﻿6 
  EDAVIN 
  S. 
  GOODRICH. 
  

  

  brandling 
  cells, 
  "which 
  do 
  not 
  appear 
  to 
  form 
  an 
  essential 
  

   part 
  of 
  the 
  rod, 
  but 
  rather 
  to 
  be 
  amoeboid 
  blood-ceils 
  creeping 
  

   over 
  it, 
  such 
  as 
  nre 
  found 
  elseAvhere 
  in 
  the 
  blood-vessels 
  

   (figs. 
  12 
  and 
  16). 
  I 
  can 
  find 
  no 
  common 
  peritoneal 
  sheath 
  

   enclosing 
  the 
  dorsal 
  branchial 
  and 
  the 
  communicating 
  vessels. 
  

  

  The 
  dorsal 
  branchial 
  vessel 
  is 
  capable 
  of 
  undergoing 
  great 
  

   expansion 
  and 
  contraction. 
  The 
  ring-cells 
  of 
  which 
  it 
  is 
  

   formed 
  consist 
  of 
  an 
  outer 
  more 
  protoplasmic 
  coat 
  and 
  an 
  

   inner 
  lining 
  of 
  homogeneous 
  refractive 
  substance. 
  When 
  the 
  

   vessel 
  is 
  expanded 
  the 
  inner 
  coat 
  appears 
  quite 
  thin; 
  on 
  the 
  

   contrary, 
  as 
  the 
  lumen 
  contracts 
  the 
  liniug 
  becomes 
  corre- 
  

   spondingly 
  thickened 
  and 
  folded. 
  In 
  transverse 
  section 
  it 
  

   then 
  acquires 
  a 
  striated 
  appearance, 
  and 
  is 
  seen 
  to 
  be 
  inter- 
  

   rupted 
  along 
  the 
  line 
  where 
  the 
  axis 
  is 
  attached 
  (figs. 
  5 
  and 
  

   10, 
  ci). 
  The 
  thick, 
  contracted, 
  inner 
  lining 
  forms 
  the 
  " 
  Hal- 
  

   bringeu 
  " 
  of 
  Vejdovsky, 
  and 
  the 
  "bague 
  cliitiueuse 
  " 
  of 
  

   Rietsch. 
  It 
  is 
  difficult 
  to 
  determine 
  whether 
  during 
  con- 
  

   traction 
  the 
  function 
  of 
  the 
  inner 
  lining 
  is 
  purely 
  passive. 
  

   The 
  real 
  agency 
  by 
  means 
  of 
  which 
  the 
  powerful 
  contraction 
  

   is 
  brought 
  about 
  seems 
  to 
  reside 
  in 
  the 
  superficial 
  network 
  of 
  

   protoplasmic 
  threads 
  in 
  the 
  outer 
  layer 
  (tig. 
  6). 
  This 
  remark- 
  

   able 
  meshwork, 
  which 
  stretches 
  across 
  uninterruptedly 
  in 
  the 
  

   living 
  tissue 
  from 
  cell 
  to 
  cell, 
  can 
  be 
  seen 
  to 
  undergo 
  

   changes, 
  the 
  threads 
  becoming 
  slenderer, 
  and 
  the 
  intervening- 
  

   spaces 
  larger 
  as 
  the 
  vessel 
  expands. 
  

  

  Peculiar 
  as 
  the 
  histological 
  structure 
  of 
  the 
  wall 
  of 
  the 
  

   dorsal 
  branchial 
  vessel 
  appears 
  to 
  be, 
  it 
  may 
  yet 
  be 
  compared 
  to 
  

   that 
  of 
  the 
  small 
  blood-vessels 
  in 
  01igocha3tes 
  so 
  well 
  described 
  

   by 
  Bergh 
  (1). 
  Here 
  also 
  we 
  have 
  small 
  contractile 
  vessels 
  

   formed 
  of 
  rows 
  of 
  ring-like 
  cells, 
  the 
  walls 
  of 
  Avliich 
  consist 
  

   of 
  an 
  inner 
  lining 
  and 
  an 
  outer 
  active 
  protoplasmic 
  net- 
  

   work. 
  But 
  in 
  the 
  case 
  of 
  Steruaspis 
  the 
  structure 
  is 
  much 
  

   more 
  highly 
  specialised.^ 
  

  

  Since 
  tills 
  was 
  written, 
  Lang 
  lias 
  published 
  liis 
  important 
  work, 
  ' 
  Beitrage 
  

   zu 
  einer 
  Tropliocceltheorie 
  ' 
  ('Jen. 
  Zeit.,' 
  1903). 
  The 
  dorsal 
  branchial 
  vessel 
  

   appears 
  to 
  correspond 
  in 
  structure 
  to 
  Lis 
  figs. 
  10 
  and 
  13, 
  pi. 
  2. 
  The 
  axial 
  rod 
  

   probably 
  develops 
  as 
  a 
  longiludinal 
  fold 
  and 
  ingrowth 
  of 
  the 
  walls 
  of 
  the 
  vessel. 
  

  

  