﻿THE 
  ANATOMY 
  OF 
  I'CECIL0CHJ5TUS, 
  CLAPAREDE. 
  129 
  

  

  enlarged, 
  forming' 
  on 
  either 
  side 
  large 
  lateral 
  pouches, 
  wliich 
  

   are 
  alternately 
  inflated 
  with 
  blood 
  and 
  emptied 
  (PL 
  7, 
  fig. 
  1; 
  

   PI. 
  11, 
  figs. 
  43 
  and 
  4^7, 
  p. 
  (If.). 
  When 
  fully 
  inflated 
  the 
  pouches 
  

   occupy 
  almost 
  the 
  whole 
  of 
  the 
  body-cavity, 
  and 
  the 
  wave 
  of 
  

   expansion 
  and 
  contraction, 
  passing 
  from 
  segment 
  to 
  segment 
  

   from 
  behind 
  forward, 
  is 
  a 
  striking 
  phenomenon. 
  

  

  The 
  forward 
  movement 
  of 
  the 
  blood 
  from 
  one 
  segment 
  to 
  

   the 
  next 
  in 
  front 
  is 
  regulated 
  by 
  a 
  series 
  of 
  valves 
  situated 
  in 
  

   the 
  dorsal 
  vessel 
  between 
  each 
  successive 
  pair 
  of 
  pouches, 
  as 
  

   well 
  as 
  immediately 
  anterior 
  to 
  the 
  first 
  pair 
  and 
  posterior 
  to 
  

   the 
  last. 
  There 
  are 
  thus 
  five 
  valves 
  altogether. 
  These 
  valves, 
  

   two 
  of 
  which 
  are 
  shown 
  in 
  sagittal 
  section 
  in 
  fig. 
  50 
  (PI. 
  11), 
  

   consist 
  of 
  somewhat 
  stout 
  membranes 
  composed 
  of 
  spindle- 
  

   shaped 
  cells, 
  attached 
  ventrally 
  to 
  the 
  wall 
  of 
  the 
  blood-vessel, 
  

   but 
  with 
  a 
  free 
  dorsal 
  edge, 
  wliich 
  pressed 
  from 
  in 
  front 
  

   comes 
  into 
  contact 
  with 
  the 
  wall 
  of 
  the 
  vessel 
  and 
  prevents 
  

   the 
  blood 
  from 
  passing 
  backwards. 
  In 
  fig. 
  50 
  the 
  anterior 
  

   valve 
  {vl. 
  seg. 
  14) 
  is 
  open, 
  whilst 
  the 
  posterior 
  valve 
  {vl. 
  seg. 
  15) 
  

   is 
  closed. 
  It 
  is 
  clear 
  that 
  contraction 
  of 
  the 
  walls 
  of 
  the 
  

   blood-vessel 
  and 
  its 
  lateral 
  pouches 
  will 
  force 
  the 
  blood 
  

   forwards, 
  whilst 
  the 
  valves 
  will 
  prevent 
  any 
  blood 
  from 
  going 
  

   in 
  the 
  opposite 
  direction. 
  

  

  It 
  must 
  be 
  pointed 
  out 
  that 
  the 
  lateral 
  pouches 
  of 
  the 
  

   dorsal 
  vessel 
  in 
  segments 
  12, 
  13, 
  14 
  and 
  15 
  are 
  not 
  swollen 
  

   lateral 
  vessels, 
  such 
  as 
  are 
  described 
  by 
  Benham 
  (1896) 
  in 
  

   Magelona, 
  for 
  in 
  sections 
  the 
  true 
  lateral 
  vessels, 
  similar 
  in 
  

   their 
  general 
  relations 
  to 
  those 
  of 
  the 
  anterior 
  segments, 
  and 
  

   like 
  them 
  giving 
  rise 
  to 
  a 
  cluster 
  of 
  finger-shaped 
  processes 
  

   in 
  the 
  segment 
  behind, 
  are 
  easily 
  seen 
  and 
  followed. 
  These 
  

   lateral 
  vessels 
  arise 
  from 
  the 
  dorsal 
  vessel 
  in 
  each 
  case 
  behind 
  

   the 
  lateral 
  pouches, 
  at 
  the 
  point 
  where 
  the 
  dorsal 
  vessel 
  

   passes 
  through 
  the 
  posterior 
  septum 
  of 
  the 
  segment, 
  and 
  they 
  

   run 
  throughout 
  the 
  greater 
  part 
  of 
  their 
  course 
  in 
  this 
  septum. 
  

  

  The 
  Posterior 
  Region. 
  — 
  In 
  the 
  posterior 
  region 
  of 
  the 
  

   body, 
  from 
  segment 
  10 
  backwards, 
  the 
  arrangement 
  of 
  the 
  

   vascular 
  system 
  undergoes 
  a 
  great 
  change. 
  The 
  dorsal 
  vessel 
  

   can 
  no 
  longer 
  be 
  distinguished 
  as 
  a 
  separate 
  organ, 
  but 
  the 
  

  

  