﻿Structure 
  of 
  Magelona. 
  449 
  

  

  longitudinal 
  sections, 
  but 
  a 
  special 
  longitudinal 
  coat 
  has 
  not 
  

   been 
  made 
  out, 
  either 
  in 
  transverse 
  or 
  longitudinal 
  sections. 
  

   The 
  longitudinal 
  muscular 
  coat 
  of 
  the 
  dorsal 
  blood-vessels, 
  

   and 
  perhaps 
  the 
  ventral 
  also, 
  may, 
  with 
  the 
  nature 
  of 
  the 
  

   wall 
  itself, 
  aid 
  in 
  this 
  respect. 
  The 
  internal 
  surface 
  from 
  

   the 
  tenth 
  segment 
  backward 
  is 
  probably 
  covered 
  with 
  cilia 
  

   (though 
  they 
  could 
  not 
  be 
  made 
  out 
  in 
  front), 
  and 
  they 
  are 
  

   well 
  seen 
  (though 
  not 
  so 
  boldly 
  marked 
  as 
  in 
  Nerine 
  and 
  

   Spio) 
  near 
  the 
  tail. 
  The 
  canal, 
  retaining 
  the 
  same 
  structure, 
  

   though 
  the 
  circular 
  coat 
  becomes 
  more 
  distinct, 
  diminishes 
  

   toward 
  the 
  tip 
  of 
  the 
  tail, 
  and 
  terminates 
  in 
  an 
  anus 
  on 
  the 
  

   dorsal 
  surface 
  a 
  little 
  within 
  the 
  margin. 
  The 
  latter 
  part 
  of 
  

   the 
  tract 
  shows 
  very 
  active 
  contractions, 
  and 
  the 
  cilia 
  at 
  the 
  

   anus 
  are 
  largely 
  developed. 
  

  

  The 
  food 
  consists 
  of 
  sand 
  containing 
  various 
  minute 
  

   nutritious 
  particles, 
  translucent 
  chitinous 
  fragments 
  of 
  

   crustaceans, 
  foraminifera, 
  and 
  other 
  debris. 
  Sand 
  is 
  very 
  

   necessary 
  for 
  the 
  existence 
  of 
  this 
  form, 
  for 
  though 
  the 
  

   animals 
  survive 
  a 
  considerable 
  period 
  in 
  captivity 
  in 
  vessels 
  

   filled 
  with 
  pure 
  sea-water, 
  they 
  thrive 
  much 
  longer 
  amongst 
  

   fine 
  sand, 
  with 
  a 
  few 
  inches 
  of 
  water 
  over 
  it. 
  No 
  parasites, 
  

   other 
  than 
  an 
  unknown 
  ovum, 
  have 
  been 
  seen 
  in 
  the 
  ali- 
  

   mentary 
  canal 
  — 
  a 
  very 
  different 
  condition 
  from 
  that 
  of 
  its 
  

   congener 
  Polydora 
  ciliata, 
  which 
  has 
  many 
  Gregarinae 
  in 
  the 
  

   same 
  organ. 
  

  

  VI. 
  The 
  Nervous 
  System. 
  

  

  The. 
  central 
  mass 
  of 
  the 
  nervous 
  system 
  lies 
  above 
  the 
  

   pre-oral 
  chamber 
  — 
  and 
  rather 
  in 
  front 
  of 
  the 
  muscular 
  

   blood-vessels 
  of 
  the 
  part 
  — 
  in 
  the 
  form 
  of 
  two 
  superior 
  

   ganglia, 
  and 
  they 
  are 
  by 
  no 
  means 
  easily 
  made 
  out 
  in 
  the 
  

   living 
  animal. 
  Each 
  consists 
  of 
  amass 
  of 
  nerve-cells 
  — 
  some 
  

   of 
  considerable 
  size, 
  with 
  a 
  commissure 
  of 
  fibres 
  — 
  best 
  

   marked 
  posteriorly. 
  In 
  the 
  hypodermic 
  region 
  below 
  and 
  

   in 
  front 
  of 
  the 
  chamber, 
  another 
  (smaller) 
  nervous 
  mass 
  is 
  

   seen 
  in 
  longitudinal 
  section, 
  but 
  the 
  actual 
  connection 
  be- 
  

   tween 
  the 
  superior 
  and 
  inferior 
  divisions 
  has 
  not 
  yet 
  been 
  

   made 
  out. 
  From 
  the 
  oblique 
  appearance 
  of 
  the 
  nerve-cords 
  

   in 
  section, 
  they 
  certainly 
  pass 
  very 
  rapidly 
  down 
  to 
  the 
  sides 
  

   of 
  the 
  body 
  from 
  the 
  central 
  mass, 
  and 
  in 
  the 
  living 
  animal 
  

   their 
  course 
  outward 
  is 
  similarly 
  abrupt. 
  There 
  is 
  no 
  trace 
  

   of 
  eye-specks, 
  but 
  the 
  animals 
  are 
  sensitive 
  to 
  light, 
  and 
  

   touch 
  is 
  highly 
  developed. 
  

  

  When 
  the 
  nerve-cord 
  reaches 
  the 
  lateral 
  region 
  it 
  is 
  found 
  

   under 
  the 
  channel 
  leading 
  to 
  the 
  tentacle, 
  and 
  somewhat 
  

  

  Ann. 
  & 
  Mag. 
  N. 
  Ilist. 
  Ser. 
  8. 
  Vol. 
  vii. 
  30 
  

  

  