﻿Structure 
  o/'Magelona. 
  435 
  

  

  stretching 
  from 
  a 
  chitinous 
  origin 
  at 
  the 
  inner 
  surface 
  of 
  the 
  

   hypoderm, 
  at 
  a 
  point 
  over 
  the 
  longitudinal 
  ventral 
  muscles, 
  

   to 
  a 
  similar 
  attachment 
  on 
  the 
  opposite 
  side. 
  In 
  contraction 
  

   it 
  usually 
  presents 
  a 
  double 
  fusiform 
  appearance 
  — 
  pointed 
  

   externally, 
  and 
  narrowed 
  at 
  the 
  middle 
  line 
  under 
  the 
  

   longitudinal 
  muscles. 
  The 
  tentacular 
  blood-vessels 
  seem 
  to 
  

   pass 
  between 
  it 
  and 
  the 
  longitudinal 
  ventral 
  in 
  this 
  region. 
  

   While 
  the 
  external 
  terminations 
  of 
  the 
  muscle 
  immediately 
  

   become 
  simplified 
  by 
  attachment 
  to 
  the 
  upper 
  end 
  of 
  the 
  

   chitinous 
  support 
  (which 
  becomes 
  continuous 
  with 
  the 
  

   dorsal 
  hypodermic 
  basement-tissue) 
  of 
  the 
  longitudinal 
  

   ventral 
  (at 
  this 
  point 
  lateral) 
  muscle, 
  the 
  central 
  region 
  is 
  

   rendered 
  complex 
  by 
  the 
  gradual 
  increase 
  of 
  the 
  dorsal 
  

   longitudinal 
  muscle, 
  which 
  causes 
  the 
  fibres 
  to 
  bend 
  down- 
  

   wards*. 
  Thus 
  a 
  narrow 
  band 
  of 
  the 
  vertical 
  muscle 
  passes 
  

   over 
  the 
  outer 
  edge 
  of 
  the 
  median 
  longitudinal, 
  and 
  a 
  strong 
  

   belt 
  of 
  the 
  dorsal 
  transverse 
  separates 
  them 
  from 
  the 
  blood- 
  

   vessel 
  beneath. 
  A 
  vertical 
  slip 
  of 
  fibrous 
  tissue 
  passes 
  from 
  

   the 
  hypoderm 
  to 
  the 
  roof 
  of 
  the 
  chamber 
  for 
  the 
  dorsal 
  

   vessels, 
  between 
  the 
  longitudinal 
  muscles. 
  Thereafter 
  the 
  

   latter 
  descend 
  entirely 
  beneath 
  the 
  transverse 
  muscle, 
  and 
  a 
  

   chitinous 
  band 
  separates 
  the 
  fibres 
  in 
  the 
  middle 
  line. 
  The 
  

   course 
  of 
  the 
  muscle 
  under 
  the 
  hypodermic 
  basement-tissue 
  

   and 
  its 
  insertion 
  are 
  similar, 
  and 
  it 
  is 
  pierced 
  by 
  the 
  fibres 
  

   of 
  the 
  oblique. 
  Further 
  backward 
  the 
  distance 
  between 
  the 
  

   dorsal 
  origins 
  of 
  the 
  muscle 
  greatly 
  increases, 
  until 
  on 
  each 
  

   side 
  its 
  divisions 
  are 
  thrust 
  into 
  a 
  dorso-lateral 
  and 
  then 
  

   a 
  lateral 
  position, 
  while 
  the 
  fibres 
  are 
  correspondingly 
  

   shortened. 
  Toward 
  the 
  ninth 
  body-segment, 
  while 
  the 
  

   longitudinal 
  dorsal 
  and 
  ventral 
  muscles 
  decrease 
  in 
  bulk, 
  

   that 
  just 
  mentioned 
  is 
  much 
  enlarged, 
  stretching 
  on 
  each 
  

   side 
  as 
  a 
  broad 
  and 
  powerful 
  mass 
  over 
  the 
  whole 
  dorso- 
  

   lateral 
  and 
  lateral 
  regions. 
  At 
  the 
  ninth 
  body-segment 
  it 
  is 
  

   still 
  large, 
  and 
  its 
  outer 
  border 
  seems 
  to 
  impinge 
  on 
  the 
  

   great 
  bristle-wings. 
  The 
  increase 
  in 
  the 
  size 
  of 
  the 
  longi- 
  

   tudinal 
  dorsal 
  and 
  ventral 
  muscles 
  in 
  the 
  posterior 
  region 
  

   of 
  the 
  body, 
  and 
  their 
  positions, 
  render 
  this 
  muscle 
  nearly 
  

   vertical 
  and 
  of 
  considerable 
  length 
  on 
  each 
  side. 
  It 
  extends 
  

   from 
  the 
  hypodermic 
  basement-tissue 
  at 
  the 
  external 
  border 
  

   of 
  the 
  dorsal 
  muscle 
  downward 
  to 
  the 
  outer 
  border 
  of 
  the 
  

   ventral 
  and 
  during 
  the 
  reproductive 
  season 
  is 
  often 
  greatly 
  

   stretched 
  by 
  the 
  vast 
  increase 
  of 
  the 
  ova. 
  

  

  In 
  the 
  living 
  animal 
  the 
  median 
  fibres 
  of 
  the 
  transverse 
  

  

  * 
  Longitudinal 
  sections 
  show 
  the 
  fibres 
  of 
  this 
  muscle 
  passing- 
  over 
  

   the 
  longitudinal 
  dorsal 
  very 
  clearly, 
  just 
  before 
  its 
  separation 
  by 
  the 
  

   median 
  chitinous 
  raphe. 
  

  

  29* 
  

  

  