorly 
  a 
  narrow 
  retractor 
  is 
  

   inserted 
  into 
  the 
  wall 
  of 
  the 
  canal 
  at 
  the 
  bend, 
  a 
  larger 
  pair 
  

   occur 
  laterally, 
  and 
  a 
  similar 
  pair 
  at 
  the 
  ventral 
  border 
  (the 
  

   specimen 
  had 
  ejected 
  its 
  proboscis 
  but 
  not 
  its 
  pharynx). 
  

   The 
  latter 
  muscles 
  pass 
  transversely 
  outward 
  to 
  be 
  attached 
  

   to 
  the 
  great 
  mass 
  at 
  the 
  raphe 
  above 
  the 
  longitudinal 
  ventral 
  

   muscles. 
  The 
  dorsal 
  region 
  of 
  the 
  chamber 
  still 
  has 
  a 
  thin 
  

   arch 
  of 
  mucous 
  tissue, 
  while 
  it 
  is 
  massive 
  at 
  the 
  sides 
  and 
  

   ventral 
  region. 
  A 
  slender 
  muscular 
  band 
  from 
  the 
  dorsal 
  

   raphe 
  (at 
  the 
  side 
  of 
  the 
  dorsal 
  longitudinal) 
  passes 
  inward 
  

  

  