808 
ME.  HANCOCK  ON  THE  OEG-ANIZATION  OF  THE  BEACHIOPODA. 
ing  minute  channels  which  run  diagonally  across  the  muscular  fibres.  And  this  belief 
is  in  some  measure  confirmed  by  the  fact,  that  no  mention  is  made  of  this  beautiful 
and  remarkable  system  of  sinuses  in  any  of  the  several  memoirs  on  the  subject  published 
by  that  observer. 
From  the  feeble  nature  of  these  brachial  muscles  it  does  not  seem  likely  that  in 
Waldheimia  the  spiral  portion  of  the  arms  can  be  unrolled.  The  true  function  of  the 
brachial  canal  would  seem  to  be  to  sustain  the  grooved  ridge  bearing  the  cirri.  As 
the  canal  is  a closed  tube,  and  as  it  undoubtedly  contains  fluid,  a very  slight  contraction 
of  its  walls  will  render  it  firm  and  rigid.  The  coil  may  also,  perhaps,  be  loosened  a 
little  by  the  same  action ; thus  giving  greater  freedom  to  the  cirri.  The  grooved  ridge 
will  in  this  way  become  firmly  seated,  so  that  its  movements  may  be  regulated  with 
precision.  These  are  effected  by  a very  complicated  piece  of  mechanism,  which  is 
rendered  obvious  on  making  a transverse  section  of  the  part,  when  the  substance  of  the 
ridge  is  found  to  be  fibrous,  and  the  fibres  to  be  arranged  in  several  distinct  gi’oupsb 
One  of  these  groups  is  situated  at  the  outer  margin  of  the  base  supporting  the  ciiri,  and 
has  the  fibres  extending  from  these  organs  downwards.  This  is  for  the  pm-pose  of 
drawing  back  or  elevating  the  fringe.  A belt  of  similar  fibres  passes  from  the  inner 
margin  of  the  cirri,  and  running  downwards,  arches  conformably  mth  the  groove,  and 
passes  up  the  inner  side  of  the  brachial  fold.  These  fibres  will  contract  the  fold  and 
draw  it  towards  the  cirri,  thus  narrowing  the  gutter.  Another  distinct  group  of  fibres 
passes  from  this  circular  belt  and  the  outer  wall  of  the  brachial  fold,  and  descends  to 
the  lower  surface  of  the  ridge,  where  there  is  a thin  stratum  of  muscular  fibres.  The 
action  of  this  group  will  be  to  pull  the  fold  forward  from  the  cirri ; and  also  when  fully 
exerted  to  draw  the  cirri  themselves  forward,  depressing  them.  The  thin  stratum  of 
fibres  will  probably  assist  in  the  latter  act. 
The  arms  of  W.  cranium  and  T.  caput-serpentis'^  are  disposed  in  the  same  manner  as 
in  W.  australis  \ and  in  the  former  the  calcareous  loop  is  precisely  similar  to  that  of  the 
latter ; but  in  T.  caput-serpentis  it  is  very  much  reduced,  the  extended  lateral  portions 
having  almost  entirely  disappeared,  little  more  than  the  transverse  portion  existing; 
and  this,  together  with  the  crural  processes,  which  are  united  below  across  the  median 
line,  forms  a collar  upon  which  the  bases  of  the  arms  rest.  In  this  species,  therefore, 
the  expanded  lateral  portions  of  the  arms  are  without  any  apophysary  support,  and 
accordingly  other  means  are  provided  for  sustaining  them.  The  two  produced  lobules 
of  the  dorsal  pallial  lobe  reach  to  the  ends  of  these  portions  of  the  ai’ms  as  in 
W.  australis ; and  are  stiffened  with  numerous,  imbedded,  calcareous  spicula  to  such  an 
extent,  that  when  the  soft  tissues  are  removed  by  maceration  the  form  of  the  pai'ts 
remains  unaltered.  The  spicula  extend  also  over  the  surface  of  the  inner  lamina  of  the 
pallial  lobe,  and  pervade  likewise  the  walls  of  the  canal,  and  even  the  cirri ; so  that  the 
brachial  apparatus  becomes  firmly  fixed,  and  in  this  way  a substitute  is  foimd  for  the 
usual  apophysary  support. 
‘ Plate  LVIII.  fig.  2. 
* Plate  LV.  fig.  2. 
