ME.  HANCOCK  ON  THE  OEGANIZATION  OE  THE  BEACHIOPODA. 
823 
transparent  and  homogeneous.  The  interior  is  entirely  devoid  of  carnese  columnse,  and 
is  perfectly  smooth.  When  this  organ  is  in  a contracted  state,  its  size  is  very  much 
reduced,  the  surface  is  occasionally  wrinkled  a little,  and  the  walls,  of  course,  are  much 
thickened. 
This  unilocular  heart  ^ in  W.  australis  receives  a large  blood-channel  or  vessel*  in  front, 
which,  running  forward  along  the  dorsal  ridge  of  the  stomach,  within  the  membrane 
denominated  mesentery,  communicates  on  each  side  by  several  minute  openings  with 
the  gastric  lacunes,  which  are  situated  between  the  walls  of  the  viscus  and  the  mem- 
branous sheath.  The  anterior  extremity  * of  this  channel  passes  down  the  dorsal  surface 
of  the  oesophagus,  and  dividing  into  two  lateral  trunks,  opens  at  each  side  into  a system 
of  large  lacunes  placed  around  the  root  of  the  alimentary  tube.  These  lacunes  will  be 
more  particularly  noticed  afterwards.  The  channel  is  the  afferent  cardiac  channel,  or 
branchio-systemic  vein. 
A little  behind  the  point  where  the  heart  receives  this  channel  two  aortic  vessels  pass 
off  laterally,  which  are  united  at  their  origin  across  the  median  line.  The  two  orifices 
communicating  with  those  vessels  are  guarded  by  sphincture  valves,  resulting  apparently 
from  the  protrusion  inwards  of  the  lining  membrane ; but  from  the  minuteness  of  the 
parts  I was  unable  to  determine  the  exact  structm’e  of  the  valve.  These  two  arterial 
trunks  are  adherent  to  the  walls  of  the  stomach,  and,  diverging  backwards,  each  divides 
at  the  side  of  this  \iscus  into  two  branches ; one  of  which  turns  forward,  and  advancing  to 
the  lower  margin  of  the  gastro-parietal  band  runs  along  it,  and  coursing  round  the  dorsal 
extremity  of  the  posterior  occlusor  muscles  reaches  the  inner  wall  of  the  outer  pallial 
sinus,  near  to  its  commencement ; it  then  bends  forward,  and  entering  the  groove  in  the 
genital  band  glides  along  the  edge  of  the  suspending  membranous  fold,  and  is  continued 
beyond  the  genital  organ  to  the  termination  of  the  minutest  ramifications  of  the  sinus.  A 
little  in  advance  of  the  point  where  this  artery  leaves  the  gastro-parietal  band  it  appears 
to  give  off  a branch,  which,  running  forward,  goes  to  the  margin  of  a membranous  fold 
that  stretches  along  the  inner  wall  of  the  inner  pallial  sinuses ; but  this  was  not  deter- 
mined with  sufficient  accuracy.  This  fold  is  similar  to  those  suspending  the  genitalia. 
The  other  division  of  the  artery  passes  backwards  along  the  lateral  margin  of  the  ilio- 
parietal  band,  and  then  running  across  the  under  smfface  of  the  laminated  portion  of  the 
o\iduct  next  the  median  line,  bifurcates.  One  of  the  branches  passes  inwards,  the  other 
outwards ; the  former,  uniting  with  its  fellow  from  the  other  side,  runs  along  the  free 
border  of  the  mesenteric  membrane,  which  extends  from  the  dorsal  aspect  of  the 
intestine,  and  passes  into  the  base  of  the  peduncle.  This  branch  seems  to  supply  that 
organ,  but  is  much  reduced  in  size  before  it  reaches  its  destination.  The  branch,  which 
passes  outwards,  continues  adherent  to  the  oviduct,  and  coursing  over  it,  at  the  junction 
of  the  two  portions,  reaches  the  anterior  wall  of  the  perivisceral  cavity;  it  then  sinks 
downwards  and  advances  to  the  loop  of  the  genital  band,  where  it  again  divides  into 
’ Plates  LII.  fig.  10 ; LIV.  fig.  1 ; LVI.  fig.  1 ; LVII.  figs.  1, 2. 
= Plate  LVI.  figs.  5,  7.  ^ Plate  LVI.  fig.  6. 
5 p 
MDCCCLVIII. 
