150  Mr.  R.  OWEN  ON  THE  ANATOMY  OF  THE  BRACHIOPODA. 
The  arms  in  Ter.  Chilensis,  when  detached  from  the  supporting  processes  and  mi- 
folded,  exceed  the  length  of  the  shell  by  two  thirds  of  that  length ;  and  their  length 
is  to  their  breadth  as  eight  to  one.  Their  stem,  from  which  the  cilia  arise,  as  it  has 
not  to  execute  the  movement  of  Lingula,  so  it  is  much  more  slender.  The  cilia, 
therefore,  are  proportionately  increased,  in  order  to  excite  the  necessary  currents  in 
the  water;  which,  being  directed  between  the  folds  of  the  arms  towards  the  mouth, 
as  to  a  focus,  carry  thither  the  nutrient  molecules,  which  are  retained  by  the  natural 
sieve  formed  by  the  decussating  cilia  of  the  terminal  processes  in  front  of  the  mouth  ; 
and  though  this  apparatus  be  apparently  less  perfect  than  in  Lingula,  it  is  evidently 
adjusted  in  due  relation  to  the  support  of  so  small  a  mass  of  body  as  exists  in  Tere- 
hratula.  The  muscular  stem,  by  means  of  its  attachment  to  the  calcareous  loop,  has 
the  power  of  acting  upon  that  part  to  the  extent  its  elasticity  admits  of,  which  is  suffi- 
cient to  produce  such  a  degree  of  convexity  in  the  reflected  part  of  the  loop,  as  to  cause 
it  to  press  upon  the  perforated  valve,  and  separate  it  slightly  from  the  opposite  one. 
This  elastic  internal  apparatus  thus  compensates  for  the  absence  of  the  thick  protruding 
arms  which  push  open  the  valves  in  Lingula,  and  for  the  want  of  the  elastic  fibres  which 
constitute  the  ligament  of  the  hinge  in  ordinary  Bivalves ;  and  it  is  apparently  the  only 
means  this  and  similar  species  of  Terebratula  possess  of  divaricating  the  valves. 
-  In  other  species  in  which  the  loop  is  wanting,  as  in  Ter.  rubicunda.  Sow.,  there  is  a 
compensating  structure ;  the  furcated  extremity  of  the  central  calcareous  process  is 
developed  to  a  great  size,  and  the  forks  can  be  approximated  and  separated  from  each 
other  to  a  small  extent,  to  effect  the  same  purpose  as  the  loop  in  the  normal  Terebratula. 
In  Ter.  vitrea,  however,  the  loop,  though  perfect,  is  too  small  to  be  capable  of  being 
made  to  press  upon  the  perforate  valve  in  the  way  in  which  I  suppose  it  to  act  in  the 
more  depressed  species,  as  Ter.  dorsata,  Ter.  dentata,  Ter.  Sowerbii,  and  Ter.  Chilensis, 
in  all  of  which  it  is  largely  developed  for  that  purpose.  It  is  probable,  therefore,  that 
the  arms  in  this  species  have  a  different  disposition,  and  possess  greater  powers  of  ex- 
tension to  compensate  for  the  small  development  of  the  internal  skeleton. 
It  is  by  such  a  modification  of  the  structure  of  the  soft  parts  that  the  opening  of  the 
shell  is  effected  in  Ter.  psittacea.  The  internal  skeleton  in  this  species  is  reduced  to 
two  small  processes,  curved  slightly  outwards,  which  are  continued  from  the  sides  of 
the  hinge  of  the  imperforate  valve.  From  these  processes  arise  two  spiral  arms ' ,  fringed 
on  their  outer  margin,  as  in  the  other  species,  but  quite  free,  excepting  at  their  origins. 
When  contracted  they  are  disposed  in  six  or  seven  spiral  gyrations,  decreasing  towards 
their  extremities  ;  and  when  completely  unfolded  they  extend  beyond  the  shell  twice 
its  longitudinal  diameter.  The  cilia  are  more  curved  than  in  Ter.  Chilensis,  and  the 
stem  which  supports  them  is  more  muscular.  The  mechanism  by  which  the  arms  are 
extended  is  simple  and  beautiful :  the  stems  are  hollow  from  one  end  to  the  other,  and 
are  filled  with  fluid,  which,  being  acted  upon  by  the  spirally  disposed  muscles  com- 
'  Fig.  14*. 
