]\IE.  HANCOCK  ON  THE  OEOANIZATION  OF  THE  BEACHIOPOHA. 
813 
side  of  the  alimentary  tube,  by  a membrane  denominated  the  mesentery.  Each  portion 
is  subdivided  into  an  anterior  and  posterior  lobe,  which  pour  their  secretion  through  the 
dorsal  wall,  into  the  cardiac  end  of  the  stomach,  by  two  or  three  short,  distinct  ducts, 
which  are  formed  by  the  confluence  of  the  various  ramuscules*  composing  the  lobes. 
The  terminal  caecal  extremities  of  the  tubes  are  rounded,  and  as  seen  at  the  surface, 
generally  give  to  the  organ  the  appearance  of  being  made  up  of  globules.  The  parts, 
though  closely  pressed  together,  are  not  united,  each  ramuscule  and  tube  being  free. 
The  anterior  lobe  is  much  the  larger,  and  extends  in  front  below,  into  the  commence- 
ment of  the  brachial  pouches ; and  above,  as  far  as  the  opening  of  the  central  pallial 
sinus ; the  posterior  extremity  extends  below  the  stomach,  reaching  backward  beyond 
the  pyloric  tennination.  The  posterior  lobe  lies  on  the  dorsal  surface  of  the  stomach 
behind  the  anterior  lobe,  and  reaches  as  far  backward  as  the  gastro-parietal  band. 
It  has  been  aheady  stated  that  the  alimentary  tube  is  composed  of  an  inner  and  an 
outer  membrane;  these  are  its  proper  walls;  there  is,  however,  another  membrane 
external  to  both,  sheathing  the  entire  tube  very  closely  throughout,  except  at  the  ante- 
rior extremity  of  the  oesophagus.  This  sheath  gives  off  several  delicate  membranous 
bands,  which  go  to  the  parietes  of  the  perivisceral  chamber,  suspending  the  alimentary 
tube  in  its  centre.  These  bands  have  been  named  respectively  by  Professor  Huxley^, 
who  flrst  described  them,  the  mesentery,  the  gastro-parietal  and  the  iho-parietal  bands. 
The  mesentery  is  divided  into  two  portions,  the  dorsal  and  ventral ; the  latter  extends 
from  the  lower  or  anterior  face  of  the  intestinal  tube  and  stomach,  to  the  posterior  face 
of  the  oesophagus  and  the  confluent  extremities  of  the  occlusor  muscles,  and  passes  above 
the  lower  portion  of  the  intestine  as  a narrow  free  edge.  The  dorsal  portion  extends 
from  the  dorsal  face  of  the  stomach  and  anterior  wall  of  the  chamber  to  its  upper  wall, 
running  along  from  end  to  end  of  the  septum  of  the  dorsal  valve.  The  mesentery, 
therefore,  sustains  the  alimentary  canal  in  the  vertical  plane,  and  together  with  it 
divides  the  anterior  portion  of  the  chamber  into  lateral  halvns. 
The  gastro-parietal  bands  are  three  in  number,  two  lateral  and  one  median.  The 
lateral  ones  pass  from  the  sides  of  the  stomach,  ascending  outwards  and  forwards  to  the 
dorsal  extremities  of  the  posterior  occlusor  muscles,  around  which  they  form  an  imper- 
fect sheath.  The  median  gastro-parietal  band  has  hitherto  escaped  notice ; it  originates 
at  the  upper  surface  of  the  stomach,  between  the  lateral  pair,  and  passes  upwards  and 
backwards  to  the  dorsal  wall,  a little  in  advance  of  the  hinge-plate ; it  is  very  short. 
All  these  three  bands  originate  in  a ridge  which  stretches  from  side  to  side  of  the  dorsal 
aspect  of  the  stomach. 
The  ilio-parietal  bands  are  prolongations  of  two  narrow  lateral  expansions,  which 
extend  from  the  sides  of  the  stomach  to  some  way  down  the  intestine ; they  pass  out- 
wards and  upwards,  and  go  to  the  sides  of  the  chamber  a little  below  the  junction  of 
the  two  pallial  lobes.  These  bands  give  support  to  the  inner  portion  of  the  peculiar 
organs  denominated  hearts  by  Professor  Owen. 
' Plate  LYIII.  figs.  9,  10.  ^ Proceedings  of  tlie  Eoyal  Society,  vol.  vii.  pp.  109,  110  (1854). 
