826 
ME.  HAifCOCK  ON  THE  OEG-ANIZATION  OE  THE  BEACHIOPODA. 
ones  have  been  followed  right  through  the  substance  of  the  posterior  adjustor  muscles. 
That  on  the  right  side  enters  the  posterior  margin  of  the  large  single  muscle,  near  to 
the  point  where  it  is  in  contact  with  the  ilio-paiietal  band,  and  thence  it  goes  directly 
to  the  opposite  margin;  it  then  becomes  isolated  and  stretches  foi-ward  between  the 
dorsal  and  ventral  ovaries  of  the  same  side,  and,  passing  round  the  external  surface  of 
the  anterior  occlusors,  turns  inward,  and  a short  way  before  it  reaches  the  median  line 
penetrates  the  lining  membrane  of  the  anterior  wall,  immediately  below  the  ridge  attached 
to  the  occlusor  muscles*.  It  here  apparently  communicates  with  the  lacunes  situated 
between  the  two  membranes  of  the  parietes.  The  external  trunk  of  the  other  side  inns 
a similar  course  after  passing  through  the  two  small  posterior  adjustors.  These  two 
trunks  probably  represent  the  ventral  pallial  or  genital  arteries  of  the  articulated  species. 
The  internal  are  most  likely  connected  with  the  external  trunks ; they  pass  from  the 
anterior  margins  of  the  posterior  adjustors,  go  directly  through  the  central  adjustors,  and 
are  lost  amidst  the  fibres  of  the  external  adjustors,  where  they  seem  to  branch. 
These  four  lateral  trunks  have  been  described  by  Professor  Owex^  as  part  of  the 
nervous  system,  and  indeed  they  bear  a strong  resemblance  to  nerves.  But  I can  find 
no  ganglia  from  which  they  originate,  and  the  external  ones  undoubtedly  pass  into  the 
anterior  wall  of  the  body  as  described : at  this  point,  certainly,  there  are  no  ganglia  nor 
anything  like  a nervous  collar.  The  trunks  themselves  are  composed  of  an  inner  and 
an  outer  tube;  the  latter  is  a continuation  of  the  lining  membrane  of  the  peririsceral 
chamber ; the  inner  is  formed  of  a very  delicate  membrane,  which,  by  the  contraction  of 
the  outer  tube,  is  thrown  into  longitudinal  folds  or  wrinkles,  haring  somewhat  the 
appearance  of  nerve-tubes.  These  trunks,  in  fact,  exactly  resemble,  in  these  respects, 
the  free  or  isolated  portion  of  the  branchio-systemic  vein®,  which  has  an  outer  envelope 
and  a delicate  internal  tunic  longitudinally  wrinkled.  So  like  are  the  two,  that  either 
might  be  figured  or  described  for  the  other. 
Haring  said  so  much  with  regard  to  the  central  organs,  it  will  now  be  necessary'  to 
examine  the  peripheral  portion  of  the  blood-system.  To  do  this  we  shall  have  to  look 
to  the  parietes  of  the  perivisceral  chamber,  and  particularly  to  the  lobes  of  the  mantle, 
to  the  various  membranous  bands  in  connexion  with  the  alimentary  tube,  and  likewise 
to  the  brachial  organs.  In  all  these  parts  will  be  found  the  extensive  system  of  lacunes. 
or  blood-channels  originally  described  by  Professor  Huxley^. 
Fu’st,  vrith  regard  to  the  walls  of  the  perivisceral  chamber:  these®  are  composed  of 
two  membranous  layers,  an  outer  and  an  inner,  between  which  the  lacunes  are  situated. 
The  outer  layer®,  of  the  dorsal  and  ventral  walls,  is  made  up  of  two  very  delicate  mem- 
branes, the  internal  of  which  is  pellucid  and  homogeneous ; the  external  one  is  reticulated 
in  a peculiar  manner,  as  if  it  were  formed  of  epithelial  scales.  The  reticulations  appear 
to  correspond  to  the  bases  of  the  prismatic  columns  composing  the  shell.  It  is  in  this 
external  membrane  that  the  csecal  prolongations  of  the  mantle  origmate,  and  the  bases 
’ Plate  LXVI.  fig.  4.  ^ Introduction  to  Dattdsok’s  ‘ British  Eossil  Brachiopoda,’  p.  12. 
^ Plate  LXY.  fig.  5.  ■*  Op.  cit.  p.  115.  ® Plate  LIX.  fig.  3.  ® Plate  LIX.  fig.  4. 
