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MR.  R.  OWEN  ON  THE  ANATOMY  OF  THE  BRACHIOPODA. 
left  exposed  by  the  retreating  tide  were  it  not  buried  in  the  sand  of  the  shore,  must 
meet  with  a  greater  variety  and  abundance  of  animal  nutriment  than  can  be  found  in 
those  abysses  in  which  Terehratula  is  destined  to  reside.  Hence  its  powers  of  pre- 
hension are  greater,  and  Cuvier  suspects  it  may  even  enjoy  a  species  of  locomotion 
from  the  superior  length  of  its  peduncle.  The  organization  of  its  mouth  and  stomach 
indicates,  however,  that  it  is  confined  to  food  of  a  minute  description ;  but  its  con- 
voluted intestine  shows  a  capacity  for  extracting  a  quantity  of  nutriment  proportioned 
to  its  superior  activity  and  the  extent  of  its  soft  parts.  A  more  complex  and  obvious 
respiratory  apparatus  was  therefore  indispensable,  and  it  is  not  surprising  that  the 
earlier  observers  failed  to  detect  a  corresponding  organization  in  genera  destined  to  a 
more  limited  sphere  of  action. 
The  respiration,  indeed,  as  well  as  the  nutrition  of  animals  living  beneath  a  pressure 
of  from  sixty  to  ninety  fathoms  of  sea  water,  are  subjects  of  peculiar  interest,  and 
prepare  the  mind  to  contemplate  with  less  surprise  the  wonderful  complexity  ex- 
hibited in  the  minutest  parts  of  the  frame  of  these  diminutive  creatures.  In  the  still- 
ness pervading  these  abysses  they  can  only  maintain  existence  by  exciting  a  perpetual 
current  around  them,  in  order  to  dissipate  the  water  already  loaded  with  their  effete 
particles,  and  bring  within  the  reach  of  their  prehensile  organs  the  animalcula  adapted 
for  their  support.  The  actions  of  Terehratula  and  Orbicula,  from  the  firm  attachment 
of  their  shells  to  foreign  substances,  are  thus  confined  to  the  movements  of  their 
brachial  and  branchial  filaments,  and  to  a  slight  divarication  or  sliding  motion  of 
their  protecting  valves ;  and  the  simplicity  of  their  digestive  apparatus,  the  corre- 
sponding simplicity  of  their  hranchice,  and  the  diminished  proportion  of  their  soft  to 
their  hard  parts,  are  in  harmony  with  such  limited  powers.  The  soft  parts  in  both 
genera  are,  however,  remarkable  for  the  strong  and  unyielding  manner  in  which  they 
are  connected  together :  the  muscular  parts  are  in  great  proportion,  and  of  singular 
complexity  as  compared  with  ordinary  Bivalves ;  and  the  tendinous  and  aponeurotic 
parts  are  remarkable  for  the  similarity  of  their  texture  and  appearance  to  those  of  the 
highest  classes.  By  means  of  all  this  strength  they  are  enabled  to  perform  the  requi- 
site motions  of  the  valves  at  the  depths  in  which,  they  are  met  with.  Terehratula,  which 
is  more  remarkable  for  its  habitat,  ■  has  an  internal  skeleton  superadded  to  its  outward 
defence,  by  means  of  which  additional  support  is  afforded  to  the  shell,  a  stronger 
defence  to  the  viscera,  and  a  more  fixed  point  of  attachment  to  the  brachial  cirri. 
The  spiral  disposition  of  the  arms  is  common  to  the  whole  of  the  Brachiopodous 
genera  whose  organization  has  hitherto  been  examined ;  and  it  is  therefore  probable 
that  in  that  remarkable  genus  Spirifer  the  entire  brachia  were  similarly  disposed,  and 
that  the  internal  calcareous  spiral  appendages  were  their  supports.  If,  indeed,  the 
hrachia  of  Ter.  psittacea  had  been  so  sustained,  this  species  would  have  presented  in  a 
fossil  state  an  internal  structure  very  similar  to  that  of  Spirifer. 
In  considering  the  affinities  of  the  Brachiopoda  to  the  other  orders  of  Mollusca,  I 
