838 
m.  ims’COCK  ox  the  oegaxizatiox  of  the  beachiopoda. 
connected  with  the  blood-system.  This  is  no  doubt  a startling  fact.  I commenced  the 
present  investigation  fully  imbued  with  the  opinion  that  these  parts  were  blood-reser- 
A'oirs  and  channels,  and  1 only  relinquished  it  when  it  became  no  longer  tenable.  Step 
by  step  the  points  relied  on  had  to  be  abandoned,  until  at  length  the  full  conviction  was 
arrived  at,  that  I had  been  seeking  to  establish  a fallacy.  I have  been  unable  to  dis- 
cover any  communication  between  the  true  blood-system  and  the  pseudo-vascular  rami- 
fications in  the  mantle,  or  the  permsceral  chamber.  Injections  were  thrown  into  this 
chamber,  but  none  of  the  fluid  found  its  way  into  any  part  of  the  lacunaiy  system.  The 
pallial  lobes  were  removed,  and  the  great  pallial  smuses  distended  to  their  fullest  capa- 
city, with  exactly  the  same  result;  and  it  was  not  until  great  pressure  was  applied, 
and  the  tissues  ruptured,  that  a little  of  the  injected  matter  was  extravasated  into  the 
peripheral  lacunes.  The  perivisceral  chamber,  then,  and  all  its  various  ramifications, 
are  in  no  way  connected  with  the  true  blood-system.  They  must  have  some  other 
peculiar  and  important  office  to  serve  in  the  economy  of  the  animal.  'SMiat  is  this 
office  1 This  shall  be  discussed  by  and  by,  and  in  the  meantime  it  is  necessary  to  revert 
to  the  structure  of  the  peripheral  lacunes  themselves. 
These,  which  answer  to  the  veins  and  capillaries  of  the  higher  animals,  are  of  a very 
peculiar  character.  They  are  for  the  most  part,  as  we  have  seen,  placed  between  two 
membranous  layers,  or  between  one  such  layer  and  the  wulls  of  the  riscera.  In  the 
former  case  they  can  be  readily  examined,  and  are  found,  how  various  soever  their  forms 
may  be,  to  be  constructed  in  the  same  manner.  With  regard  to  those  of  the  outer 
system  of  the  mantle,  it  has  been  shown  that  they  are  nothing  more  than  intercommu- 
nicating spaces  left  between  two  membranes,  which  are  only  partially  united.  In  this 
wuy  they  are  all  formed,  but  in  some  the  channels  take  a linear  disposition,  and  compose 
more  or  less  open  networks  of  various  degrees  of  minuteness,  always  sharply  defined. 
In  well-preserved  specimens  the  lacunes  are  perfectly  distinct,  and  are  generally  well 
charged  with  blood-corpuscles,  which  are  liable  to  adhere  to  each  other,  forming  oval 
pellets*.  These  channels,  moreover,  do  not  appear  to  be  entirely  free,  but  to  be  encum- 
bered with  cellular  tissue ; so  that,  when  one  of  the  membranes  is  removed,  the  coagu- 
lated blood  remains  adherent  to  the  other,  retaining  the  form  of  the  chamiels  in  all 
their  distinctness^. 
It  has  been  pointed  out  that  the  alimentary  canal  is  encased  within  a sheath  forming 
the  system  of  visceral  lacunes,  and  that  it  apparently  coats  the  liver,  following  all  its 
minute  subdivisions.  Now,  it  is  the  continuation  of  this  sheath,  reflected  upon  the 
w'alls  of  the  perivisceral  chamber,  that  forms  the  lining  membrane  or  inner  layer 
between  which  and  the  true  walls  of  the  body  the  parietal  lacunes  are  situated.  And 
moreover  it  is  pretty  evident  that  this  membrane  also  sheaths  the  muscles,  and  is  con- 
tinued throughout  the  great  pallial  sinuses,  enclosing  the  genitalia  within  a fold.  The 
various  bands,  too,  passing  from  the  sheath  of  the  alimentary  tube  to  the  parietes  are 
* Plate  LX.  fig.  11.  2 Plate  LIX.  fig.  3. 
