MR.  R.  OWEN  ON  THE  ANATOMY  OF  THE  BRACHIOPODA. 
151 
posing  the  parietes  of  the  canal,  is  forcibly  injected  towards  the  extremity  of  the  arm, 
which  is  thus  unfolded  and  protruded  outwards. 
My  opinion  of  the  uses  of  the  complex  internal  testaceous  apparatus  of  Ter.  Chilensis 
and  its  congeners,  was  at  first  shaken  by  observing  that  it  was  wanting  in  the  more 
globose  species,  as  Ter.  vitrea,  Ter.  rubicunda,  and  Ter.  psittacea,  where  it  ought  rather  to 
have  been  proportionately  developed,  in  order  to  act  upon  the  valves.  But  the  re- 
lations of  the  soft  parts  to  the  loop,  as  exhibited  in  Ter.  Chilensis,  showed  that  the 
increase  of  that  part  requisite  to  perform  the  same  office  in  the  globose  species,  would 
have  been  incompatible  with  the  limited  proportion  of  the  soft  parts  which  characterizes 
the  genus  Terebratula,  and  the  opening  of  the  shell  is  therefore  effected  by  other  means. 
It  is  interesting  to  observe  that  the  globose  figure  is  assumed  by  those  species  which 
have  the  weakest  valves,  in  order  to  enable  them  to  resist  surrounding  pressure,  while 
in  Ter.  dentata,  Ter.  dorsata,  Ter.  Sowerbii,  and  Ter.  Chilensis,  in  which  the  imperforate 
valve  is  more  or  less  flattened,  the  whole  shell  is  characterized  by  its  superior  thick- 
ness and  strength. 
Under  the  microscope  the  brachial  cilia  are  seen  to  be  of  a  transparent  horny  texture ; 
and  the  muscular  stem  to  have  no  vascular  trunk  accompanying  it,  which  would  cer- 
tainly be  the  case  if  the  blood  of  the  animal  was  distributed  through  this  part  for  its 
renovation :  so  that  the  absence  of  the  adequate  organization  in  the  brachia,  together 
with  the  above-described  structure  of  the  mantle-lobes,  leaves  no  doubt  as  to  the  true 
position  of  the  branchics. 
Although  the  pedicle  is  the  fixed  point  to  which  the  muscles  are  attached,  I  shall 
consider  the  extremities  going  to  that  part  as  the  insertions.  Two  pairs  of  muscles 
arise  from  each  valve.  Those  of  the  imperforate  valve  arise  at  a  distance  from  each 
other  :  the  anterior  pair  come  ofi"  just  behind  the  middle  of  the  valve,  fleshy  ;  they  soon 
diminish  to  thin  shining  tendons,  which  converge  and  unite  below  the  stomach,  and 
then  again  separate  and  pass  through  the  foramen  of  the  perforated  valve  to  be  inserted 
in  the  pedicle.  The  posterior  pair  are  very  short,  and  wholly  carneous ;  they  arise 
from  the  lateral  depressions  in  the  base  of  the  central  process  of  the  hinge,  and  are 
inserted  into  the  pedicle.  The  muscles  of  the  perforated  valve  arise  close  together, 
so  as  to  leave  only  a  single  muscular  impression  on  either  side^  The  anterior  pair 
soon  diminish  to  slender  tendons,  which  are  inserted  into  the  base  of  the  imperforate 
valve ;  the  posterior  pair  pass  exclusively  to  the  pedicle. 
The  pedicle  is  surrounded,  except  where  it  is  attached  to  foreign  substances,  by  a 
tubular  prolongation  of  the  superior  lobe  of  the  mantle.  When  this  membrane  is  de- 
tached, the  surface  beneath  is  found  to  be  smooth ;  and  the  fibres,  when  separated,  ex- 
hibit some  of  the  lustre  of  the  tendons  of  which  it  seems  to  be  composed.  At  its  ex- 
tremity these  become  partially  decomposed,  are  of  a  black  colour,  and  separated  irre- 
gularly from  each  other,  so  as  to  form  an  expanded  base  of  attachment. 
'  Fig.  3. 
