834 
ilE.  HANCOCK  ON  THE  OEGANIZATION  OE  THE  BEACHIOPODA. 
base  of  the  brachial  fold.  This  canal,  which  is  of  no  great  size,  is  close  to  the  roots  of 
the  cirri,  which  are  somewhat  expanded,  and  open  into  it  at  the  side ; it  also  commu- 
nicates with  cellular  interspaces  situated  in  the  thickness  of  the  brachial  fold.  Further 
than  this  I have  not  succeeded  in  determining  the  anatomy  of  these  parts,  owing  to  their 
opacity  and  minuteness. 
Having  now  gone  over  all  that  I have  been  able  to  ascertain  with  respect  to  the  cen- 
tral and  peripheral  portions  of  the  circulatory  apparatus,  and  having  also  examined  the 
lacunes  and  blood-canals  of  the  brachial  organs,  it  vrill  not  be  difficult  to  follow  the  flow 
of  the  blood  throughout  its  entire  course  in  Waldheimia ; and  as  it  is  in  it,  so  will  it  be, 
in  all  probability,  in  all  other  Brachiopods. 
It  has  been  shown  that  the  heart'  is  a simple,  unilocular,  p^niform  vesicle,  suspended 
fi'om  the  dorsal  aspect  of  the  stomach,  and  projecting  freely  into  the  peiiHsceral  cham- 
ber ; that  there  is  neither  auricle  nor  pericardium,  unless  the  membrane  which  closely 
invests  it  can  be  so  called ; that  it  is  hardly  more  complex  in  structure  than  the  pulsa- 
tile vessel  of  the  Tunicata ; and  that  in  Lingula^  indeed,  it  scarcely  at  all  difiers  from 
the  heart  of  those  lowly  organized  mollusks.  This  vesicle,  or  heart,  propels  the  blood 
through  four  arterial  trunks  or  channels  to  the  reproductive  organs  and  mantle,  and 
probably  also  to  the  alimentary  tube,  and  is  apparently  assisted  by  four  or  more  pulsa- 
tile vesicles  in  connexion  with  these  principal  trunks.  The  blood  thus  conveyed  by  the 
genital  or  pallial  arteries  will  escape  by  the  lacunes  in  the  membranes  suspending  the 
genitalia,  into  the  plexus  in  the  floor  of  the  great  pallial  sinuses.  Thence  it  will  And 
its  way  into  the  outer  lacunary  system  of  the  pallial  lobes,  and  into  that  of  the  dorsal 
and  ventral  walls  of  the  body,  as  well  as  into  the  lacunes  of  the  anterior  parietes. 
Having  saturated  all  these  parts  of  the  peripheral  system,  it  uill  divide  itself  into  two 
currents,  one  of  which  will  set  backwards  in  the  direction  of  the  membranous  bands 
connecting  the  alimentary  tube  to  the  parietes,  and  will  flow  through  their  channels  into 
the  system  of  visceral  lacunes,  which  encircle  the  alimentary  canal  within  the  sheath, 
and  which  probably  carry  blood  to  the  liver.  This  current  will  also  supply  the  lacunes 
nourishing  the  muscles.  The  blood  thus  directed  will  reach  the  branchio-systemic  vein, 
either  by  the  great  oesophageal  lacunes,  or  through  the  foramina  which  penetrate  the 
sides  of  the  channel  as  it  runs  along  the  dorsal  ridge  of  the  stomach. 
The  other  blood-current  will  set  forward  in  the  direction  of  the  base  of  the  arms,  and 
some  of  it  will  pass  into  these  organs  through  their  general  system  of  lacunes ; but  the 
principal  portion  will  be  carried  by  the  afferent  brachial  canal  to  the  extensive  plexus 
of  lacunes  in  those  parts,  and  will  circulate,  in  the  manner  before  pointed  out,  uithin  the 
walls  of  the  great  brachial  canal.  The  blood  will  then  be  drawn  up  one  side  of  the  cirri 
through  the  vessels,-— the  afferent  brachial  arteries,— -originating  in  the  great  brachial 
plexus,  and  returning  down  the  other,  will  be  poured  into  the  efferent  brachial  canal, 
and  thus  reach  the  lateral  efferent  sinuses  at  the  root  of  the  oesophagus.  Thence  it  will 
> Plate  LXIII.  fig.  1. 
