﻿ON 
  THE 
  BRANCHIAL 
  VESSELS 
  OV 
  STERNASPIS. 
  7 
  

  

  The 
  contractile 
  dorsal 
  branchial 
  vessel 
  and 
  its 
  contained 
  

   inner 
  axis 
  form 
  a 
  most 
  efficient 
  apparatus 
  for 
  propelling- 
  the 
  

   blood 
  forcibly 
  from 
  one 
  end 
  of 
  the 
  vessel 
  to 
  the 
  other 
  as 
  

   waves 
  of 
  contraction 
  pass 
  down 
  it. 
  When 
  fully 
  contracted 
  

   the 
  lumen 
  is 
  entirely 
  obliterated 
  by 
  the 
  closing 
  of 
  the 
  wall 
  on 
  

   to 
  tlie 
  axial 
  rod 
  (figs. 
  10 
  and 
  13). 
  

  

  Passing 
  down 
  to 
  the 
  base 
  of 
  the 
  gill 
  filament 
  we 
  find 
  that 
  

   the 
  two 
  vessels, 
  the 
  dorsal 
  branchial 
  and 
  the 
  communicatino- 
  

   vessel, 
  pass 
  directly 
  into 
  the 
  filament 
  through 
  the 
  pore 
  in 
  the 
  

   branchial 
  plate, 
  but 
  that 
  the 
  axial 
  rod 
  reaches 
  only 
  to 
  the 
  level 
  

   of 
  the 
  pore, 
  where 
  it 
  disappears, 
  merging 
  into 
  the 
  septum 
  

   which 
  separates 
  the 
  two 
  gill 
  vessels. 
  

  

  Following 
  the 
  vessels 
  upwards 
  and 
  forwards 
  towards 
  the 
  

   intestine, 
  it 
  is 
  seen 
  that 
  at 
  the 
  beginning 
  of 
  what 
  has 
  been 
  

   termed 
  above 
  the 
  second 
  region 
  the 
  axial 
  rod 
  suddenly 
  

   diminishes 
  to 
  a 
  thin 
  thread, 
  which 
  runs 
  along 
  the 
  wall 
  of 
  the 
  

   dorsal 
  branchial 
  vessel 
  and 
  then 
  gradually 
  expands 
  again 
  into 
  

   a 
  second 
  short 
  axial 
  rod 
  similar 
  to 
  that 
  in 
  the 
  posterior 
  thii-d 
  

   region 
  (figs. 
  14 
  and 
  15). 
  This 
  short 
  axial 
  piece 
  again 
  thins 
  

   out 
  to 
  a 
  delicate 
  strand 
  with 
  a 
  nucleus 
  here 
  and 
  there, 
  which 
  is 
  

   continued 
  forwards 
  into 
  the 
  expanded 
  first 
  region 
  of 
  the 
  vessel 
  

   attached 
  to 
  its 
  inner 
  surface 
  (figs. 
  9 
  and 
  14). 
  Near 
  the 
  place 
  

   where 
  the 
  branchial 
  vessel 
  opens 
  by 
  a 
  slightly 
  narrowed 
  neck 
  

   into 
  the 
  large 
  limb 
  of 
  the 
  dorsal 
  vessel 
  the 
  fine 
  axial 
  strand 
  

   swells 
  again 
  into 
  a 
  large 
  plug 
  of 
  vacuolated 
  tissue. 
  The 
  plug 
  

   lies 
  loose 
  in 
  the 
  vessel, 
  kept 
  in 
  place 
  by 
  its 
  posterior 
  attach- 
  

   ment, 
  and 
  acts 
  as 
  a 
  valve 
  (figs. 
  7 
  and 
  14). 
  

  

  At 
  the 
  point 
  where 
  the 
  first 
  joins 
  the 
  second 
  region 
  of 
  the 
  

   branchial 
  vessel 
  the 
  communicating 
  vessel 
  opens 
  into 
  it 
  by 
  

   an 
  aperture 
  protected 
  by 
  a 
  thin 
  flap 
  acting 
  as 
  a 
  valve, 
  so 
  as 
  to 
  

   prevent 
  blood 
  passing 
  back 
  into 
  the 
  communicating 
  vessel 
  

   when 
  the 
  other 
  contracts. 
  

  

  We 
  have 
  seen, 
  then, 
  that 
  two 
  vessels 
  from 
  the 
  dorsal 
  system 
  

   pass 
  to 
  the 
  base 
  of 
  each 
  gill 
  filament. 
  

  

  Now, 
  the 
  fine 
  ventral 
  branchial 
  vessels, 
  veins 
  of 
  Vejdovsky, 
  

   also 
  run 
  to 
  the 
  base 
  of 
  the 
  gill 
  filaments. 
  These 
  delicate 
  

   capillaries 
  pass 
  in 
  near 
  the 
  skin 
  between 
  the 
  dorsal 
  branches. 
  

  

  