﻿TDK 
  ANATOMY 
  OF 
  PCECILOCH.^ITUS, 
  CLAI'AKEDE. 
  131 
  

  

  have 
  no 
  gills, 
  but 
  the 
  loo}3 
  just 
  mentioned 
  would 
  seem 
  to 
  

   represent 
  in 
  a 
  rudimentary 
  way 
  the 
  loops 
  of 
  the 
  hiteral 
  vessels 
  

   which 
  supply 
  the 
  gill 
  filaments 
  in 
  the 
  posterior 
  gill-bearing 
  

   segments. 
  

  

  The 
  Structure 
  of 
  the 
  Walls 
  of 
  the 
  Blood-vessels. 
  — 
  

   The 
  different 
  layers 
  of 
  the 
  walls 
  of 
  the 
  blood-vessels 
  attain 
  

   their 
  greatest 
  development 
  in 
  the 
  dorsal 
  vessel. 
  This 
  vessel 
  

   is 
  lined 
  internally 
  by 
  an 
  epithelial 
  layer 
  consisting 
  of 
  flattened 
  

   cells, 
  the 
  general 
  height 
  of 
  which 
  is 
  less 
  than 
  the 
  diameter 
  of 
  

   their 
  nuclei, 
  so 
  that 
  the 
  portion 
  of 
  the 
  cell 
  in 
  the 
  immediate 
  

   neighbourhood 
  of 
  a 
  nucleus 
  often 
  appears 
  to 
  protrude 
  into 
  the 
  

   blood-space. 
  The 
  bodies 
  of 
  the 
  cells 
  generally 
  remain 
  clear 
  

   and 
  unstained. 
  

  

  Proceeding 
  outwards 
  from 
  this 
  epithelial 
  layer, 
  one 
  finds 
  

   a 
  layer 
  of 
  longitudinal 
  muscle-fibres, 
  which 
  is 
  followed 
  by 
  

   several 
  layers 
  of 
  annular 
  muscles, 
  this 
  part 
  of 
  the 
  wall 
  being 
  

   especially 
  developed 
  in 
  the 
  dorsal 
  vessel. 
  The 
  whole 
  vessel 
  is 
  

   covered 
  externally 
  by 
  a 
  layer 
  of 
  coelomic 
  epithelial 
  cells, 
  

   which 
  form 
  the 
  lining 
  of 
  the 
  body-cavity. 
  Like 
  that 
  of 
  the 
  

   cells 
  lining 
  the 
  vessel 
  internally, 
  the 
  protoplasm 
  of 
  these 
  

   external 
  cells 
  remains 
  clear 
  and 
  unstained 
  (PI. 
  11, 
  fig. 
  50). 
  

  

  The 
  differences 
  met 
  with 
  in 
  the 
  structure 
  of 
  the 
  walls 
  of 
  

   the 
  other 
  blood-vessels 
  of 
  the 
  body 
  are 
  due 
  to 
  the 
  reduction 
  

   of 
  the 
  various 
  layers, 
  more 
  especially 
  of 
  the 
  muscular 
  layers. 
  

   In 
  the 
  ventral 
  vessel, 
  as 
  well 
  as 
  in 
  the 
  lateral 
  pouches 
  of 
  the 
  

   dorsal 
  vessel, 
  the 
  epithelial 
  layers 
  are 
  well 
  developed, 
  but 
  the 
  

   muscular 
  layers, 
  though 
  still 
  obvious, 
  are 
  greatly 
  reduced. 
  In 
  

   the 
  lateral 
  vessels 
  and 
  their 
  various 
  branches, 
  especially 
  when 
  

   extended 
  with 
  blood, 
  only 
  a 
  thin 
  membrane 
  in 
  which 
  an 
  

   occasional 
  nucleus 
  is 
  seen 
  can 
  generally 
  be 
  recognised. 
  It 
  is 
  

   probable, 
  however, 
  that 
  both 
  epithelial 
  layers 
  are 
  present, 
  

   whilst 
  the 
  muscular 
  layer 
  has 
  almost, 
  if 
  not 
  entirely, 
  dis- 
  

   appeared. 
  

  

  In 
  the 
  external 
  wall 
  of 
  the 
  intestinal 
  blood-sinus 
  the 
  two 
  

   epithelial 
  layers 
  can 
  be 
  made 
  out, 
  with 
  a 
  layer 
  of 
  muscle- 
  

   fibres 
  between 
  them. 
  Internally 
  the 
  bases 
  of 
  the 
  intestinal 
  

   epithelial 
  cells 
  appear 
  to 
  be 
  separated 
  from 
  the 
  blood-space 
  by 
  

  

  