﻿128 
  E. 
  J. 
  Al.LEN. 
  

  

  Shortly 
  after 
  leaving 
  the 
  dorsal 
  vessel, 
  each 
  lateral 
  vessel 
  

   gives 
  off 
  a 
  branch, 
  which 
  breaks 
  up 
  upon 
  the 
  wall 
  of 
  the 
  

   oesophagus 
  and 
  pharynx, 
  uniting 
  with 
  and 
  helping 
  to 
  form 
  a 
  

   rich 
  network 
  of 
  blood-vessels, 
  which 
  extends 
  over 
  the 
  

   surface 
  of 
  these 
  organs 
  (PI. 
  7, 
  fig. 
  1). 
  This 
  network 
  also 
  

   ffives 
  rise 
  to 
  vessels 
  which 
  start 
  from 
  the 
  under 
  surface 
  of 
  

   the 
  oesophagus 
  aud 
  phai-ynx 
  and 
  pass 
  directly 
  downwards 
  to 
  

   the 
  ventral 
  vessel 
  (fig. 
  48, 
  int. 
  v.) 
  Blood 
  can 
  thus 
  pass 
  

   either 
  directly 
  from 
  the 
  dorsal 
  to 
  the 
  ventral 
  vessel 
  through 
  

   the 
  laterals, 
  or 
  indirectly 
  after 
  passing 
  through 
  the 
  network 
  

   on 
  the 
  walls 
  of 
  the 
  alimentary 
  canal. 
  

  

  At 
  its 
  anterior 
  end, 
  immediately 
  behind 
  the 
  brain, 
  the 
  

   dorsal 
  vessel 
  bifurcates 
  (PI. 
  7, 
  fig. 
  1), 
  sending 
  a 
  large 
  vessel 
  to 
  

   each 
  of 
  the 
  palps. 
  These 
  large 
  vessels 
  pass 
  along 
  the 
  axes 
  of 
  

   the 
  palps 
  (PI. 
  10, 
  fig. 
  30), 
  and 
  in 
  the 
  living 
  worm 
  are 
  subject 
  to 
  

   rhythmical 
  pulsations, 
  which 
  keep 
  the 
  blood 
  within 
  them 
  con- 
  

   stantly 
  in 
  motion. 
  The 
  palps 
  appear 
  to 
  be 
  one 
  of 
  the 
  principal 
  

   organs 
  of 
  respiration 
  of 
  the 
  worm 
  (see 
  p. 
  86). 
  

  

  Immediately 
  after 
  entering 
  the 
  palp 
  the 
  large 
  blood-vessel 
  

   gives 
  off 
  a 
  branch, 
  which 
  passes 
  downwards 
  and 
  backwards 
  

   through 
  the 
  first 
  and 
  second 
  segments. 
  It 
  sends 
  one 
  secondary 
  

   branch 
  to 
  the 
  oesophageal 
  network 
  and 
  another 
  through 
  the 
  

   posterior 
  septum 
  of 
  the 
  second 
  segment 
  to 
  form 
  a 
  cluster 
  of 
  

   blind, 
  Hnger-shaped 
  vessels 
  in 
  the 
  third 
  segment, 
  and 
  then, 
  pass- 
  

   ing 
  below 
  the 
  oesophagus, 
  joins 
  with 
  its 
  fellow 
  of 
  the 
  opposite 
  

   side 
  to 
  form 
  the 
  anterior 
  end 
  of 
  the 
  ventral 
  vessel. 
  These 
  

   structures 
  can 
  best 
  be 
  understood 
  from 
  an 
  examination 
  of 
  fig. 
  1. 
  

  

  As 
  only 
  one 
  blood-vessel 
  passes 
  along 
  the 
  axis 
  of 
  each 
  palp, 
  

   it 
  would 
  seem 
  that 
  the 
  pulsations 
  of 
  the 
  vessel 
  itself 
  nnist 
  take 
  

   place 
  in 
  such 
  a 
  manner 
  as 
  alternately 
  to 
  drive 
  blood 
  in 
  and 
  

   then 
  out 
  of 
  the 
  vessel, 
  but 
  owing 
  to 
  the 
  readiness 
  with 
  which 
  

   the 
  palps 
  are 
  thrown 
  off 
  on 
  the 
  slightest 
  irritation, 
  direct 
  

   observations 
  on 
  the 
  point 
  are 
  not 
  easy 
  to 
  make. 
  

  

  The 
  Middle 
  Region.— 
  The 
  modification 
  of 
  the 
  vascular 
  

   system 
  in 
  segments 
  12, 
  13, 
  14 
  and 
  15, 
  the 
  middle 
  region 
  of 
  

   the 
  body, 
  is 
  of 
  special 
  aud 
  peculiar 
  interest. 
  

  

  In 
  each 
  oi 
  these 
  sefjrments 
  the 
  dorsal 
  vessel 
  is 
  itself 
  much 
  

  

  