802 
ME.  HANCOCK  ON  THE  OEG-ANIZATION  OE  THE  BEACHIOPODA. 
perivisceral  chamber  are  also  well  supplied  with  fibres.  In  front  a powerful  muscular 
band,  in  connexion  with  the  parietes,  extends  along  each  side,  above  the  oviducts,  from 
the  anterior  wall  of  the  chamber  to  behind  the  dorsal  extremities  of  the  adjustors : these 
are  the  anterior  parietals.  As  the  walls,  with  which  these  two  sets  of  muscles  are  con- 
nected, arch  outwards  when  they  are  relaxed,  it  is  evident  that  when  in  action  they  vdll 
straighten  the  curvature,  and  thus  diminish  the  capacity  of  the  perivisceral  chamber. 
The  posterior  parietals  will  also  assist  in  drawing  together  the  umbonal  regions  of  the 
valves.  Now  when  the  divaricator  muscle  contracts  and  the  anterior  occlusors  relax, 
the  fluid  contained  in  this  chamber  will  be  pressed  forward ; the  anterior  parietals  then 
coming  into  play  compress  the  space  in  front,  and  the  lateral  walls  behind  losing  their 
curvature  and  becoming  firm  and  resistent,  from  the  action  of  the  posterior  parietals, 
the  valves  will  be  constrained  to  open  anteriorly.  This,  there  can  be  little  doubt,  is  the 
mechanism  employed  in  opening  the  valves  in  lAngula ; and  it  is  probably  the  same  in 
all  the  unarticulated  Brachiopods : we  know  that  in  Crania  and  Discina  the  muscles  are 
arranged  after  the  same  plan,  and  the  muscular  impressions  in  the  fossil  species  would 
seem  to  indicate  that  in  them  also  they  were  similarly  disposed. 
The  pair  of  central  adjustors  are  attached  to  the  ventral  valve,  by  fine  points,  between 
the  posterior  occlusors  in  front ; they  are  placed  close  together,  one  on  each  side  of  the 
median  line : sweeping  round  the  inner  border  of  these  muscles  they  diverge  posteriorly, 
and,  increasing  in  size  as  they  go,  ascend  towards  the  dorsal  valve,  to  which  they  become 
adherent,  one  at  each  side  immediately  within  the  parietes  of  the  body. 
The  external  adjustors  arise  from  the  ventral  valve,  at  the  outside  of  the  posterior 
occlusors,  and  in  contact  with  them ; they  are  at  first  pretty  stout,  but  on  passing  out- 
wards and  backwards  they  enlarge  a little,  and  ascending  are  inserted  into  the  dorsal 
valve,  one  on  each  side  immediately  behind  the  central  pair. 
The  posterior  adjustors  are  large  and  powerful  muscles,  and  though  they  may  be  con- 
sidered as  a pair,  they  are  asymmetrical,  there  being  two  on  one  side,  and  only  one  on 
the  other.  As  they  pass  across  from  valve  to  valve  they  intersect  each  other,  the  single 
one  passing  between  the  other  two.  These  are  the  decussating  muscles  of  authors. 
The  single  one  is  as  large  as  the  other  two  put  together,  and  is  attached  to  the  left  side 
of  the  ventral  valve  about  midway  between  the  divaricator  and  the  anterior  occlusor. 
From  this  point  it  passes  diagonally  upwards  and  forwards  between  the  reproductive 
organs,  having  the  alimentary  canal  above  it ; and  on  reaching  the  opposite  side  of  the 
dorsal  valve,  has  the  other  end  inserted  into  the  latter,  immediately  within  the  posterior 
terminations  of  the  external  and  central  adjustors  of  the  same  side.  At  the  points  of 
attachment  the  three  muscles  are  pressed  so  close  together,  that  they  appear,  at  fii'st 
sight,  as  only  one. 
The  two  opposite  posterior  adjustors  take  their  origin  from  the  right  side  of  the 
ventral  valve,  considerably  apart ; but  both  of  them  close  to  the  lateral  parietes  of  the 
body,  one  only  a little  in  advance  of  the  divaricator,  and  the  other  a short  distance 
further  forward.  They  converge  as  they  penetrate  the  visceral  mass,  and,  sloping 
