ME.  HANCOCK  ON  THE  OEGANIZATION  OE  THE  BEACHIOPODA. 
827 
of  these  enigmatical  organs  are  distinctly  visible  rising  up  from  the  surface  as  short, 
delicate  cylinders.  And  it  may  here  be  observed,  that,  contrary  to  expectation,  they  do 
not  appear  to  penetrate  even  this  external  membrane,  and  consequently  can  have  no 
communication  with  the  lacunes  or  blood-channels  lying  between  the  two  layers  of  the 
wall,  or  within  those  of  the  pallial  lobes,  as  stated  by  Dr.  Caepentee\  This  interesting 
point  will  be  more  particularly  adverted  to  in  the  sequel.  The  inner  layer  of  the  wall 
is  thin,  transparent,  and  minutely  granular,  and  is  lined  with  a granular  epithelium. 
Through  these  transparent  layers  the  blood-channels,  which  lie  between  them,  are  seen, 
in  well-preserved  specimens,  in  all  their  details. 
The  anterior  wall  of  the  chamber  has  likewise  its  blood-channels  between  two  layers, 
both  of  which  are  clothed  with  a granular  epithelium.  The  lacunes  of  this  portion  of 
the  tegumentary  envelope  form  an  open  network,  with  the  meshes  elongated  in  the 
direction  of  the  arms. 
The  pallial  lobes  ^ are  each  composed  of  two  laminae,  an  external  and  an  internal  one. 
The  external  lamina  of  both  the  dorsal  and  ventral  lobes  is  a continuation  of  only  the 
external  layer  of  the  upper  and  under  walls  of  the  body;  the  internal  lamina  is  an 
expansion,  in  like  manner,  of  the  anterior  wall.  But  those  portions  of  the  laminae  that 
form  the  walls  of  the  great  pallial  sinuses  are  composed  of  both  layers  of  the  parietes. 
Hence  the  pallial  lobe  consists,  above  and  below  the  sinuses,  as  well  as  in  the  spaces 
between  them,  of  two  layers,  and  the  lacunes  are  situated  everywhere  between  the  layers, 
and  intercommunicate.  In  other  words,  the  pallial  lobe  may  be  described  to  be  formed 
of  a fold  of  the  external  layer  of  the  parietes,  the  internal  layer  lining  the  great  pallial 
sinuses  throughout. 
The  external  lamina  of  the  mantle  is  formed  of  two  membranes,  exactly  resembling 
those  of  the  parietes  of  the  body,  the  outer  being  reticulated,  the  inner  transparent  and 
homogeneous.  The  membrane  of  the  internal  lamina  is  of  the  same  character  as  the 
latter ; but  it  exhibits  a few  large,  scattered,  oval,  granular  cells.  The  pallial  nerves  are 
in  connexion  with  this  membrane,  which  is  provided,  on  its  external  surface,  with  a 
stout,  gi’anular  epithelium®.  The  layer  lining  the  great  pallial  sinuses  presents  two 
different  characters ; that  portion  which  lies  next  the  shell  is  clear  and  homogeneous,  that 
w'hich  rests  on  the  lower  wall  or  floor  is  slightly  granular,  and  almost  entirely  composed 
of  transverse  muscular  fibres.  This  layer  is  lined  throughout  with  a granular  epi- 
thelium. 
The  blood-channels  or  lacunes  lying  immediately  below  the  external  layer,  and  which 
may  be  called  the  outer  pallial  lacunes,  are  continuous  with  those  in  the  parietes  of  the 
body ; and  throughout  this  extensive  superficial  system  they  have  the  same  peculiar 
character.  The  channels  are  exceedingly  wide,  and  in  the  spaces  between  the  great 
pallial  sinuses  they  are  of  considerable  depth ; in  fact  the  whole  of  this  system  may  be 
considered  as  one  vast  lacune,  the  walls  of  which  coming  together  are  united  at  certain 
* Proceedings  of  the  Eoyal  Society,  vol.  vii.  p.  34  (1854).  ^ Plates  LIX.  fig.  1 ; LVIII.  figs.  5,  7. 
* Plate  LXI.  fig.  5. 
