844 
ME.  HAJS^COCK  ON  THE  OEGANIZATION  OF  THE  BEACHIOPODA. 
ney ; whilst  the  other,  that  is,  the  lining  membrane  of  the  perivisceral  chamber,  and  its 
various  ramifications,  must  execute  the  complementary  renal  office.  It  has  been  pointed 
out  that  this  chamber  is  everywhere  in  connexion  with  the  various  systems  of  blood- 
lacunes ; the  ramifications  in  the  mantle  follow  these  systems  even  to  the  circumpallial 
vessel,  which  also  lies  in  the  midst  of  blood-channels.  It  therefore  seems  impossible 
not  to  conclude  that  this  peculiar  arrangement  and  excessive  development  of  the  atrium 
is  to  facilitate  the  evacuation  of  some  matter  from  the  blood,  probably  little  more  than 
pure  water. 
Some  fluid  undoubtedly  fills  the  perivisceral  chamber,  and  as  it  communicates  externally, 
it  might  be  supposed  that  water  would  be  received  in  this  way.  It  has  been  stated, 
however,  that  the  passages  are  so  constructed  that  they  would  not  readily  admit  fluid, 
though  it  might  easily  be  discharged  by  them,  the  arrangement  being  veiy  similar  to 
that  which  obtains  at  the  points  of  entrance  of  the  ureters  into  the  bladder  of  the  higher 
animals.  And,  moreover,  the  ramifications  of  this  chamber  into  the  pallia!  lobes  are 
inexplicable,  unless  it  be  assumed  that  they  are  connected  with  the  discharge  of  some 
matter  from  the  blood-system.  It  must  be  admitted,  nevertheless',  that  the  perivisceral 
chamber  with  its  ramifications  is  apparently  the  equivalent  of  that  which  in  the  other 
Mollusks  is  usually  denominated  the  water-system,  and  into  which  the  external  water  is 
supposed  to  be  admitted. 
The  atrial  membrane  will  probably  be  found  in  all  the  Mollusca  in  various  states  of 
development,  in  some  performing  the  two  functions  above  distinguished  by  separate 
portions,  in  others  having  these  portions,  and  therefore  their  functions,  more  or  less 
united  or  blended  together,  as  is  probably  the  case  in  Doris,  in  which  the  great  sinus, 
designated  the  kidney’,  stretching  along  the  liver,  and  following  the  ramifications  of  the 
arteries,  is  throughout  more  or  less  glandular.  This  renal  sinus  is  also  remarkable  for 
the  richness  of  the  vascular  network  in  its  membranous  walls,  reminding  us  forcibly  of 
the  peculiar  condition  of  the  parietes  of  the  perivisceral  chamber. 
Again,  in  the  Cephalopods,  the  renal  follicles  appended  to  the  v'enous  trunks  in  the 
so-called  pericardium  appear  to  perform  the  office  of  a kidney,  while  the  extension  of 
this  cavity  is  probably  the  equivalent  of  the  perivisceral  chamber  and  its  prolongations, 
having  perhaps  to  discharge  fluid  eliminated  from  the  blood  instead  of  receiving  it  from 
the  exterior.  Without  wishing  to  press  unduly  this  suggestion,  for  it  is  little  more,  it 
may  be  asked,  whence  the  necessity  for  supposing  that  water  is  admitted  direct  mto  the 
so-called  water-system  1 
Be  this,  however,  as  it  may,  it  may  perhaps  be  concluded  that  the  atrial  wall  of  the 
Ascidian,  and  the  lining  membrane  of  the  perivisceral  chamber  in  the  Brachiopod,  are 
the  homologues  of  the  kidney  and  membrane  composing  what  is  usually  designated  the 
water-system  in  the  Mollusca  generally. 
1 Hancock;  and  Embleton,  “ On  the  Anatomy  of  Doris,”  Philosophical  Transactions,  1852,  pp.  227,  228. 
