﻿288 
  W. 
  BLAXLAND 
  BENHAM. 
  

  

  this 
  region 
  over 
  a 
  great 
  part 
  of 
  its 
  extent 
  exhibits 
  the 
  

   following 
  features 
  (see 
  fig. 
  22, 
  which 
  is 
  the 
  sixth 
  section 
  

   below 
  that 
  drawn 
  in 
  fig. 
  21) 
  : 
  

  

  The 
  wall 
  of 
  the 
  sac 
  consists 
  of 
  a 
  thick 
  coat 
  of 
  circular 
  

   muscles, 
  covered 
  externally 
  by 
  a 
  flattened 
  caslomic 
  epithe- 
  

   lium, 
  which 
  forms 
  a 
  distinctly 
  recognisable 
  membrane 
  with 
  

   flattened 
  nuclei. 
  

  

  A 
  considerable 
  space 
  is 
  enclosed 
  by 
  this 
  sac, 
  in 
  which 
  lie 
  

   the 
  sections 
  of 
  the 
  atrium 
  — 
  one, 
  two, 
  or 
  three, 
  according 
  

   to 
  the 
  region 
  involved. 
  Passing 
  across 
  this 
  space 
  are 
  

   numerous 
  muscle-fibres 
  (r. 
  m.), 
  the 
  direction 
  of 
  which 
  is 
  for 
  

   the 
  most 
  part 
  radial. 
  These 
  fibres 
  appear 
  to 
  be 
  developed 
  

   as 
  processes, 
  or 
  at 
  any 
  rate 
  as 
  fibrous 
  refringent 
  modifica- 
  

   tions 
  of 
  the 
  cytoplasm 
  of 
  certain 
  cells 
  (m. 
  c.) 
  which 
  are 
  

   attached 
  to 
  the 
  inner 
  surface 
  of 
  the 
  wall 
  of 
  the 
  sac. 
  The 
  

   nucleus 
  of 
  such 
  a 
  cell 
  is 
  oval, 
  and 
  takes 
  the 
  stain 
  well 
  ; 
  the 
  

   body 
  is 
  only 
  very 
  faintly 
  stained. 
  

  

  Some 
  of 
  these 
  muscle-cells 
  are 
  seen 
  in 
  figs. 
  22, 
  23, 
  especially 
  

   well 
  at 
  m. 
  c. 
  The 
  general 
  form 
  of 
  the 
  cell 
  is 
  usually 
  spindle 
  

   shaped, 
  one 
  extremity 
  of 
  which 
  lies 
  against 
  the 
  wall 
  of 
  the 
  sac 
  

   while 
  the 
  other 
  is 
  " 
  frayed 
  out 
  ^' 
  into 
  fibres, 
  which 
  pass 
  

   across 
  the 
  space 
  to 
  be 
  inserted 
  into 
  the 
  wall 
  of 
  the 
  contained 
  

   atrium; 
  others 
  pass 
  from 
  one 
  part 
  of 
  the 
  wall 
  to 
  another; 
  

   others, 
  again, 
  appear 
  to 
  pass 
  from 
  one 
  coil 
  (or 
  section) 
  of 
  

   the 
  atrium 
  to 
  another. 
  

  

  In 
  some 
  sections, 
  less 
  favourable 
  than 
  this, 
  where 
  portions 
  

  

  Tlie 
  term 
  " 
  atrium 
  " 
  is 
  usually, 
  and 
  it 
  seems 
  to 
  me 
  most 
  conveniently, 
  em- 
  

   ployed 
  to 
  indicate 
  the 
  tube 
  resulting 
  from 
  liie 
  union 
  of 
  tlie 
  spermiducal 
  gland 
  

   (" 
  prostate") 
  and 
  the 
  sperm-duct, 
  as 
  I 
  attempted 
  to 
  point 
  out 
  in 
  1890. 
  This 
  

   is 
  the 
  sense 
  in 
  which 
  Beddard 
  himself 
  uses 
  the 
  term 
  in 
  his 
  Monograph 
  in 
  

   reference 
  to 
  Tubifex 
  and 
  others. 
  The 
  terminology 
  of 
  this 
  region, 
  in 
  spite 
  of 
  

   Beddard's 
  own 
  articles 
  on 
  it, 
  still 
  requires 
  a 
  revision. 
  Neither 
  Beddard 
  nor 
  

   Michaelsen, 
  in 
  their 
  monographs, 
  appear 
  to 
  be 
  quite 
  consistent 
  in 
  the 
  terms 
  

   employed. 
  The 
  fact 
  that 
  the 
  upper 
  part 
  of 
  the 
  atrium 
  in 
  Phreodrilus 
  

   beddardi 
  and 
  P. 
  subterraneus 
  is 
  ciliated 
  is 
  quite 
  in 
  agreement 
  with 
  the 
  

   condition 
  in 
  Tubifex, 
  of 
  which 
  Beddard 
  writes 
  in 
  his 
  monograph, 
  p. 
  105, 
  

   "the 
  elongated 
  atrium 
  has 
  its 
  proximal 
  part 
  ciliated; 
  its 
  distal 
  part 
  not 
  

   ciliated 
  ; 
  the 
  latter 
  forms 
  a 
  protrusible 
  penis." 
  

  

  