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ME.  HAIN^COCK  ON  THE  OEGANIZATION  OE  THE  BEACHIOPODA. 
points.  These  points  of  union  are  circular,  and  are  arranged  in  groups  of  various  forms 
and  sizes,  generally  more  or  less  rounded. 
When  the  specimen  is  in  good  condition,  this  heautiful  system  of  peripheral  lacunes 
is  well  displayed,  the  channels  or  spaces  beirrg  then  for  the  most  part  filled  with  blood- 
corpuscles,  which  give  to  the  lacunes  air  opake,  yellouish  hue,  rendering  them  as 
distinct  and  sharp  as  though  they  had  been  injected.  And  as  the  circular  points  of 
union  of  the  two  walls  are  transparent,  and  are  consequently  liable  to  appear  of  a darkish 
tint,  they  show  like  spots  on  a light  ground,  not  altogether  unlike  the  markings  on  a 
leopard’s  skin.  In  other  lights  the  whole  has  a beautiful  lace-like  delicacy.  These 
lacunes  occasionally  assume  a branched  or  dendritic  character,  particularly  as  they 
approach  the  margin  of  the  mantle,  where  they  become  minute  and  run  almost  parallel 
to  each  other.  These  minute  twigs  pass  on  to  the  external  margin  of  the  lobe. 
The  inner  lacunes,  or  those  of  the  inner  wall  or  floor  of  the  great  pallial  sinuses,  have 
a very  different  character ; they  resemble,  however,  considerably  the  lacunes  in  the  ante- 
rior wall  of  the  body  with  which  they  are  in  direct  communication.  They  are  in  the 
form  of  numerous,  narrow  channels,  which,  anastomosing  at  various  points,  compose  a 
network  of  very  long,  transverse  meshes ; thus  most  of  the  channels  cross  the  direction 
of  the  sinuses  and  run  parallel  to  each  other.  They  communicate  at  the  sides  of  the 
sinuses  with  the  other  system  of  pallial  lacunes,  and  along  the  centre  with  that  of  the 
membrane  suspending  the  genitalia.  This  membrane,  like  all  the  others,  consists  of  two 
layers,  and  has  its  own  system  of  anastomosing  lacunes,  which,  on  the  one  hand,  is  in 
connexion  with  the  genital  or  pallial  artery;  and  on  the  other,  as  just  stated,  with  the 
inner  pallial  lacunes. 
The  lacunes  in  the  floor  of  the  inner  sinuses  of  the  dorsal  lobe  exhibit  a slight  modi- 
fication; here  they  are  not  in  regular  transverse  order,  but  form  a pretty  even  net- 
work. 
A few  words  more  will  complete  the  description  of  the  pallial  lobes.  The  margins ' of 
these  organs  are  complicated  in  then.'  structure.  ■ Both  the  membranes  of  the  external 
lamina  seem  to  extend  to  the  extreme  edge  of  the  shell,  and  the  margin  of  the  outer 
reticulated  one  is  apparently  continuous  with  the  periostracum.  A little  within  the 
edge  of  the  shell  the  inner  lamina  is  thickened  and  forms  a fold,  which  is  free  in  fr'ont 
and  capable  of  extension  and  contraction.  It  is  within  this  fold  that  the  marginal  setse 
are  developed ; they  originate  in  follicles,  fr:om  which  occasionally  two  or  three  setfe 
issue,  though  more  frequently  only  one,  and  project  from  depressions  in  the  margin.  In 
connexion  with  the  inner  membrane  of  the  external  lamina,  numerous  muscular  fibres 
are  formed,  which,  increasing  in  number  as  they  extend  forward,  pass  into  the  marginal 
fold,  and  when  in  action  will  withdraw  its  free  border ; while  a narrow  muscular  cord, 
which  runs  round  immediately  within  the  margin  of  the  lobe,  will  probably  control  the 
lateral  movements  of  the  setse.  The  setae  ^ themselves,  which  are  placed  a little  apart 
from  each  other,  taper  gradually  to  exceedingly  fine  points,  and  are  transparent,  glisten- 
* Plates  LYIII.  figs.  5,  6 ; LIX.  fig.  1.  - Plate  LVII.  fig.  5. 
