ME.  HAJS^COCK  ON  THE  OEGANIZATION  OF  THE  BEACHIOPODA. 
797 
appears  to  have  been  first  pointed  out  by  Professor  Quenstedt  ' ; and  this  characteristic 
feature  of  the  articulated  Brachiopods  was  commented  upon  and  fully  appreciated  by  its 
discoverer. 
Though  this  is  undoubtedly  the  primary  function  of  the  two  pairs  of  divaricators,  yet 
as  they  are  intimately  united  to  the  capsule  of  the  peduncle,  when  in  action  they  will 
likewise  pull  it  do^vnwards  towards  the  ventral  valve,  thus  giving  room  for  the  cardinal 
process  to  pass  inwards,  which  it  must  always  do  when  these  muscles  are  in  action. 
And  moreover,  from  the  position  of  the  ventral  attachment  of  the  accessory  divarica- 
tors, and  from  their  coursing  round  the  base  of  the  peduncular  capsule  to  their  supe- 
rior attachment,  it  is  evident  that  whenever  exerted  they  will  have  a tendency  to  thrust 
out  the  peduncle.  Thus  the  opening  of  the  valves  must  always  be  accompanied  by 
an  elevation  of  the  animal  upon  its  support,  though  this  can  never  be  to  any  great 
extent. 
The  peduncle,  being  the  medium  by  which  the  Brachiopod  becomes  attached  to 
foreign  bodies,  is  the  fixed  point  upon  which  the  limited  movements  of  the  shell  are 
efifected ; and  the  adjustor  muscles,  having  one  of  their  extremities  attached  to  this 
organ,  may  be  considered  as  taking  their  origin  in  it.  It  will  therefore  be  well,  before 
describing  them,  to  examine  the  structure  of  the  peduncle  itself.  This  organ ^ is  com- 
posed of  a dense  muscular  or  semi-cartilaginous  mass  of  a cylindrical  form,  with  one 
extremity  exserted  through  the  foramen  of  the  ventral  valve,  and  the  other  resting 
within  its  umbonal  region.  The  protruded  extremity  is  protected  by  a thick,  horn-like 
covering  of  a brownish  colour;  it  is  short  and  stout,  possessing  little  or  no  elasticity, 
and  has  a fibrous  expanded  base  of  attachment.  The  inner  extremity  is  enlarged,  and 
is  received  into  a capsule,  which  is  formed  by  the  invagmation  of  the  tegumentary 
envelope  at  the  margin  of  the  foramen.  The  peduncle  is  attached  to  the  bottom  of  this 
capsule,  through  which  it  sends  a wide,  narrow  belt  of  muscular  fibres,  to  be  inserted 
into  the  valve  immediately  behind  the  accessory  divaricators.  This  muscle  is  short  and 
white,  hke  the  muscular  portion  of  the  peduncle,  and  appears  at  the  surface  of  the  tegu- 
mentary envelope  as  a narrow,  transverse,  pale  patch,  extending  almost  across  the  um- 
bonal region.  This  is,  as  previously  stated,  the  “ capsularis  ” muscle  of  Professor  Owen. 
Its  function,  however,  is  not  merely  to  attach  the  capsule,  but  is  in  an  especial  sense  to 
unite  the  peduncle  to  the  shell,  as  proved  by  the  homologous  muscle  in  Lingula^  and  by 
the  dfrection  of  its  fibres,  which  are  the  continuation  of  those  forming  the  core  of  the 
peduncle,  or  more  properly  the  peduncle  itself,  running  from  one  end  of  the  organ  to 
the  other.  When  in  action,  it  will  have  the  effect  of  erecting  the  shell  and  steadying  it 
upon  the  peduncle. 
The  movements  of  the  shell,  however,  upon  this  support,  will  be  chiefly  effected  by 
the  action  of  the  adjustors,  of  which  we  have  seen  there  are  two  pairs,  the  dorsal  and 
' “Heber  das  (Effnen  und  Schliessen  der  Bracniopoden,”  Wiegmann’s  Arcliiv,  vol.  ii.  pp.  220-222,  pi.  4. 
figs.  4,  5,  6,  1835. 
^ Plate  LYII.  figs.  2,  3,  4. 
