ME.  HANCOCK  ON  THE  OEG.AN-IZATION  OF  THE  BKACHIOPODA. 
837 
It  is  not  easy  to  explain  this  phenomenon  on  the  supposition  that  tlie  corpuscles  ar(; 
really  blood-globules ; but  the  difficulty  would  at  once  disappear  if  it  were  assumed  that 
some  local  disease  had  produced  this  abnormal  condition  in  the  contents  of  the  caeca ; 
in  which  case  it  must  be  concluded  that  the  corpuscles  form  an  integral  part  of  the  caeca 
themselves,  or  rather  that  they  are  permanently  placed  within  these  organs,  and  do  not 
flow  through  them,  as  they  must  do  were  the  caeca  connected  with  respiration. 
It  is  also  a significant  fact,  that  the  Terehratulidce  are  liable  to  be  covered  with  extra- 
neous matter.  Several  of  the  indi\dduals  of  W.  australis  that  have  passed  through  my 
hands  were  extensively  encrusted  with  zoophytic  and  other  parasitic  growths ; and  one 
or  two  of  T.  caput-sei'pentis  had  both  valves  entirely  enveloped  in  a dense  downy  coat  of 
some  species  of  sponge.  This  appears,  from  what  Foebes  and  Hanley  state  in  their 
‘ British  MoUusca,’  to  be  frequently  the  case,  and  to  have  gained  for  the  specimens  so 
clothed  the  specific  name  of  pubescens.  Were  the  caeca  respiratory  organs,  even  in  a 
secondary  capacity,  this  would  hardly  be  the  case. 
The  best  mode  of  investigating  these  organs'  is  to  dissoh'e  the  shell,  and  then  they 
are  exposed,  in  various  stages  of  growth,  adhering  to  the  margin  of  the  mantle.  They 
are  arranged  in  rows,  and  are  cylindrical,  with  the  distal  extremity  obtusely  rounded, 
and  are  pedunculated  from  the  first ; the  peduncle  is  long  and  narrow ; the  cseca  at  the 
extreme  edge  are  small,  but  rapidly  increase  in  size  backward ; the  terminal  or  enlarged 
portion  is  almost  constantly  stufled  full  with  the  so-called  blood-corpuscles.  When 
observed  in  this  way,  these  organs  have  very  much  the  character  of  secerning  follicles, 
hut  what  function  they  really  subserve  is  difficult  to  determine ; it  may  be  that  they 
have  something  to  do  with  the  growth  and  reparation  of  the  shell,  though  it  is  not  easy 
to  understand  how.  They  are  probably,  as  suggested  by  Professor  Huxley®,  the  homo- 
logical  representatives  of  the  vascular  processes  that  penetrate  the  test  of  the  Ascidian  ; 
and  if  so,  it  would  seem  likely  that  they  have  lost  much  of  then'  functional  importance ; 
and  in  fact  their  entire  absence  in  forms  closely  allied  to  those  in  which  they  are  highly 
developed,  augurs  that  they  are  not  of  any  high  functional  signification.  It  may  there- 
fore be  here,  as  it  is  in  the  Ascidice,  that  they  maintain  a vitality  in  the  external  cover- 
ing or  shell ; low,  indeed,  in  comparison  to  that  of  the  test  of  the  latter,  but  sufficient, 
perhaps,  to  repair  the  shell  should  it  be  fractured;  and  as  the  Brachiopods  do  not,  to 
any  great  extent,  thicken  their  shells  by  successive  internal  layers,  some  such  provision 
appears  necessary  for  the  purpose. 
The  reticulated  membrane,  which  is  closely  adherent  to  the  homogeneous  layer  beneath, 
and  in  which  the  caeca  originate,  has  very  much  the  character  of  an  epithelial  layer.  It 
is  therefore  not  unlikely  that  they  may  be  productions  merely  of  the  epithelium. 
From  the  foregoing  account  of  the  circulatory  apparatus,  it  is  evident  that  the  peri- 
visceral chamber,  and  its  various  so-called  vascular  ramifications  in  the  mantle,  are  not 
■ Plate  LVIII.  fig.  8. 
* Introduction  to  Davidson’s  ‘ British  Fossil  Brachiopoda,’  p.  30. 
