﻿Structure 
  o/Magelona. 
  433 
  

  

  lateral 
  (external) 
  muscle 
  ; 
  and 
  it 
  is 
  clear 
  that 
  this 
  powerful 
  

   band 
  is 
  the 
  chief 
  agent 
  in 
  narrowing 
  the 
  snout 
  from 
  side 
  to 
  

   side. 
  Proceeding 
  backward 
  it 
  is 
  found 
  to 
  increase 
  in 
  

   strength, 
  while 
  the 
  vertical 
  fibres 
  in 
  the 
  inferior 
  space 
  

   diminish 
  (a 
  few 
  occurring 
  laterally). 
  Then 
  a 
  stronger 
  

   vertical 
  series, 
  springing 
  from 
  the 
  middle 
  of 
  the 
  chitinous 
  

   arch 
  superiorly 
  — 
  between 
  two 
  lateral 
  channels 
  — 
  and 
  spreading 
  

   out 
  in 
  a 
  fan-like 
  manner 
  through 
  the 
  powerful 
  transverse 
  

   plate 
  above 
  mentioned, 
  reaches 
  the 
  ventral 
  hypoderm 
  with 
  

   its 
  base 
  and 
  obliterates 
  the 
  inferior 
  space. 
  The 
  two 
  muscles 
  

   are 
  evidently 
  antagonistic 
  in 
  action, 
  and 
  their 
  important 
  

   functions 
  in 
  connection 
  with 
  the 
  vascular 
  system 
  may 
  be 
  

   further 
  exemplified 
  by 
  contrasting 
  specimens 
  in 
  which 
  the 
  

   region 
  is 
  contracted 
  with 
  those 
  in 
  which 
  it 
  is 
  largely 
  dilated. 
  

   The 
  inner 
  wall 
  of 
  the 
  chamber 
  being 
  composed 
  of 
  elastic 
  

   chitinous 
  tissue, 
  very 
  great 
  expansion 
  is 
  permitted. 
  In 
  

   longitudinal 
  sections 
  the 
  transverse 
  muscle 
  is 
  well 
  seen 
  in 
  

   its 
  space 
  just 
  in 
  front 
  of 
  the 
  mouth, 
  with 
  the 
  vessels 
  at 
  its 
  

   upper 
  border. 
  The 
  space 
  or 
  cavity 
  is 
  comparatively 
  short 
  

   (antero-posteriorly) 
  ; 
  indeed, 
  it 
  is 
  confined 
  to 
  the 
  pre-oral 
  

   region. 
  Further, 
  the 
  vertical 
  fibres 
  from 
  the 
  chitinous 
  

   superior 
  arch 
  seem 
  to 
  meet 
  over 
  the 
  transverse 
  muscle 
  and 
  

   send 
  buudles 
  in 
  front 
  of 
  and 
  behind 
  it 
  — 
  the 
  latter 
  being 
  the 
  

   stronger. 
  The 
  fibres 
  of 
  the 
  vertical 
  muscle 
  are 
  attached 
  to 
  

   a 
  chitinous 
  plate, 
  which 
  springs 
  from 
  the 
  anterior 
  border 
  of 
  

   the 
  superior 
  transverse 
  muscle, 
  beneath 
  which 
  the 
  dorsal 
  

   blood-vessels 
  pass. 
  The 
  latter 
  muscle 
  in 
  some 
  preparations 
  

   is 
  narrow 
  and 
  deep 
  in 
  the 
  middle, 
  and 
  spreads 
  out 
  at 
  each 
  

   side. 
  

  

  The 
  next 
  muscles 
  that 
  come 
  under 
  notice 
  are 
  the 
  longi- 
  

   tudinal 
  dorsal, 
  which, 
  when 
  viewed 
  from 
  above, 
  take 
  origin 
  

   between 
  the 
  forks 
  of 
  the 
  chitinous 
  process 
  over 
  the 
  mouth 
  as 
  

   narrow 
  ribbons, 
  widen 
  till 
  about 
  the 
  sixth 
  set 
  of 
  bristles, 
  

   then 
  diminish 
  to 
  the 
  ninth, 
  and 
  again 
  spread 
  out 
  thereafter. 
  

   In 
  section 
  they 
  are 
  found 
  to 
  commence 
  in 
  front 
  of 
  the 
  

   vascular 
  space 
  as 
  two 
  small 
  slips 
  surrounded 
  by 
  the 
  usual 
  

   chitinous 
  basement-tissue 
  situated 
  over 
  the 
  transverse 
  

   muscle. 
  They 
  then 
  pass 
  below 
  the 
  latter, 
  increase 
  in 
  size, 
  

   and 
  form 
  the 
  superior 
  arch 
  of 
  the 
  chamber 
  for 
  the 
  dorsal 
  

   blood-vessel. 
  Proceeding 
  backward 
  they 
  gradually 
  extend 
  

   outward 
  and 
  enlarge 
  — 
  the 
  deepest 
  part 
  of 
  each 
  being 
  toward 
  

   the 
  middle 
  of 
  the 
  muscle, 
  and 
  the 
  thinnest 
  near 
  the 
  mesial 
  

   line, 
  where 
  a 
  raphe 
  occurs. 
  Externally 
  is 
  the 
  hypodermic 
  

   basement-tissue, 
  internally 
  (in 
  the 
  median 
  line) 
  the 
  dorsal 
  

   blood-vessel 
  and 
  the 
  oesophagus 
  at 
  the 
  sides 
  ; 
  laterally 
  each 
  

   abuts 
  on 
  the 
  origin 
  of 
  the 
  external 
  lateral 
  muscle. 
  Toward 
  

   Ann. 
  & 
  Mag. 
  N. 
  Hist. 
  Ser. 
  8. 
  Vol. 
  vii. 
  29 
  

  

  