MR.  R.  OWEN  ON  THE  ANATOMY  OF  THE  BRACHIOPODA. 
155 
thick  and  strong  muscles  which  form  the  anterior  and  posterior  pairs  above  noticed, 
do  not  decussate  each  other,  but  pass  a  little  obliquely  from  one  valve  to  the  other. 
On  the  lower  valve  they  are  attached  to  the  margin  of  the  elevation  caused  by  the 
oval  depression  noticed  on  the  exterior  of  the  shell.  Some  of  the  fibres  of  the  large 
anterior  pair  pass  through  the  chink  in  the  perforated  valve,  and  expand  into  the  organ 
of  adhesion.  Within  the  space  included  by  the  above  pairs  of  muscles,  there  are  two 
slender  pairs  of  muscles  which  decussate  each  other.  The  superior  pair  ^  take  their 
origin  from  the  anterior  part  of  the  strong  membrane  that  circumscribes  and  protects 
the  viscera  below  the  stomach,  and  between  the  insertions  of  the  anterior  shell-muscles  ; 
they  then  ascend,  diverge  on  either  side  the  alimentary  canal,  and  are  inserted  into 
the  opposite  valve  outside  the  posterior  shell-muscles.  The  inferior  pair 2  arise  from 
the  sides  of  the  membranous  circle,  and  converge,  as  they  pass  below  the  preceding,  to 
be  inserted  into  the  perforated  valve  on  the  inner  side  of  the  posterior  shell-muscles. 
While,  therefore,  the  larger  muscles  have  the  more  important  office  of  guarding  the 
animal  by  closing  the  shell,  the  smaller  muscles  would  admit  the  water  by  sliding  the 
margin  of  one  valve  over  the  other ;  and  they  are  also  calculated  to  produce  a  com- 
pression of  the  viscera. 
The  labial  processes,  or  brachia,  are  scarcely  more  adapted  to  protrude  externally  than 
in  Terebratula  Chilensis,  the  only  parts  that  are  free  being  the  short  spiral  extremities ; 
but  in  the  more  muscular  character  of  their  basis  or  stem,  they  exhibit  a  closer  affinity 
with  Lingula.  Considering  the  arms  as  a  pair,  the  stems  are  then  joined  below  the 
mouth,  forming  on  that  aspect  a  transverse  semilunar  fleshy  basis,  fringed  and  convex 
anteriorly.  This  is  attached  to  the  anterior  part  of  the  tendinous  belt  of  the  viscera^. 
At  the  sides  of  this  basis  the  arms  make  a  sudden  bend  upon  themselves  towards  the 
mouth,  above  and  in  front  of  which  the  extremities  make  a  spiral  turn  and  a  half^. 
The  bent  portions  are  closely  adherent  to  each  other,  not  free  as  in  Lingula.  These 
parts  of  the  arms,  by  contracting  from  the  angle  of  flexion  towards  the  mouth,  would 
necessarily  become  thicker,  and  so  press  upon  and  open  the  shell  a  little  way,  in  a 
manner  analogous  to  what  I  have  supposed  to  take  place  in  the  calcareous  loop  of  Ter. 
Chilensis ;  the  arms  in  Orbicula  are  not,  however,  supported  by  an  internal  calcareous 
process.  The  muscular  basis,  when  cut  into,  exhibits  on  each  side  a  well-defined  cylin- 
drical cavity^,  which  commences  near  the  mesial  plane  in  the  transverse  part  below  the 
mouth,  and  is  continued  into  the  spiral  extremity.  I  injected  these  canals,  but  could 
not  in  that  way  perceive  that  they  had  any  connexion  with  the  vascular  system :  no 
part  of  the  fluid  entered  the  filaments  composing  the  fringe.  The  parts  being  hardened 
by  long  maceration  in  spirits,  prevented  the  unfolding  of  the  arms  by  any  force  I  could 
use ;  but  I  conclude,  nevertheless,  that  the  canals  serve  to  extend  outwards  the  free 
spiral  extremities,  by  being  forcibly  distended  with  fluid  propelled  along  them, — a 
' /i.  Figg.  7.  8.  Fig.  7.  8,  '  A'.  Fig.  8.  " Figg.  9.  10.  f  m.  Figg.  9.  12. 
VOL.  I.  Y 
