830 
ME.  HANCOCK  ON  THE  OEGANIZATION  OE  THE  BEACHIOPODA. 
matter  over  the  trunks  of  the  great  pallial  sinuses.  Elsewhere  they  are  sparingly  dis- 
tributed, except  towards  the  margin  of  the  lobes,  where  they  are  rather  numerous. 
In  Megerlia  truncata^  the  spicula  have  much  the  same  character,  hut  the  branches 
are  so  much  flattened  or  spread  out,  that  they  become  extensively  fused,  so  as  to  form 
plates  of  irregular  forms ; and  as  the  margins  of  these  plates  are  in  contact,  the  whole 
compose  a tolerably  compact  shield  with  sharply  defined  borders,  corresponding  very 
closely  in  form  to  the  pallial  sinuses,  to  which,  no  doubt,  they  give  protection,  resisting 
the  pressure  of  the  external  fluid. 
In  both  species  the  spicula  are  continued  from  the  mantle  into  the  arms,  where  the 
branches  become  interwoven  so  as  to  compose  a sort  of  sponge-like  tissue  of  calcareous 
matter,  which  firmly  supports  these  organs.  In  the  cirri  the  spicula  retain  theii- 
branched  character,  and  curving  round  them  enclose  their  lower  portions  in  an  open  net- 
work. Were  these  species  fossilized,  it  is  quite  possible  that  their  arms  might  be  so 
preserved  as  to  give  the  appearance  of  their  having  been  sustained  by  some  apophysan' 
apparatus;  and  in  that  case  the  roots  of  the  cirri  would  be  found  projecting  much  in 
the  same  way  as  the  salient  spines  or  processes  on  the  calcareous  spii’als  of  the  fossil 
Spiriferina  rostmta ; and  perhaps  these  spines  were  formed  by  similar  imbedded  spicula. 
No  spicula  are  found  in  Waldheimia,  Mliynchonella^  or  lAngula. 
From  the  description  already  given  of  the  peripheral  lacunes,  it  would  seem  that  the 
walls  of  the  body,  and  the  laminm  of  the  pallial  lobes,  present  one  great  system  of  blood- 
channels  or  lacunes,  the  various  parts  of  which  freely  communicate  vrith  each  other. 
There  still  remains,  however,  to  be  examined  the  peripheral  system  in  the  membranous 
bands  of  the  perivisceral  chamber,  and  in  the  brachial  apparatus ; but  in  the  fii'st  place 
it  will  be  well  to  refer  to  the  sheath  of  the  alimentary  tube.  As  before  stated,  this 
tube  is  encased  from  end  to  end  in  a close-fitting  membranous  sheath ; between  which 
and  the  proper  walls  of  the  tube,  the  blood,  which  nourishes  this  discus,  undoubtedly 
flows  in  lacunes ; but  from  the  opacity  of  the  parts  the  exact  character  of  the  channels 
could  not  be  determined.  Blood-corpuscles,  however,  were  found  strewed  about  between 
the  sheath  and  the  walls  of  the  organ,  apparently  occupying  a network  of  channels. 
Towards  the  root  of  the  oesophagus,  where  the  sheath  is  not  so  much  constricted,  large 
blood-channels  or  lacunes^  are  situated  between  it  and  the  walls  of  the  tube.  These 
may  be  denominated  the  great  oesophageal  lacunes ; while  those  of  the  other  portion  of 
the  alimentary  tube  may  be  named  the  visceral  lacunes,  as  they  seem  to  form  part  of 
a system  which  extends  to  the  liver  and  heart. 
Now,  all  the  bands  which  pass  from  the  alimentary  tube  to  the  parietes  are,  in  fact, 
duplicatures  of  this  sheath,  and  accordingly  they  are  found  to  be  composed  of  two  layers, 
each  with  its  proper  epithelial  covering.  The  layers  are  exceedingly  delicate,  transpa- 
rent, and  homogeneous ; and  between  them  are  situated  numerous  blood-lacunes,  which 
are  narrow  anastomosing  channels.  Those  in  the  gastro-parietal  bands  run  for  the  most 
part,  in  the  direction  of  their  length,  and  communicate  at  one  extremity  mth  the  ’sisceral 
* Plates  LII.  fig,  9 ; LIII.  figs.  6,  7.  ^ Plate  LV.  fig.  4.  ^ Plate  LTI.  fig.  8. 
