﻿126 
  E. 
  J. 
  ALLEN. 
  

  

  alternate 
  filling 
  and 
  emptying- 
  of 
  the 
  larger 
  vessels 
  produces 
  

   an 
  appearance 
  of 
  rapid 
  colour-cliange. 
  A 
  further 
  change 
  of 
  

   colour 
  is 
  seen 
  also, 
  which 
  is 
  due 
  to 
  changes 
  in 
  the 
  chemical 
  

   character 
  of 
  the 
  blood. 
  If 
  a 
  worm 
  be 
  allowed 
  to 
  remain 
  for 
  

   some 
  time 
  in 
  a 
  vessel 
  containing 
  only 
  a 
  small 
  quantity 
  of 
  sea- 
  

   water, 
  the 
  bright 
  scarlet 
  of 
  the 
  blood 
  changes 
  to 
  a 
  dull 
  purple- 
  

   red^ 
  but 
  the 
  oi'iginal 
  colour 
  immediately 
  reappears 
  on 
  the 
  

   addition 
  of 
  a 
  new 
  supply 
  of 
  water, 
  as 
  it 
  does 
  under 
  similar 
  

   circumstances 
  in 
  Magelona 
  (cf. 
  Beuham, 
  1896). 
  The 
  red 
  

   colour 
  therefore 
  would 
  seem 
  to 
  be 
  due 
  to 
  the 
  presence 
  in 
  the 
  

   blood 
  of 
  one 
  of 
  the 
  respiratory 
  pigments. 
  

  

  The 
  general 
  arrangements 
  of 
  the 
  principal 
  vessels 
  is 
  

   illustrated 
  in 
  fig. 
  1, 
  which 
  has 
  been 
  constructed 
  from 
  obser- 
  

   vations 
  on 
  the 
  living 
  worm, 
  corrected 
  and 
  extended 
  by 
  the 
  

   examination 
  of 
  sections. 
  For 
  the 
  purposes 
  of 
  description 
  

   the 
  body 
  of 
  the 
  worm 
  must 
  be 
  divided 
  into 
  three 
  regions, 
  an 
  

   anterior 
  region 
  consisting 
  of 
  segments 
  1 
  to 
  11, 
  an 
  inter- 
  

   mediate 
  region, 
  comprising 
  the 
  four 
  segments 
  12, 
  13, 
  14, 
  and 
  

   15, 
  and 
  a 
  posterior 
  region 
  from 
  segment 
  16 
  to 
  the 
  end 
  of 
  the 
  

   body. 
  

  

  Anterior 
  Region. 
  — 
  Between 
  the 
  alimentary 
  canal 
  and 
  

   the 
  dorsal 
  body-wall 
  there 
  is 
  in 
  the 
  anterior 
  region 
  a 
  large, 
  

   muscular, 
  dorsal 
  vessel 
  of 
  cylindrical 
  shape 
  capable 
  of 
  very 
  

   considerable 
  expansion, 
  waves 
  of 
  expansion 
  and 
  contraction 
  

   passing 
  along 
  it 
  from 
  behind 
  forwards. 
  

  

  Corresponding 
  with 
  each 
  of 
  the 
  body 
  segments 
  from 
  the 
  

   third 
  to 
  the 
  eleventh, 
  a 
  lateral 
  vessel 
  is 
  given 
  off 
  on 
  each 
  

   side 
  from 
  the 
  dorsal 
  vessel, 
  and 
  runs 
  outwards 
  and 
  downwards 
  

   in 
  the 
  posterior 
  septum 
  of 
  the 
  segment. 
  Owing 
  to 
  the 
  fact 
  

   already 
  described 
  that 
  the 
  posterior 
  septa 
  of 
  segments 
  5 
  

   to 
  11 
  are 
  pushed 
  backwards, 
  the 
  origins 
  of 
  the 
  lateral 
  vessels 
  

   in 
  these 
  segments, 
  running, 
  as 
  the 
  vessels 
  do 
  during 
  the 
  first 
  

   part 
  of 
  their 
  course, 
  actually 
  in 
  the 
  septum, 
  are 
  also 
  carried 
  

   backwards, 
  the 
  vessels 
  being 
  in 
  consequence 
  much 
  elongated 
  

   and 
  running 
  forwards 
  (PI. 
  7, 
  fig. 
  1). 
  In 
  this 
  way 
  the 
  lateral 
  

   vessels 
  belonging 
  to 
  segment 
  7 
  arise 
  from 
  the 
  dorsal 
  vessel 
  at 
  

   about 
  the 
  plane 
  of 
  the 
  junction 
  of 
  the 
  ninth 
  and 
  tenth 
  para- 
  

  

  