ME.  HAJS^COCK  ON  THE  OEGANIZATION  OF  THE  BEACHIOPODA. 
807 
These  large  canals  at  the  roots  of  the  arms  are  continuous  with  those  of  the  spirals,  and 
terminate  in  blind  sacs,  one  at  each  side  of  the  oesophagus,  close  to  the  mouth.  On 
making  a transverse  section  of  this  part  of  the  arm',  the  enlarged  terminal  portion  of  the 
brachial  canal  is  seen  connected  with  the  external  edges  of  the  dorsal  and  ventral 
members  of  the  loop ; and  the  pallial  lobule,  stretching  between  the  inner  edges,  forms 
a sort  of  flattened  inner  tube.  This  inner  tube  opens  widely  into  the  perivisceral 
chamber,  is  in  fact  a prolongation  of  this  chamber,  and  terminates  at  the  anterior 
extremity  of  the  lateral  portion  of  the  loop,  forming  what  may  be  designated  the  bra- 
chial pouches. 
At  the  proximal  extremity  of  the  arms  the  semi-cartilaginous  grooved  ridge  is  continued 
across  the  median  line^.  The  cirri  are  inserted  into  an  elevated  base  behind  the  groove, 
and  in  front  of  the  latter  there  is  a stifiish  membranous  fold,  which  runs  along  with  it 
and  the  cirri  to  the  distal  extremity.  This  brachial  fold  rising  from  the  edge  of  the 
groove  forms  it  into  a deep,  narrow  gutter,  within  which  the  mouth  is  situated  at  the 
posterior  junction  of  the  arms,  the  cirri  of  course  being  behind,  the  fold  in  front.  The 
cirri  are,  there  can  be  little  doubt,  clothed  with  vibratile  cilia  as  in  the  Polyzoa,  and  the 
currents  they  excite  will  draw  the  nutrient  molecules  down  into  the  gutter,  along  which 
they  will  be  hurried  to  the  mouth,  the  gutter  itself  being  undoubtedly  likewise  ciliated. 
The  cirri  are  very  fine,  somewhat  compressed  on  the  contiguous  sides,  stiff  and  rigid 
towards  the  base,  and  tapering  and  delicate  towards  the  tips,  which  are  rounded ; they 
are  close-set  and  arranged  in  a double  series,  the  cirri  in  each  alternating.  Hitherto 
they  have  been  generally  described  as  forming  only  a single  series ; but  this  is  a matter 
of  such  easy  demonstration,  a single  transverse  section^  of  the  arm  being  sufficient  to 
prove  that  there  are  two,  that  it  is  difficult  to  understand  how  it  ever  came  to  be  asserted 
that  there  was  only  one.  The  series  is  double  throughout,  not  only  in  W.  australis, 
but  in  all  the  other  Brachiopods  which  have  been  examined  on  this  occasion,  namely, 
W.  cranium,  Terehratulina  caput-serpentis,  Mhynclionella  psittacea.  Lingula  anatina, 
L.  affinis,  and  Crania  anomala. 
The  cirri  are  undoubtedly  contractile  to  some  extent,  and  are  also  endowed  with  the 
power  of  motion  individually,  as  proved  by  the  observations  of  Mr.  BAEEETTb  It  would 
likewise  appear  that  the  entire  fringe  of  cirri  can  be  elevated  or  depressed,  there  being 
ample  means  prorided  for  the  purpose.  The  walls  of  the  brachial  canal  are  tolerably 
well  supplied  with  delicate  muscular  fibres,  which  run  diagonally  round  the  tube, 
and  are  most  strongly  developed  towards  the  sides  near  the  grooved  ridge.  An  indi- 
stinct band  of  exceedingly  delicate,  longitudinal  fibres  may  also  be  observed  nearly 
opposite  to  it.  I have,  however,  completely  failed  to  discover  anything  like  the  double 
spiral  arrangement  of  fibres  described  by  Professor  Owen®,  either  in  Waldheimia  or 
Rhynchonella,  and  cannot  but  believe  that  this  distinguished  anatomist  has  been  misled 
by  the  blood-sinuses,  which  are  very  numerous  in  the  wall  of  the  brachial  canal,  form- 
' Plate  LVI.  fig.  2.  = Plate  LV.  figs.  1,  2.  ® Plates  LVI.  fig.  3 ; LVIII.  figs.  1,  2. 
^ Op.  cit.  ‘ Introduction  to  Davidson’s  ‘British  Fossil  Brachiopoda,’  p.  10. 
AIDCCCLVIII.  5 N 
