848 
3^IE.  HA^s^COCK  OX  THE  OEGAXIZATIOX  OE  THE  BEACHIOPODA. 
plied  with  nervous  filaments,  which  stream  in  a radiating  manner  towards  the  margin ; 
those  more  particularly  mentioned  by  Professor  Owex  as  supplying  the  muscles,  have 
been  described  as  blood-channels  for  reasons  pre\iously  given. 
Having  now  gone  over  the  whole  anatomy  of  these  animals,  it  is  necessary  to  make  a 
few  concluding  remarks  as  to  their  affinities.  But  before  entering  upon  these,  a word 
or  two  may  be  said  respecting  the  growth  of  the  shell ; to  add  anything  on  its  structure 
would  be  mere  supererogation.  Dr.  Caepextee^  having  fully  and  accurately  treated  on 
this  interesting  portion  of  the  subject.  In  respect  to  the  increment,  this  takes  place, 
as  in  all  mollusks,  mainly  at  the  margin  of  the  shell,  and  here  the  mantle  in  the  Tere- 
hratulidce  presents  a peculiar  appearance.  When  the  shell  is  dissolved  in  acid,  the  free 
border,  which  projects  beyond  the  marginal  fold,  and  which  is  applied  to  the  extreme 
edge  of  the  shell,  can  be  examined  with  advantage.  The  pallial  cseca  are  then  com- 
pletely exposed  appended  to  the  membrane  in  various  stages  of  development,  and  the 
spaces  between  them  are  found  studded  all  over  with  rather  large,  clear,  oval,  cell-like 
spots'^,  which  are  arranged  mth  considerable  regularity  in  rows,  so  that  those  in  the 
approximate  rows  alternate.  These  spots  apparently  correspond  to  the  bases  of  the 
prismatic  columns  of  the  shell ; and  if  it  be  allowed  that  they  represent  spaces  in  which 
calcareous  granules  had  been  accumulated,  it  is  easy  to  understand  how  the  fibrous  or 
columnar  structure  is  formed.  A succession  of  layers  of  such  accumulated  granules, 
deposited  one  after  the  other,  would  result  in  the  peculiar  shell  formation  of  the  Bra- 
cMopoda. 
It  has  been  stated  by  Dr.  Caepentee®,  and  the  statement  is  perfectly  correct,  that  the 
extremities  of  the  prisms  are  not  visible  on  the  external  sm’face.  He  supposes  that  this 
arises  from  the  incorporation  of  the  periostracum  with  the  terminations  of  the  prisms. 
But  there  appears  to  be  a thin,  homogeneous,  external  layer  or  stratum  of  calcareous 
matter  observable  at  the  margin  of  the  valves  in  young  indi^iduals,  particularly  of  T. 
caput-serjpentis,  in  which  this  layer  is  apparently  thicker  than  usual.  If  tliis  be  correct, 
and  I can  scarcely  doubt  it,  the  shell  of  some  species,  at  least,  is  composed  of  two  layers, 
in  which  the  structure  is  different ; and  thus  in  the  Brachiopods  the  outer  and  inner 
layers  of  the  Lamellibranchs  would  seem  to  be  represented. 
It  may  likewise  be  remarked  that  this  investigation  appears  to  establish  that  there 
are  two  great  types  of  form  in  the  Brachiopoda^  namely,  the  articulated  and  the  unar- 
ticulated ; which  are  characterized,  not  merely  by  the  presence  of  a hinge  in  the  one, 
and  the  deficiency  of  it  in  the  other,  but  by  several  other  important  points  in  their 
structure.  In  the  articulated  division,  the  valves  are  opened  by  the  muscles  acting 
through  the  agency  of  a lever ; in  the  unarticulated,  by  the  pressure  of  fimd  contained 
in  the  perivisceral  cavity ; hence  the  difference  in  the  muscular  arrangement  previously 
described.  In  the  former,  too,  the  intestine  terminates  in  a blind  sac,  while  in  the 
latter  there  is  an  ample  anal  aperture.  These  and  many  other  points  might  be  cited, 
’ Introduction  to  Davidson’s  ‘ British  Eossil  Brachiopoda,’  p.  23. 
Plate  LVIII.  Hg.  8.  ^ Op.  cit.  p.  2G. 
