﻿Structure 
  of 
  Magelona. 
  423 
  

  

  the 
  cuticle 
  thinner, 
  and 
  there 
  are 
  indications, 
  e.g. 
  the 
  lateral 
  

   frill, 
  that 
  the 
  mouth 
  is 
  approaching. 
  The 
  lateral 
  expansions 
  

   of 
  the 
  snout 
  (which 
  resemble 
  alas 
  in 
  transverse 
  section) 
  also 
  

   diminish 
  considerably, 
  their 
  shape 
  alters 
  from 
  the 
  lanceolate 
  

   to 
  a 
  more 
  or 
  less 
  cylindrical 
  condition, 
  and 
  then, 
  by 
  a 
  basal 
  

   constriction, 
  they 
  resume 
  a 
  clavate 
  appearance. 
  The 
  cuticle 
  

   on 
  these 
  expansions 
  is 
  much 
  thicker 
  dorsally 
  than 
  ventrally, 
  

   and, 
  while 
  the 
  chitinous 
  raphe 
  is 
  placed 
  in 
  the 
  lower 
  third 
  

   instead 
  of 
  the 
  middle 
  of 
  the 
  processes, 
  the 
  hypodermic 
  fibres 
  

   preserve 
  the 
  same 
  arrangement 
  in 
  front. 
  Immediately 
  

   behind 
  the 
  transverse 
  space 
  alluded 
  to 
  above, 
  and 
  in 
  a 
  line 
  

   with 
  the 
  commencement 
  of 
  the 
  dorsal 
  longitudinal 
  muscles, 
  

   the 
  hypoderm 
  somewhat 
  increases 
  in 
  thickness 
  toward 
  the 
  

   base 
  of 
  the 
  expansion, 
  slightly 
  narrows 
  at 
  the 
  dorsal 
  arch, 
  

   and 
  again 
  expands 
  before 
  the 
  decided 
  narrowing 
  occurs 
  over 
  

   the 
  dorsal 
  longitudinal 
  muscles. 
  From 
  the 
  inferior 
  border 
  

   of 
  the 
  base 
  of 
  the 
  lateral 
  expansion 
  a 
  chitinous 
  septum 
  joins 
  

   the 
  raphe 
  at 
  a 
  somewhat 
  acute 
  angle, 
  and 
  cuts 
  off 
  a 
  narrow 
  

   strip 
  of 
  hypoderm, 
  to 
  which 
  and 
  the 
  great 
  lobe, 
  projecting 
  

   beneath 
  the 
  expansion, 
  it 
  acts 
  as 
  a 
  party- 
  wall. 
  The 
  ventral 
  

   hypoderm 
  has 
  now 
  merged 
  into 
  the 
  foliaceous 
  surface 
  of 
  the 
  

   oral 
  region. 
  

  

  The 
  lateral 
  expansions 
  of 
  the 
  snout 
  then 
  decrease 
  (in 
  

   transverse 
  section) 
  to 
  simple 
  processes 
  in 
  which 
  the 
  raphe 
  is 
  

   barely 
  visible 
  and 
  then 
  disappears. 
  They 
  spring 
  from 
  a 
  

   thick 
  mass 
  of 
  hypoderm 
  marked 
  by 
  certain 
  large 
  areola}, 
  and 
  

   which 
  gradually 
  diminishes 
  dorsally 
  until 
  the 
  middle 
  line 
  is 
  

   reached. 
  A. 
  slender 
  neck 
  of 
  hypoderm 
  proceeds 
  downward 
  

   along 
  the 
  now 
  slightly 
  projecting 
  process 
  beneath 
  the 
  ex- 
  

   pansion, 
  and 
  then 
  enlarges 
  to 
  form 
  a 
  bulbous 
  ventral 
  mass, 
  

   which 
  contains 
  the 
  nerve-cord 
  with 
  its 
  neural 
  canal 
  (situated 
  

   externally). 
  It 
  then 
  becomes 
  continuous, 
  by 
  a 
  narrow 
  strip, 
  

   with 
  the 
  buccal 
  region. 
  The 
  expansion 
  soon 
  shrinks 
  to 
  a 
  

   blunt 
  process, 
  and 
  the 
  hypodermic 
  band 
  between 
  the 
  dorsal 
  

   and 
  ventral 
  regions 
  elongates. 
  The 
  ventral 
  hypoderm 
  also 
  

   increases 
  and 
  its 
  cuticular 
  investment 
  is 
  thicker. 
  At 
  the 
  

   oiigin 
  of 
  the 
  tentacles 
  the 
  hypoderm 
  of 
  the 
  snout 
  forms 
  an 
  

   arch 
  over 
  their 
  base, 
  the 
  outer 
  edge 
  being 
  thick, 
  while 
  the 
  

   inner 
  part 
  of 
  the 
  arch 
  is 
  more 
  translucent. 
  In 
  some 
  views 
  

   it 
  simulates 
  an 
  aperture. 
  

  

  Behind 
  the 
  oral 
  region 
  the 
  hypoderm 
  still 
  forms 
  a 
  com- 
  

   plete 
  ring. 
  A 
  narrow 
  arch 
  occurs 
  over 
  the 
  dorsum, 
  but 
  it 
  

   widens 
  at 
  each 
  side 
  and 
  at 
  intervals 
  runs 
  into 
  the 
  lateral 
  

   lamellae, 
  which, 
  with 
  the 
  exception 
  of 
  the 
  delicate 
  cuticular 
  

   investment, 
  are 
  almost 
  wholly 
  formed 
  of 
  this 
  tissue. 
  A 
  

   somewhat 
  thick 
  layer 
  courses 
  down 
  the 
  lateral 
  region 
  and 
  

  

  