ME.  HANCOCK  ON  THE  OE&ANIZATION  OF  THE  BRACHIOPODA. 
805 
with  the  perivisceral  chamber  by  a narrow  passage  which  turns  round  the  right  side  of 
the  thvaricator  muscle.  But  as  the  peduncle  had  been  detached  before  the  specimen 
came  into  my  possession,  I cannot  speak  with  certainty  as  to  these  points. 
The  peduncular  muscle  itself  is  rather  small  and  short,  and  is  transversely  elongated ; 
it  is  formed  by  a continuation  of  the  fibres  of  the  muscular  cylinder,  and  has  its  insertion 
immediately  withm  the  umbo  of  the  ventral  valve  and  close  behind  the  divaricator ; 
thus  in  position  it  exactly  resembles  the  same  muscle  in  the  Terehratulidoe^  and  originates 
in  precisely  the  same  manner. 
The  wall  of  the  peduncle  is  united  to  the  external  surface  of  the  pallial  expansion  at 
the  umbonal  region;  and  just  within  the  margin,  a slight  fold*,  which  is  a little  wider 
than  the  peduncular  muscle,  indicates  a rudimentary  capsule.  This  organ  would  at 
once  be  formed  if  the  mantle  were  invaginated  at  the  fold,  the  peduncle  thrust  a little 
inwards,  and  the  attachment  of  its  muscle  advanced.  In  this  case  the  divaricator  would 
also  have  to  be  pushed  forward,  and  the  adjustors  becoming  confounded  with  the 
peduncle  and  its  capsule,  the  muscles  of  lAngula  would,  to  a great  extent,  assume  the 
arrangement  of  those  of  the  TerehratulidcE. 
The  brachial  and  pallial  muscles  still  claim  attention ; but  these  will  be  more  con- 
veniently described  when  we  come  to  speak  of  those  parts. 
The  muscles  in  the  Terehratulidm  and  Rhynclionellidce  are  peculiar  for  their  enlarged, 
fleshy  extremities,  and  for  the  attenuated,  tendinous  character  of  their  intermediate 
portions^.  The  extremities®  are  mostly  of  a yellow  flesh-colour,  and  are  comparatively 
soft,  with  the  fibres  arranged  in  separate  radiating  bundles ; so  that  a needle  may  be 
passed  between  them  almost  as  far  as  the  centre  of  the  muscle.  The  tendinous  portions 
are  dense,  white,  glistening,  firm  and  rigid,  and  are  permeated  with  blood  channels,  the 
larger  of  which  run  in  the  direction  of  the  length  of  the  muscle.  On  making  a trans- 
verse section  of  the  occlusors  in  W.  cmstralis,  several  such  channels  are  observed ; some 
of  considerable  size,  with  others  branching  in  various  directions  containing  blood-cor- 
puscles. Amidst  the  fibres  of  the  enlarged  extremities  similar  corpuscles  are  strewed 
about.  The  fibres  easily  separate  from  each  other ; they  are  cylindrical,  homogeneous, 
transparent  and  devoid  of  cross  striae  ■*,  with  the  exception  of  those  of  the  posterior  occlu- 
sors, in  which  transverse  striations®  are  vividly  displayed,  and  give  to  them  a very  striking 
appearance.  The  fibres®  are  generally  formed  of  three,  four  or  more  fibrillae ; but  single 
fibrillae  are  frequently  observed  perfectly  isolated,  in  which  the  cell-like  structure  is  beau- 
tifully distinct.  It  will  afterwards  be  seen  that  these  muscles,  with  cross  striae,  are  richly 
supphed  with  nervous  filaments. 
The  muscles  in  lAngiila’^  are  fleshy  and  of  equal  thickness  throughout,  with  the  single 
exception  of  the  central  adjustors,  which  taper  to  their  ventral  attachments.  The 
bundles  of  fibres,  though  very  distinct,  are  not  separated  as  in  the  Ter-ehratulidoe,  and  in 
no  instance  did  the  fibres  display  transverse  striae. 
’ Plate  LXVI.  fig.  2.  ^ Plates  LYII.  figs.  1,  2 ; LXI.  figs.  1,  2. 
^ Plate  LXII.  figs.  4,  5,  14, 15.  ^ Plate  LXII.  figs.  8,  9. 
® Plate  LXII.  fig.  6.  **  Plate  LXII.  fig.  7.  ^ Plate  LXV.  figs.  2,  3. 
