ME.  HANCOCK  ON  THE  OEOANIZATION  OE  THE  BEACHIOPODA. 
809 
The  arms  of  B.  jpsittacea  are  * totally  deprived  of  calcareous  support  except  at  their 
origin,  where  they  are  sustained  by  the  two  hinge-processes,  or  oral  laminee,  the  points 
of  which  reach  as  far  forward  as  their  external  margins.  They  fill  up  the  greater 
portion  of  the  pallial  chamber;  and  in  their  arrangement  accurately  resemble  the 
calcareous  spirals  of  Atrypa  reticularis^  a Silurian  fossil,  only  their  approximate  sides 
are  not  flattened.  The  arm  throughout  is  composed  of  a slightly  depressed  tube  or 
canal,  carrying  along  its  outer  margin  the  semi-cartilaginous  grooved  edge,  bearing  the 
fringe  of  cirri  as  in  W.  australis.  The  brachial  fold  in  front  of  the  groove  is  largely 
developed,  and  completely  overlaps  it.  The  tube  or  great  brachial  canal  terminates  at 
the  side  of  the  oesophagus  in  a delicate  membranous  sac  of  no  great  extent,  which  pro- 
jects into  the  perivisceral  chamber,  as  first  noticed  by  Professor  Huxley.  The  use  of 
these  sacs  is  not  obvious,  though  perhaps  they  may  be  in  some  way  connected  with  the 
limited  movements  of  the  arms ; or  it  is  not  altogether  unlikely  that  they  are  for  the 
purpose  of  increasing  the  surface  through  which  the  fluids  contained  in  the  canal  and 
chamber  may  act  upon  each  other. 
The  parietes^  of  the  great  canal  are  somewhat  stouter  than  in  W.  australis ; the  mus- 
cular fibres,  however,  are  arranged  in  the  same  manner  as  in  it,  but  are  more  numerous, 
particularly  the  longitudinal  ones,  which  form  a well-defined  band  towards  the  proximal 
extremity  of  the  arm.  There  is  not  here,  any  more  than  in  the  Terehratulidce,  any  trace 
of  a double  spiral  disposition  of  fibres ; nor  is  there  any  more  reason  to  suppose  that  the 
brachial  organs  can  be  um’olled.  The  primary,  if  not  the  only,  use  of  the  contractility 
of  the  walls  of  the  great  canal,  would  appear  to  be,  in  Bhynchonella  as  well  as  in  Wald- 
heimia.,  to  reduce  the  calibre,  and  by  the  pressure  of  the  contained  fluid  to  stitfen  the 
organ  so  that  the  grooved  ridge  and  cirri  may  be  brought  into  play,  A slight  opening 
of  the  cod.  most  likely  takes  place,  and  the  band  of  longitudinal  fibres  seems  to  indicate 
that  the  first  coil,  at  least,  may  be  to  some  extent  drawn  in,  and  thus  perhaps  the  whole 
brought  closer  up  to  the  posterior  wall  of  the  palhal  chamber.  But  it  must  not  be 
overlooked  that  longitudinal  fibres  appear  necessary  to  the  complete  action  of  those 
destined  to  compress  the  canal  by  preventing  any  tendency  to  the  unrolling  of  the  coil. 
Professor  Owen,  however,  has  expressed  an  opinion  that  the  multispiral  arms  of 
Bhynchonella  and  Lingula  can  be  unrolled  and  thrust  out  of  the  pallial  chamber ; and 
that  in  the  latter  genus  they  probably  assist  in  opening  the  valves ; likewise  that  the 
coiled  portion  of  the  arms  of  the  Terehratulidoe  can  be  so  far  unrolled,  as  in  like  manner 
to  react  upon  the  closed  valves  of  the  shelP.  Such  an  opinion  seems  scarcely  tenable. 
A large  development  of  the  brachial  apparatus  seems  necessary  in  the  economy  of  the 
animal,  and  the  various  ways  in  which  it  is  folded  up  and  disposed  within  the  pallial 
chamber  are  only  so  many  methods  of  arranging  within  a limited  space  the  requisite 
extent  of  organ.  And  there  is  sufficient  proof,  in  several  fossil  genera,  that  the  unrolling 
’ Plates  LV.  fig.  3 ; LX.  fig.  3 ; LXI.  fig.  2.  ^ Plate  LX.  fig.  9. 
* Trans.  Zool.  Soc,  vol.  i.  p.  150,  1835;  and  Introduction  to  Davidson’s  ‘British  Fossil  Brachiopoda,’ 
pp.  10,  11. 
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