832 
ME.  H'i]S-COCK  OX  THE  OEGAXIZATIOX  OE  THE  BEACHIOPODA. 
distinctly  obsen^ed  in  fK  australis,  but  is  sufficiently  conspicuous  in  B.  jpsittacea^,  in 
which  it  was  traced  from  the  junction  of  the  arms  to  a considerable  distance  along  them, 
charged  with  blood-corpuscles.  In  this  species  the  channels  of  the  anterior  wall  of  the 
body  run  in  the  direction  of  these  canals. 
The  walls  of  the  great  brachial  canal  like  those  of  the  body,  are  composed  of  two 
layers,  the  inner  displaying  muscular  fibres,  as  already  stated,  the  outer  being  homo- 
geneous : both  are  clothed  with  an  epithelium.  That  lining  the  canaP  is  distinctly  formed 
of  irregular  polygonal  scales  with  scattered  granules.  The  external  epithelium  is  stouter 
than  the  other,  and  is  more  crowded  with  granules.  In  the  space  between  the  two  layers 
there  are  numerous  lacunes  resembling  those  in  the  anterior  parietes ; but  they  form 
pretty  regular  parallel  channels  of  limited  width,  which  occasionally  anastomosing  run 
diagonally  round  the  canal.  These  channels  take  their  origin  in  a minute  plexus  situated 
directly  above  the  membrane  connecting  the  spirals.  The  afferent  canal  runs  along  at 
the  other  side  of  or  below  this  membrane,  and  the  network  of  channels  excavated  in  its 
substance,  which  is  thick  and  of  a semi-cartilaginous  appearance,  becomes  the  medium 
of  communication  between  the  afferent  canal  and  the  minute  plexus  just  mentioned. 
At  the  points  of  communication  the  channels  in  the  membrane  form  minute  networks. 
Towards  the  interior  of  its  substance  the  meshes  are  much  enlarged.  After  ■v\indhig 
round  the  canal,  the  brachial  blood-channels  are  again  lost  in  a similar  minute  plexus 
situated  at  the  base  of  the  semi-cartilaginous  ridge  supporting  the  cirri.  This  plexus  is 
in  connexion  with  another  system  permeating  the  substance  of  the  ridge,  which  conducts 
the  blood  through  amidst  the  roots  of  the  cirri  to  a peculiar  network  of  chaimels,  in 
like  manner  excavated  in  the  substance  of  the  ridge  next  the  brachial  fold.  This,  the 
great  brachial  plexus'*,  is  supplied  with  central  trunks  which  give  off  branches;  some  of 
these  pass  towards  the  base  of  the  ridge,  and,  anastomosing  as  they  go,  result  in  a 
I’egular  series  of  transverse  channels,  which,  on  reaching  the  small  canal  running  along 
at  the  bases  of  the  cirri,  attain  proper  walls,  and  as  distinct  vessels®  pass  through  the 
wall  of  this  canal,  and,  turning  in  the  direction  of  the  cirri,  enter  the  bases  of  these 
organs  in  regular  parallel  order.  These  vessels  are  the  afferent  brachial  arteries. 
The  small  canal  at  the  bases  of  the  cirri  is  situated  within  the  large  brachial  canal, 
and  is  apparently  formed  by  a duplicature  of  the  lining  membrane  of  the  latter.  The 
septum,  thus  dividing  the  two  canals,  is  composed  in  the  usual  Avay  of  two  layers,  and 
is  provided  with  its  system  of  anastomosing  channels  which  open  into  those  of  the  great 
canal.  The  small  one  is  the  efferent  brachial  canal.  The  bases  of  the  cirri,  which  ai’c 
situated  in  a groove,  open  freely  into  it  in  a double  series.  The  cirri  themselves®  are 
tubular  with  their  apices  rounded  and  imperforate;  they  are  a little  fiattened  at  the 
approximate  sides,  and  are  apparently  lined  throughout  by  the  continuation  of  a deli- 
cate granular  epithelium  which  coats  the  efferent  canal.  Towai’ds  the  base  the  walls 
are  stout,  and  have  a ridged,  horny  character;  they  gradually  become  attenuated 
' Plate  LX.  fig.  10.  * Plate  LIX.  fig.  2.  * Plate  LXI.  fig.  6. 
^ Plate  LVIII.  fig.  J.  ‘ Plate  LVIII.  fig.  3.  ‘ Plate  LYIIl.  fig.  4. 
