MR.  R.  OWEN  ON  THE  ANATOMY  OF  THE  BRACHIOPODA. 
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shall  compare  them  in  the  first  place  with  the  Lamellibranchiate  Bivalves,  to  which  they 
present  the  most  obvious  relations  in  the  nature  and  forms  of  their  organs  of  defence. 
To  these  they  are  in  some  respects  superior.    The  labial  arms  are  more  complex  pre- 
hensile organs  than  the  corresponding  vascular  lamina  on  either  side  the  mouth  of  the 
Lamellibranchiata.    The  whole  muscular  system  is  more  complex ;  and  the  opening 
as  well  as  the  closing  of  the  shell  being  regulated  by  muscular  action,  indicates  a  higher 
degree  of  organization  than  where  the  antagonizing  power  results  from  a  property  of 
the  cardinal  ligament,  which  is  independent  of  vitality,  viz.  elasticity.    With  respect, 
however,  to  the  respiratory  organs,  the  modifications  which  these  have  presented  in 
Orbicula  and  Terehratiila  show  the  Brachiopods  to  be  still  more  inferior  to  the  Lamelli- 
branchiata than  was  to  be  inferred  from  the  structure  of  the  branchics  in  Ldngula  :  and 
notwithstanding  the  division  of  the  systemic  heart,  I  consider  that  there  is  also  an 
inferiority  in  the  vascular  system.    Each  heart,  for  example,  in  the  Brachiopoda  is  as 
simple  as  in  Ascidia,  consisting  of  a  single  elongated  cavity,  and  not  composed  of  a 
distinct  auricle  and  ventricle,  as  in  the  ordinary  Bivalves :  for  in  these  even  when,  as 
in  the  genus  Area,  the  ventricles  are  double,  the  auricles  are  also  distinctly  two  in 
number ;  and  in  the  other  genera,  where  the  ventricle  is  single,  it  is  mostly  supplied 
by  a  double  auricle.    The  two  hearts  of  the  Brachiopoda,  which  in  structure  resemble 
the  two  auricles  in  the  above  Bivalves,  form  therefore  a  complexity  or  superiority  of 
organization  more  apparent  than  real. 
Having  been  thus  led  to  consider  the  circulating  as  well  as  respiratory  systems  as 
constructed  on  an  inferior  plan  to  that  which  pervades  the  same  important  systems  in 
the  Lamellibranchiate  Bivalves,  I  infer  that  the  position  of  the  Brachiopoda  in  the  natural 
system  is  inferior  to  that  order  of  Acephala. 
Among  the  relations  of  the  Brachiopoda  to  the  Tunicated  Acephala,  and  more  especially 
to  the  Ascidia,  we  may  first  notice  an  almost  similar  position  of  the  extended  respiratory 
membranes  in  relation  to  the  mouth,  so  that  the  currents  containing  the  nutrient  mo- 
lecules must  first  traverse  the  vascular  surface  of  that  membrane  before  reaching  the 
mouth ;  the  simple  condition  also  to  which  the  branchice  are  reduced  in  Orbicula  and 
Terebratula  indicates  their  close  affinity  to  the  Ascidia.  But  in  consequence  of  the  form 
of  the  respiratory  membranes  in  the  Brachiopoda,  which  is  so  opposite  to  that  of  the 
sacciform  branchia  of  the  Ascidia,  the  digestive  system  derives  no  assistance  from  that 
part  as  a  receptacle  for  the  food,  and  the  superaddition  of  prehensile  organs  about  the 
mouth  became  a  necessary  consequence.  The  Brachiopods  again  are  stationary,  like  the 
Ascidia,  and  resemble  the  Boltenia  in  the  pedunculated  mode  of  their  attachment  to 
foreign  bodies. 
With  the  Cirripeds  their  relation  is  one  of  very  remote  analogy ;  their  generative, 
nervous,  and  respiratory  systems  being  constructed  on  a  different  type,  and  their  brachia 
manifesting  no  trace  of  the  articulate  structure.  In  all  essential  points  the  Brachiopoda 
closely  correspond  with  the  Acephalous  Mollusca,  and  I  consider  them  as  being  in- 
