ME.  HANCOCK  ON  THE  OEOANIZATION  OF  THE  BEACHIOPODA. 
793 
limited,  and  that  on  separating  them  fracture  ensues.  In  the  unarticulated  forms  the  valves 
do  not  move  upon  each  other,  for  when  open  no  part  of  their  margins  are  in  contact ; 
there  is  no  hinge,  properly  so  called,  and  like  the  former  group  their  gape  is  extremely 
limited ; at  least  such  is  the  case  in  Crania,  which  has  been  observed  in  a living  state. 
The  two  divisions  of  the  Brachiopods  thus  indicated,  at  the  outset  of  our  inquiry,  wUl 
be  found,  as  we  proceed,  stiQ  more  strongly  marked  by  their  internal  organization. 
The  valves,  as  generally  acknowledged,  are  placed  on  the  dorsal  and  ventral  surfaces 
of  the  animal ; the  imperforate  being  the  dorsal,  the  perforate  the  ventral  valve  in  the 
Terehratulidce  and  theii’  immediate  alhes*.  Thus  that  side  which  is  helow  when  the 
animal  is  attached  by  its  peduncle  is  really  the  back.  If  this  were  ever  doubted,  the 
discovery  of  the  tme  heart,  as  we  shall  afterwards  see,  confirms  the  accuracy  of  Professor 
Owen’s  determination  of  these  points.  In  Lingiila,  the  large  valve,  or  that  to  which  the 
peduncle  is  attached,  corresponds  to  the  perforate  or  ventral  valve  of  the  Terehratulidce, 
the  small  one  to  the  dorsal. 
To  facilitate  the  examination  of  the  soft  parts,  it  is  necessary  to  reduce  the  valves  by 
dissolving  them  in  dilute  acid.  When  the  calcareous  matter  has  entirely  disappeared, 
and  the  decalcified  shell-membrane  has  been  removed,  the  transparent  tegumentary 
envelope  of  the  animal  is  exposed  to  "view;  and  this,  when  placed  in  water,  assumes 
the  form  of  the  shell The  pallial  lobes  extend  forward,  but  do  not  project  beyond  the 
sides  of  the  body,  where  they  become  united,  the  junction  being  marked  by  a groove, 
bordered  by  a ridge  on  each  side.  They  are  very  delicate  and  transparent,  so  that  the 
great  pallial  sinuses  can  be  distinctly  traced,  even  to  their  terminal  ramifications ; and 
the  genitaha,  which  he  within  them,  appear  through  theh  walls  of  a full  yellow  colour. 
Towards  the  posterior  or  umbonal  region  the  enlarged  extremities  of  the  shell-muscles 
are  always  apparent,  clustered  together,  on  each  side  of  the  antero-posterior  line,  form- 
ing reddish  fiesh-colouind  patches.  On  the  dorsal  surface  the  liver  is  seen  occupying 
the  space  between  the  muscles,  and  on  the  ventral,  the  termination  of  the  intestine 
makes  its  appearance  as  a minute,  circular  point,  immediately  behind  the  central  or 
occlusor  muscles  in  Waldheimia  australis.  In  Bhynclionella  it  is  seen  in  the  same  posi- 
tion enlarged,  rounded  and  doubled  upon  itself®. 
On  turning  the  pallial  lobes  back,  the  arms*  are  found  to  occupy  almost  the  entire 
palhal  ca\ity,  which  is  very  extensive,  reaching  backwards  for  two-thirds  the  length  of 
the  shell ; the  posterior  third  being  filled  up  with  the  body  of  the  animal.  The  mouth 
opens  upwards,  on  the  median  line,  in  the  brachial  groove ; and  on  each  side  of  it,  a 
little  below,  in  the  anterior  wall  of  the  body,  there  is  an  opening  which  leads  into  the 
abdominal  cavity  or  perivisceral  chamber.  These  orifices  are  situated  at  the  apices  of 
the  two  elongated  organs  designated  hearts  by  Professor  Owen®.  These  so-called  hearts 
‘ Note  2,  p.  851.  ^ Plates  LII.  and  LIII.  figs.  1,  2. 
® Plate  LX.  fig.  2.  Plate  LV.  figs.  1,  2. 
® Introduction  to  Mr.  Davidsox’s  ‘British  Fossil  Brachiopoda,’  published  by  the  Palseontographical 
Society,  p.  14,  1853. 
5l2 
