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MR.  R.  OWEN  ON  THE  ANATOMY  OF  THE  BRACHIOPODA. 
the  fissure  in  order  to  avoid  disturbing  the  soft  parts,  the  vascular  lobe  of  the  mantle 
with  similar  free  margins  is  exposed,  but  the  viscera  are  quite  concealed  by  the  dilated 
disk  or  foot^ 
Each  lobe  of  the  mantle  can  be  reflected  from  before  backwards  to  the  extent  of  five 
lines,  and  from  behind  forwards  to  the  extent  of  half  a  line,  but  they  adhere  too  closely 
to  the  visceral  mass  to  be  detached  without  laceration.  When  so  reflected,  the  branchial 
vessels  may  be  seen  in  rich  profusion  on  their  inner  surface. 
On  the  lobe 2  of  the  mantle  which  lines  the  imperforate  valve,  these  vessels  are 
seen  converging  from  the  respiratory  margin  to  four  trunks,  which  are  much  shorter 
than  the  corresponding  ones  in  Terebratula :  on  the  opposite  mantle-lobe^  the  branchial 
vessels  form  only  two  such  trunks^. 
The  principal  trunks  in  both  mantle-lobes  unite,  and  terminate  in  two  sinuses  or 
hearts^,  situated  close  to  two  strong  tendinous  membranes^,  which  circumscribe  the 
visceral  mass,  and  to  which  the  mantle-lobes  firmly  adhere.  The  arteries  continued 
from  the  hearts  pass  obliquely  through  the  membrane,  and  may  be  plainly  seen  distri- 
buting ramuli  over  the  liver  and  ovary.  In  one  of  the  specimens  I  succeeded  in  in- 
jecting the  vessels  of  one  lobe  of  the  mantle  from  one  of  the  ventricles  in  the  retrograde 
course  of  the  circulation :  the  solution  of  carmine  which  I  used  pervaded  the  numerous 
small  ramuli  given  off"  from  the  larger  branches  of  the  veins,  to  the  extent  shown  in 
the  magnified  view  (Fig.  11.)  of  the  recent  preparation,  which  is  now  in  the  Museum 
of  the  Royal  College  of  Surgeons. 
In  subjecting  this  injected  preparation  to  high  magnifying  power,  there  evidently 
appeared  a  small  uninjected  line'',  as  in  the  Terebratula,  accompanying  each  of  the 
larger  branchial  veins,  running  along  the  centre  of  every  trunk ;  and  these  lines  I 
conclude  to  be  branchial  arteries  :  if  they  were  retractile  muscles  of  the  mantle,  they 
might  be  expected  to  have  a  straighter  course.  At  the  margins  of  the  lobes,  near  the 
roots  of  the  cilia,  lateral  ramulets  are  given  off"  at  right  angles,  which  form  a  chain  or 
circular  vessel  aU  round  the  margin. 
The  cilia,  besides  being  longer  and  more  closely  set  than  in  Terebratula,  are  seen 
under  a  high  magnifier  to  be  themselves  beset  with  smaller  seta,  a  structure  which 
probably  gives  them  greater  power  in  exciting  the  respiratory  currents^. 
In  this  profuse  distribution  of  vessels  over  a  plain  membranous  expansion,  we 
perceive  the  simplest  construction  of  the  water-breathing  organ,  or  branchia ;  and, 
while  it  proves  the  close  affinity  of  the  Brachiopoda  to  the  Ascidice,  it  presents,  at  the 
same  time,  a  beautiful  analogy  with  the  elementary  forms  of  the  air-breathing  organ, 
as  it  exists,  for  example,  in  the  pulmoniferous  Gasteropods. 
The  muscular  system  of  Orbicula  differs  in  some  respects  from  that  of  Terebratula. 
Eight  distinct  muscles  may  be  perceived,  without  including  the  labial  arms.    The  four 
'  Fig-  6.  2  c.  Fig.  5.  s     pig.  g.  *  n.  Figg.  7.  &  8. 
*D.  Fig.  11.  «  2.     Figg,  7.  8.  ^n.  Fig.  13.  «  Fig.  13. 
