ME.  HANCOCK  ON  THE  OEGANIZATION  OF  THE  BEACHIOPODA. 
839 
duplicatures  of  this  membrane.  Thus  it  appears  that  the  various  systems  of  peripheral 
lacunes  may  be  considered  as  one  great,  continuous  blood-reservoir,  notwithstanding  the 
multiplicity  of  its  parts.  Those  of  the  brachial  apparatus  may  perhaps  be  an  exception 
to  this,  and  form,  to  some  extent,  a distinct  system,  though  they  undoubtedly  commu- 
nicate with  the  lacunes  of  the  parietes.  This  distinctness,  however,  is  probably  more 
apparent  than  real,  and  the  great  canals  of  the  arms  are  perhaps  only  prolongations  of 
the  general  cavity,  though  they  are  cut  otf  from  it  by  a delicate  membrane.  The  two 
membranes  composing  the  walls  of  these  canals  would  seem  to  indicate  this. 
It  is  also  worthy  of  remark,  that  the  channels  which  have  been  denominated  arteries 
soon  lose  the  appearance  of  having  proper  walls.  At  their  origin  in  the  Terebratulidoe, 
they  seem  to  be  within  the  visceral  sheath,  bulging  it  out ; and  as  they  pass  over  the 
parietal  bands,  they  have  a peculiar  texture  and  are  somewhat  different  in  colour.  After 
reaching  the  genitalia  their  character  is  a little  changed,  they  having  assumed  the  ap- 
pearance of  being  formed  by  the  membrane  suspending  these  organs,  as  if  they  had 
become  mere  channels  left  within  the  two  layers  composing  it.  This  is  also  the  character 
of  the  branchio-systemic  vein,  which  seems  to  be  nothing  more  than  a space  left  between 
the  layers  of  the  mesenteric  membrane,  though,  as  it  approaches  the  heart,  it  has  pro- 
bably proper  walls.  The  efferent  brachial  canal  has  all  the  appearance  of  being  formed 
by  a duplicature  of  the  lining  membrane  of  the  great  brachial  canal ; but  in  Lingula  it  is 
excavated  in  the  substance  of  the  brachial  ridge.  There  can  be  little  doubt,  however, 
that  the  vessels  which  have  been  termed  afferent  brachial  arteries,  those  vessels  which 
drain  the  great  brachial  plexus  and  pass  up  the  cnri,  have  true  walls. 
This  latter  fact  seems  to  favour  the  opinion  that  the  lacunes  themselves  may  have 
proper  walls,  and  that  all  these  channels  are  connected  from  end  to  end  with  the  central 
organs  by  continuity  of  tissue;  to  favour,  in  fact,  John  Huntee’s  views  with  regard  to 
the  blood-sinuses  in  the  lower  animals.  I have  failed,  however,  to  discover  any  such 
continuity  of  tissue  demanded  by  the  hypothesis  of  our  great  anatomist.  If  such  had 
any  other  than  a hypothetical  existence,  it  ought,  one  would  think,  to  be  demon- 
strable in  the  Terehratulidm  \ for  perhaps  in  no  other  molluscous  animal  are  the  peri- 
pheral blood-channels  so  beautifully  displayed,  and,  from  the  peculiarity  of  their  arrange- 
ment, so  easily  submitted  to  examination  by  even  the  highest  powers  of  the  microscope. 
The  tissues  are  very  transparent,  and  the  blood-channels  or  lacunes  most  distinctly  de- 
fined ; the  only  problem  being,  are  these  latter  lined  with  a membrane  % They  contain 
apparently  cellular  tissue ; does  the  membrane  of  continuity  thread  the  meshes  of  this 
tissue  \ or  may  the  tissue  itself  turn  out  to  be  the  required  walls  These  questions  must 
for  the  present  remain  unanswered,  and  in  the  meantime  it  would  appear  best  to  deno- 
minate the  channels  lacunes,  as  has  been  done  throughout  this  communication,  for  to 
use  the  term  sinus  would  be  to  imply  that  they  had  true  or  proper  walls ; a matter,  to 
say  the  least,  of  extreme  doubt. 
5 E 
MDCCCLVIII. 
