ME.  HANCOCK  ON  THE  OEGANIZATION  OF  THE  BEACHIOPODA. 
845 
Neevous  System. 
This,  like  all  the  other  organs,  has  been  hitherto  imperfectly  described,  though  its 
examination  does  not  present  any  peculiar  difficulties ; on  the  contrary,  the  nerves  are 
very  distinctly  displayed,  and  can  be  easily  traced.  The  nervous  element,  however,  on 
the  whole,  exhibits  a rather  low  state  of  development,  lower,  undoubtedly,  than  that 
of  the  Lamellibranchs,  and  not  very  much  higher  than  that  of  the  Ascidians.  As  in  all 
true  Mollusks,  the  ganglionic  centres  are  placed  in  connexion  with  a nervous  collar 
surrounding  the  oesophagus. 
In  TK  australis  the  collar'  is  situated  at  the  commencement  of  the  alimentary  tube, 
and  there  are  five  nervous  centres,  three  of  which,  on  account  of  their  superior  size,  may 
be  assumed  to  be  the  principal  oesophageal  ganglia.  These  lie  amidst  blood-lacunes, 
between  the  two  membranes  forming  the  anterior  wall  of  the  perivisceral  cavity  imme- 
diately below  the  oesophagus,  or  rather,  on  account  of  the  mouth  being  bent  down, 
behind  it.  One,  the  largest,  is  central  and  anterior ; the  other  two  form  a pair,  and  are 
lateral  and  posterior.  The  former  is  fiattened  or  depressed,  and  elongated  transversely 
across  the  median  line ; the  anterior  and  posterior  margins  are  parallel,  the  sides  slope 
inwards  and  backwards ; at  each  side  in  front  it  is  prolonged  into  a stout  nerve,  which 
immediately  di\ides  into  an  upper  and  a lower  trunk.  The  latter  is  considerably  the 
smaller,  and  goes  to  supply  the  brachial  apparatus,  running  along  the  brachial  ridge 
below  the  cirri. 
The  other  trunk  does  not  send  any  of  its  branches  to  the  ventral  pallial  lobe,  as  it  has 
been  stated  to  do ; but  it  supplies  all  the  nerves  of  the  dorsal  lobe,  besides  others  to  the 
anterior  wall  of  the  periAsceral  chamber  and  to  the  occlusor  muscles.  A single  nerve 
or  two  only  are  sent  to  the  wall  of  the  chamber,  and  these  pass  from  the  root  of  the 
trunk ; the  trunk  then  advances  upwards  within  the  parietes,  and  on  reaching  the 
external  margin  of  the  muscles  the  filaments  separate  and  form  a loop®,  from  which 
numerous  twigs,  dividing  and  subdmding,  enter  both  sides  of  the  extremity  of  the 
posterior  occlusor,  richly  supplying  it.  This  rather  peculiar  and  beautiful  arrangement 
of  nerves  has  hitherto  escaped  observation,  although  the  loop  has  been  described  and 
figured®.  The  trunk-nerve  is  continued  from  the  end  of  the  loop  round  the  outer  margin 
of  the  anterior  occlusor  muscle,  giving  to  it  a few  twigs  similar  to  those  supplied  to  the 
posterior  occlusor.  It  then  gives  off  seven  pallial  nerves ; other  two  pallial  nerves  pass 
off  from  the  loop ; thus  there  are  nine  sent  to  each  side  of  the  dorsal  lobe,  all  of  which 
run  in  a radiating  manner  in  connexion  with  the  outer  layer  of  the  inner  lamina,  dividing 
dichotomously  three  or  four  times,  though  not  very  regularly.  I have  entirely  failed 
to  detect  the  circumpallial  nerve  described  by  Professor  Owen'*,  and  cannot  but  suppose 
that  the  delicate  muscular  cord®,  previously  mentioned  as  running  within  the  margin  at 
• Plates  LII.  fig.  10 ; LYI.  fig.  1 ; LVII.  fig.  2 ; LXII.  fig.  11 ; LXIII.  fig.  2. 
^ Plate  LXII.  figs.  11,  15. 
* Introductiou  to  Davidsos’s  ‘ British  Fossil  Brachiopoda,’  p.  12.  pi.  2.  fig.  2. 
^ Ibid.  ® Plate  LVIII.  fig.  G. 
