rni.  HA^TOCK  ON  TIIE  OEGANIZATION  OF  THE  BEACHIOPODA. 
849 
shomng  the  propriety  of  di\iding  these  animals  into  two  great  groups,  as  was  proposed 
in  1836  by  M.  DESHATES^  Some  objections  might,  perhaps,  be  raised  to  this,  on  account 
of  Prodiictus  and  Aulosteges,  as  neither  have  teeth  or  sockets,  though  both  undoubt- 
edly belong  to  the  articulated  group.  But  notwithstanding  these  deficiencies,  the  valves 
are  related  to  each  other  as  if  hinged,  and  the  cardinal  process  is  so  locked  into  the  beak 
of  the  opposite  valve,  as  to  have  prevented  any  lateral  movement.  The  distinctive  feature, 
however,  is  not  in  the  mere  fact  of  articulation,  but  in  the  mode  of  opening  the  valves ; 
and  in  these  two  genera  there  was  apparently  no  deviation  in  this  respect  from  the  rest 
of  the  group. 
With  regard  to  the  affinities  of  the  Brachiopods,  it  is  impossible  not  to  be  struck  with 
the  strong  resemblance  that  exists  between  the  organization  of  these  animals  and  that 
of  the  Tunicata  and  Polyzoa.  In  the  first  place,  they  are  all  three,  with  a few  excep- 
tions, attached  to  foreign  bodies ; and  in  all,  the  respiratory  organ  is  appended  to  the 
mouth,  ser-ving  at  the  same  time  the  purpose  of  sustentation.  The  intestine  has  a 
neural  flexure  in  the  Brac]iioj)oda  and  Polyzoa,  and  the  arms  of  the  former  are  con- 
structed upon  the  same  plan  as  those  of  a Hippocrepian  Polyzoon,  as  is  at  once  obvious 
on  the  examination  of  the  lophophore  of  Plwnatella,  for  instance.  It  is  interesting  to 
mark  the  gradual  change  of  these  organs  from  their  simplest  form  to  their  full  and 
complete  development  in  the  BracMojJoda.  In  the  lower  forms  of  the  Polyzoa  the  cirri 
or  tentacles  are  arranged  in  a simple  circle  around  the  mouth,  but  in  Fredericella  the 
base  upon  which  they  are  supported  is  angulated  a little  at  each  side.  In  Plumatella 
the  angles  are  produced  so  as  to  give  to  the  base  a horseshoe  form ; m fact,  to  provide 
two  rudimentary  arms,  along  both  sides  of  which  the  cirri  are  arranged  in  a single 
series.  In  the  next  step  in  the  development  of  these  organs  the  cirri  of  the  two  sides 
are  approximated,  forming  a close-set  double  series ; the  arms  are  much  elongated  and 
spirally  coiled,  and  they  are  thus  at  once  transformed  into  the  brachial  apparatus  of  the 
Brachiopod : the  arms  of  Lingula  and  Rhynchonella  are  complete,  only  they  are  turned 
forward  instead  of  backward. 
The  relation  of  the  two  forms  is  also  manifested  by  the  muscular  system.  The  polype 
retractors  and  the  opercular  muscles  of  Paludicella  seem  to  be  homologous  with  the 
adjustor  muscles,  their  position  being  very  similar;  and  it  is  also  interesting  to  find 
that  their  function  is  not  altogether  different,  for  in  addition  to  the  retraction  of  the 
polype,  they  have  the  power  of  depressing  and  moving  from  side  to  side  the  lophophore, 
much  in  the  same  way  as  the  adjustors  move  the  shell  in  the  Terehratulidee  upon  the 
peduncle.  It  has  likewise  been  pointed  out  by  Professor  Huxley^,  that  the  occlusor  and 
divaricator  muscles  of  these  latter  animals  act  in  exactly  the  same  manner  as  the  muscles 
similarly  named,  which  open  and  close  the  beaks  or  valves  of  the  avicularium  or  bird’s- 
head  process  of  the  Cheilostomatous  Polyzoa.  Thus  in  these  curious  organs  we  see  the 
same  peculiar  disposition  of  muscles  which  characterizes  the  articulated  division ; and 
' Lamaeck,  Animaux  sans  Vertebres,  ed.  tom.  vii.  p.  309. 
* English  Cyclopsedia,  Nat.  Hist.  vol.  iii.  column  859,  1855. 
5s2 
