842 
ME.  HAJ^COCK  ON  THE  OEGANIZATION  OE  THE  BEACHIOPODA. 
side,  numerous  irregular  ramuscules.  The  outer  ones  stretch  to  the  free  edge  of  the 
pallial  border,  the  inner  extend  almost  to  the  lateral  wall  of  the  body. 
In  L.  affinis  the  branches  of  the  pallial  sinuses^  taper  gradually  and  are  a good 
deal  subdivided,  and  though  considerably  bulged  out  do  not  assume  the  appearance  of 
laminae.  The  two  main  trunks  of  the  ventral  lobe  turn  suddenly  inwards,  and  meet  on 
the  median  line,  forming,  as  it  were,  an  arch  from  which  the  branches  radiate  to  the 
pallial  border. 
The  branches  of  the  pallial  sinuses  are  frequently  choked  up  with  a reddish-brown 
matter,  which  on  examination  with  a microscope  is  found  to  be  entirely  composed  of 
large,  roundish,  scale-like  bodies,  no  doubt  from  the  epithelial  lining  of  the  chamber  and 
its  ramifications,  which  lining  in  all  the  Brachiopods  is  exceedingly  deciduous,  and 
rarely  observed  in  its  natural  position.  In  the  Terebratulidoe  the  sinuses  are  also  not 
unfrequently  clogged  with  matter,  which  has  been  taken  for  coagulated  blood ; but  on 
attentive  inspection  it  is  found  to  be  made  up  of  similar  epithelial  scales. 
It  has  been  already  stated  that  this  complicated  chamber  is  lined  throughout  with  a 
membrane  as  well  as  all  the  ramifications  of  the  pallial  sinuses ; and  that  folds  of  this 
membrane  form  the  various  bands  which  pass  from  the  parietes  of  the  body  to  the  vis- 
ceral sheath,  which  is  itself  apparently  an  involution  of  the  same  membrane.  The 
muscles  are  likewise  sheathed  by  it.  To  arrive  at  an  understanding  of  the  function  of 
this  chamber,  so  formed,  it  is  necessary  in  the  first  place  to  determine  the  natiu’e  of  this 
lining  membrane.  Where  shall  we  look  for  its  homologue  1 Does  it  form  an  essential 
element  in  molluscan  organization!  For  answers  to  these  important  questions  I am  in- 
debted to  Professor  Huxley,  a gentleman  from  whom  I have  derived  much  and  important 
information  in  connexion  with  the  subject  of  this  paper,  and  who  has  with  the  utmost 
candour  and  liberality  communicated  to  me  his  views  on  these  very  interesting  points, — 
interesting,  for  they  relate  to  all  that  is  most  anomalous  in  the  anatomy  of  these  ani- 
mals, consequently  to  that  which  particularly  demands  our  attention. 
Before  these  questions  can  be  solved  it  is  necessary  to  refer  to  the  Ascidice,  wliich 
rank  with  the  lowest  organized  Mollusks.  These  animals  are  provided  with  several 
envelopes  or  tunics ; in  the  first  place  there  is  the  external  covering  or  test ; next  the 
lining  membrane  or  outer  tunic;  and  within  it  the  thu'd  tunic  enclosing  the  space 
termed  the  atrium  by  Professor  Huxley^.  The  space  thus  circumscribed  communicates 
externally  through  the  excurrent  tube ; and  the  blood  flows  between  the  outer  tunic  on 
the  one  hand,  and  the  third  tunic  and  the  intestine  on  the  other. 
Now  in  the  Brachiopods  we  hav^e  the  shell  which  is  equal  to  the  test  of  the  Ascidian ; 
then  comes  the  mantle  in  the  place  of  the  outer  tunic ; but  there  is  nothing  to  represent 
the  third  tunic,  unless  the  lining  membrane  of  the  perivisceral  chamber  does  so ; and 
that  it  is  really  the  homologue  of  the  third  tunic  there  can  be  little  doubt.  We  have 
seen  that  the  blood  flows  between  this  lining  membrane  and  the  mantle,  and  between 
Plate  LXVI.  figs.  1,  2,  3. 
' Eeport  Brit.  Assoc.  1852,  Transactions  of  the  Sections,  p.  76. 
