14^  MR.  R.  OWEN  ON  THE  ANATOMY  OF  THE  BRACHIOPODA. 
margin,  are  formed  the  large  trunks  above  described.  The  size  of  these  vessels  at 
once  suggests  their  subserviency  to  other  purposes  than  that  of  merely  returning  the 
blood  necessary  for  the  nutrition  of  the  mantle.  The  four  vessels  of  the  perforated 
lobe  of  the  mantle  form  two  trunks  near  the  visceral  mass,  which  pass  exterior  to  the 
muscular  disks,  and  joining  those  of  the  opposite  lobe,  enter  the  two  hearts,  or  dilated 
sinuses,  which  are  situated  exterior  to  the  liver,  and,  in  Ter.  Chilensis  and  Ter.  Sowerbii, 
just  within  the  origins  of  the  internal  calcareous  loop.  Corresponding  to  the  large 
branchial  veins  there  appear  under  the  microscope  much  smaller  vessels,  which  I 
regard  as  the  branchial  arteries ;  these  run  parallel  with  the  middle  of  the  branchial 
veins,  and  terminate  in  the  margins  of  the  mantle  from  which  the  veins  commence. 
These  margins  present  the  following  appearances  when  viewed  with  a  high  magnifying 
power: — they  are  puckered  at  regular  distances,  the  puckerings  being  apparently 
caused  by  the  insertions  of  delicate  cilia,  which  pass  as  far  within  the  mantle  as  they 
project  beyond  it :  in  the  interspaces  of  the  cilia  the  margin  of  the  mantle  is  minutely 
fringed:  and  within  this  fringe  is  a  canal,  which  extends  along  the  whole  circumference 
of  the  lobe,  and  from  which  the  branchial  veins  appear  to  take  their  origin :  the 
marginal  canal  is  contracted  where  the  cilia  are  inserted  into  it,  which  gives  it  a 
sacculated  appearance,  like  that  of  the  canal  of  Petit  in  the  human  eye. 
The  uniform  results  of  repeated  observations  on  all  the  specimens  of  Terebratula 
which  I  had  at  my  disposal,  convinced  me  that  the  vascular  mantle  was  the  chief,  it" 
not  the  sole  respiratory  organ ;  and  the  utility  of  the  marginal  cilia  in  reference  to  this 
function  can  now  be  readily  appreciated,  in  consequence  of  the  discovery  of  the  remark- 
able property  which  cilia  possess  of  exciting  determinate  currents  in  the  surrounding 
water, — a  discovery  for  which  the  scientific  world  is  indebted  to  the  observations  of 
Dr.  Grants  Dr.  Sharpey2,  and  M.  RaspaiP. 
The  imperforate  valve  in  many  species  of  Terebratula  is  characterized,  as  is  well 
known,  by  a  peculiar,  complex,  and  extremely  delicate  testaceous  apparatus,  attached 
to  its  internal  surface :  now,  as  in  those  recent  specimens  with  the  soft  parts  which  I 
have  examined,  and  in  which  this  structure  existed,  it  was  found  to  give  attachment 
to  the  arms,  it  becomes  necessary  to  describe  it  before  speaking  of  those  parts. 
The  principal  part  of  this  internal  skeleton,  as  it  may  be  termed,  consists  of  a 
slender,  flattened,  calcareous  loop,  the  extremities  of  which  are  attached  to  the  lateral 
elevated  ridges  of  the  hinge ;  the  crura  of  the  loop  diverge,  but  again  approximate  to 
each  other  as  they  advance  for  a  greater  or  less  distance  towards  the  opposite  margin 
of  the  valve ;  the  loop  then  suddenly  turns  towards  the  perforate  valve,  and  is  bent 
back  upon  itself  for  a  greater  or  less  extent  in  different  species.  When  the  loop  is  very 
short  and  narrow,  as  in  Ter.  vitrea,  Brug.,  there  is  but  a  small  tendency  towards  a  re- 
flected portion ;  but  where  the  loop  is  of  great  length  and  width,  as  in  Ter.  Chilensis^ 
'  Edinburgh  Phil.  Journal,  vol.  xv.  p.  150.    Brewster's  Journal,  vol.  vii.  p.  121. 
f  Edinburgh  Journ.  of  Nat.  and  Geogi-.  Science,  vol.  ii.  p.  334.  ^  Chimie  Organique,  p.  247. 
