ME.  HANCOCK  ON  THE  OEOANIZATION  OF  THE  BEACHIOPODA. 
811 
these  orifices  open  into  the  cavity  close  behind  the  oesophagus,  and  directly  above  a 
transverse  fold  of  the  anterior  wall  of  the  body.  It  is  not  altogether  unlikely  that  these 
chambers  likewise  communicate  with  the  posterior  canals,  but  I have  failed  to  ascertain 
the  fact;  and  Vogt^  also  states  that  the  two  canals  of  the  arm  are  cut  off  from  the 
perivisceral  cavity.  Above  the  orifices  there  is  another  delicate  membranous  fold  or 
fiap,  which  seems  capable  of  closing  them  by  being  pressed  down  against  the  parietal 
fold,  to  which  the  occlusor  muscles  are  attached,  acting  in  the  manner  of  a valve. 
The  membranous  wall  of  the  posterior  canals  is  composed  of  two  layers,  an  outer 
homogeneous,  and  an  inner  muscular  layer,  each  having  its  proper  epithelium.  The 
fibres  of  the  latter  layer  run,  for  the  most  part,  diagonally  across  the  canal,  becoming 
transverse  towards  the  part  furthest  from  the  grooved  ridge.  Here  there  is  also  pro- 
jecting into  the  canal  a stout  muscular  band  of  a brown  colour  with  the  fibres  longi- 
tudinal, and  extending  the  whole  length  of  the  organ.  Throughout  it  is  firmly  attached 
to  the  wall  of  the  canal,  and  terminates  at  the  root  of  the  arm. 
The  additional  mechanism  in  connexion  with  the  arms  of  Lingula  undoubtedly  implies 
some  peculiar  function ; and  in  this  genus  it  may  be  fairly  inferred  that  the  brachial 
organs  are  capable  of  being  evolved,  if  not  entii’ely,  at  least  to  a considerable  extent. 
The  posterior  canals,  which  are  only  found  in  Lingula,  appear  to  be  the  principal 
agents  in  this  operation.  These  vrill  be  able  to  act  vrithout  interfering  with  the 
movements  of  the  grooved  ridge  and  cirri ; when  their  walls  are  contracted  and  the  longi- 
tudinal muscular  belt  relaxed,  the  fiuid,  which  they  undoubtedly  contain,  will  cause  the 
arms  to  uncoil,  and  on  the  walls  relaxing  and  the  muscle  coming  into  play,  they  will 
again  be  withdrawn  and  coiled  up.  But  the  animal  must  be  seen  in  a living  state 
before  it  can  be  ascertained  to  what  extent  these  organs  are  um-olled  and  protruded. 
This  appears  to  be  the  explanation  of  the  action  of  these  parts ; but  whether  exactly 
correct  or  not,  it  seems  clear  enough  that  the  arms  of  lAngula  have  the  power  of 
extension  in  a greater  or  less  degree.  And  from  the  deficiency,  in  the  Terehratulidoe 
and  Bhynchonellidce,  of  the  apparatus  here  employed,  it  may  safely  be  concluded  that  in 
them  the  brachial  organs  are  deprived  of  any  such  movement. 
The  anterior  canals  in  Lingula  will  perform  the  same  office  as  the  great  canals  do  in 
the  other  Brachiopods ; that  is,  they  will  give  firmness  to  the  parts,  so  that  the  cirri  and 
brachial  fold  may  be  brought  into  play.  When  the  arms  are  retracted,  the  walls  of 
these  canals,  which  it  will  be  remembered  are  closed  tubes,  vrill  relax  a little  to  relieve 
themselves  from  the  pressure  of  the  contained  fluid ; when  they  are  extended  the  walls 
will  contract  to  maintain  the  required  pressure.  Thus  the  cirri  and  brachial  fold  will 
be  under  the  control  of  their  proper  muscles,  however  much  or  little  the  arms  may  be 
extended. 
The  arms  of  L.  affinis^  are  perfectly  similar  to  those  of  L.  anatina,  only  the  thickened 
portion  of  the  wall  of  the  anterior  canal  is  much  more  limited. 
The  mouth*,  in  all  the  Brachiopods,  as  previously  stated,  is  situated  in  the  brachial 
' Op.  cit.  ^ Plates  LXV.  fig.  6 ; LXVI.  figs.  2,  3.  ^ Plates  LV.  figs.  2,  3 ; LXIV.  fig.  3. 
