im.  HANCOCK  ON  THE  OEGANIZATION  OP  THE  BEACHIOPODA. 
835 
enter  the  great  oesophageal  lacunes,  and  there  meeting  with  the  other  cuiTent  of  return- 
ing blood  from  the  \isceral  lacunes,  will  be  carried  to  the  heart  by  the  branchio-systemic 
vein  along  the  dorsal  side  of  the  stomach. 
Thus  it  is  perceived  that  the  blood  finds  its  way  back  to  the  central  organ  in  a mixed 
condition.  That  which  is  conveyed  by  the  gastro-parietal  and  other  bands  will  be 
imperfectly  aerated,  having  only  flowed  through  the  pallial  membranes,  which  must  be 
looked  upon  as  but  accessory  oxygenating  agents.  The  arms  undoubtedly  perform  the 
office  of  gills,  and  are  true  respiratory  organs.  The  blood  which  circulates  througli 
them  will  consequently  be  returned  in  a perfectly  aerated  condition,  to  be  mixed,  how- 
ever, vdth  that  in  a less  pure  state  from  the  visceral  lacunes  before  it  enters  the  heart. 
This  mixed  state  of  the  blood  is  not  by  any  means  peculiar  to  these  animals,  for  it 
obtains  in  many  of  even  the  higher  mollusks. 
To  prove  that  the  brachial  organs  subseiwe  the  function  of  gills,  as  well  as  that  of 
sustentation,  it  is  only  necessary  to  refer  to  the  manner  in  which  the  blood  circles  round 
the  arms  and  is  carried  to  the  cirri ; but  more  particularly  to  its  circulating  through 
these  latter  organs,  and  to  its  return  direct  from  them  to  the  heart.  There  can  be  no 
doubt  that  the  cirri  are  pro\ided  with  vibratile  cilia,  and  indeed  this  fact  seems  to  be 
established  by  the  observations  of  Mr.  Baeeett  * ; and  a considerable  portion  of  their 
extremities  is  sufficiently  delicate  to  admit  of  the  blood  being  aerated  through  their 
walls.  The  inner  lamina  of  the  mantle,  and  more  particularly  that  portion  of  it  form- 
ing the  floor  of  the  great  pallial  sinuses,  will  undoubtedly  assist  in  purifying  the  blood ; 
but  like  the  mantle  of  many  of  the  Nudibranchs,  it  can  only  play  a secondary  part. 
This  may  fairly  be  infen’ed  from  the  distribution  of  the  blood-channels  within  the 
lamina.  In  T.  capiit-serpentis^,  howm’er,  this  portion  of  the  mantle  is  so  filled  with 
calcareous  spicula,  that  in  this  instance  it  must,  even  in  this  secondary  capacity,  have 
its  efficiency  much  impaired ; and  in  Megei'lia  truncata^  the  floor  or  inner  wall  of  the 
trunks  of  the  great  sinuses  is  completely  filled  with  spicula  to  such  an  extent,  that  the 
membrane  seems  entirely  displaced. 
It  is  generally  asserted  that  the  mantle  in  Lingula  has  assumed  the  character  of  a 
rudimentary  gill ; and  in  this  genus,  indeed,  it  perhaps  attains  its  highest  development 
as  an  accessory  breathing  organ.  I find,  however,  nothing  agreeing  to  the  so-called 
vascular  loops  of  Baron  Cuviee^  and  Professor  Owen®.  The  mantle®  does  not  appear  to 
differ  at  all  from  that  of  the  articulated  Brachiopods,  except  that  the  inner  walls  of  the 
pallial  sinuses  are  bulged  out,  forming,  as  it  were,  transverse  or  radiating  plaits : there 
are,  however,  no  branchial  loops ; but  the  surface  over  which  the  inner  lacunes  are 
distributed  being  thus  increased,  the  lobes  of  the  mantle  in  Lingula  may,  to  this  extent, 
be  considered  specialized  breathing  organs.  Nevertheless  there  is  no  reason  for  sup- 
posing any  modification  in  the  internal  arrangement  of  the  blood-channels,  that  there 
' Op.  cit.  ^ Plate  LII.  fig.  G.  ® Plate  LII.  fig.  9.  * Op.  cit.  p.  5. 
‘ Trans.  Zool.  Soc.  vol.  i.  p.  157.  pi.  2.3.  fig.  16.  ® Plates  LXIV.  fig.  3 ; LXVI.  fig.  5, 
