﻿Structure 
  of 
  Magelona. 
  429 
  

  

  inferior 
  margin 
  of 
  the 
  central 
  muscular 
  area, 
  and 
  sends 
  a 
  

   bar 
  to 
  meet 
  its 
  fellow 
  of 
  the 
  opposite 
  side. 
  The 
  chitinous 
  

   supports 
  of 
  the 
  vascular 
  area 
  and 
  the 
  septa 
  between 
  the 
  

   muscular 
  spaces 
  are 
  strong. 
  Lastly, 
  the 
  median 
  muscular 
  

   investment 
  disappears, 
  leaving, 
  about 
  the 
  origin 
  of 
  the 
  

   tentacles, 
  only 
  the 
  external 
  chamber, 
  which 
  sends 
  a 
  process 
  

   outward 
  to 
  the 
  base 
  of 
  the 
  tentacle, 
  and 
  another 
  from 
  the 
  

   same 
  point 
  to 
  join 
  the 
  ventral 
  cuticle 
  ; 
  while 
  between 
  these 
  

   lie 
  the 
  hypodermic 
  nerve-area 
  and 
  the 
  neural 
  canal 
  (supe- 
  

   riorly 
  and 
  externally). 
  The 
  chitinous 
  investment 
  does 
  not 
  

   closely 
  surround 
  the 
  external 
  muscle, 
  but, 
  sending 
  its 
  lower 
  

   bar 
  upward 
  and 
  inward, 
  a 
  considerable 
  space 
  is 
  by-and-by 
  

   formed. 
  Thereafter 
  the 
  chitinous 
  layer 
  is 
  continued 
  back- 
  

   ward 
  as 
  the 
  external 
  support 
  of 
  the 
  muscle, 
  and 
  may 
  for 
  the 
  

   present 
  be 
  left. 
  

  

  The 
  chitinous 
  basement-system 
  thus 
  constitutes 
  a 
  kind 
  of 
  

   endo 
  skeleton 
  for 
  the 
  entire 
  snout, 
  giving, 
  with 
  the 
  cuticle, 
  

   firmness 
  to 
  the 
  organ 
  in 
  its 
  ceaseless 
  thrusts 
  into 
  the 
  sand, 
  

   supporting 
  its 
  muscles, 
  and 
  confining 
  their 
  action 
  (in 
  lieu 
  of 
  

   a 
  circular 
  coat) 
  to 
  the 
  most 
  favourable 
  lines, 
  and 
  with 
  its 
  

   flexible 
  plates 
  — 
  even 
  more 
  than 
  the 
  beams 
  of 
  a 
  ship 
  — 
  lending 
  

   the 
  necessary 
  power 
  of 
  resistance 
  to 
  the 
  yielding 
  tissues 
  of 
  

   the 
  snout 
  and 
  protecting 
  the 
  blood-vessels. 
  Moreover, 
  the 
  

   continuity 
  between 
  the 
  base 
  of 
  the 
  tentacles 
  and 
  the 
  chitinous 
  

   support 
  of 
  the 
  ventral 
  longitudinal 
  muscles 
  gives 
  a 
  com- 
  

   pleteness 
  to 
  the 
  system 
  which 
  most 
  admirably 
  adapts 
  the 
  

   whole 
  anterior 
  region 
  for 
  its 
  special 
  functions. 
  

  

  It 
  is 
  found 
  — 
  to 
  continue 
  the 
  account 
  of 
  the 
  median 
  longi- 
  

   tudinal 
  muscles 
  — 
  that 
  they 
  are 
  not 
  simple 
  bands, 
  but, 
  as 
  

   might 
  be 
  inferred 
  from 
  their 
  chitinous 
  investment 
  and 
  the 
  

   arrangement 
  of 
  the 
  fibres, 
  they 
  act 
  to 
  a 
  certain 
  extent 
  

   obliquely 
  as 
  well 
  as 
  longitudinally. 
  Before 
  the 
  appearance 
  

   of 
  the 
  lateral 
  muscles 
  at 
  the 
  tip 
  of 
  the 
  snout, 
  and 
  when 
  the 
  

   spectacle-like 
  arrangement 
  of 
  the 
  pair 
  exists 
  in 
  transverse 
  

   section, 
  each 
  consists 
  of 
  a 
  thick 
  dorsal 
  arch 
  of 
  fibres, 
  which 
  

   proceeds 
  a 
  short 
  distance 
  downward 
  at 
  each 
  end, 
  while 
  the 
  

   ventral 
  part 
  of 
  the 
  chamber 
  is 
  occupied 
  by 
  granular 
  tissue. 
  

   The 
  muscular 
  fasciculi 
  are 
  nearly 
  linear 
  and 
  slant 
  from 
  

   above 
  downward, 
  with 
  varying 
  degrees 
  of 
  obliquity 
  according 
  

   to 
  the 
  condition 
  of 
  the 
  preparation. 
  When 
  the 
  spaces 
  

   assume 
  the 
  form 
  of 
  a 
  crown, 
  fasciculi 
  pass 
  from 
  the 
  arch 
  

   nearly 
  at 
  right 
  angles; 
  others 
  extend 
  from 
  the 
  upper 
  part 
  

   of 
  the 
  median 
  septum 
  downward 
  and 
  inward. 
  Moreover, 
  

   the 
  outer 
  fasciculi 
  from 
  the 
  external 
  septum 
  join 
  the 
  larger 
  

   which 
  pass 
  to 
  the 
  bottom, 
  then 
  curve 
  inward 
  along 
  the 
  

   ventral 
  chitinous 
  floor, 
  and 
  meet 
  the 
  descending 
  fasciculi 
  at 
  

  

  