ME.  HANCOCK  ON  THE  OEOANIZATION  OE  THE  BEACHIOPODA. 
843 
an  involution  of  it  and  the  viscera,  exactly  as  it  would  do  if,  in  the  Ascidian,  the  atrial 
membrane  were  a little  more  extensively  developed;  only  in  the  former  case  the 
membrane  closely  invests  the  viscera,  and  is  in  contact  with  the  mantle,  leaving 
defined  blood-channels  or  lacunes.  In  both  cases  the  space  circumscribed  by  this 
membrane  opens  externally,  and  in  both  cases,  too,  it  receives  the  products  of  the 
genitalia  on  their  passage  outwards.  Much  more  might  be  urged  in  confirmation  of 
this  view ; but  it  is  not  intended,  on  the  present  occasion,  to  enter  upon  the  details  of 
this  branch  of  the  subject.  Indeed  my  knowledge  of  the  anatomy  of  the  Ascidice  is  too 
limited  to  permit  me  to  do  so  satisfactorily.  I can  therefore  only  hope  that  Mr.  Huxley 
will  shortly  publish  his  researches  bearing  upon  this  matter,  for  assuredly  no  one  is 
better  prepared  than  he  is  to  discuss  it,  as  no  one  has  a more  extended  knowledge  of 
the  morphology  of  the  Mollusca^  and  a more  complete  and  accurate  acquaintance  with  the 
anatomy  of  the  Ascidice. 
Taking  it  for  granted,  then,  that  the  atrium  and  the  perivisceral  chamber  are  homo- 
logous, the  question  arises,  what  light  does  this  fact  throw  upon  the  function  of  the 
latter!  In  the  Ascidice.,  the  atrium  communicating  freely  with  the  surrounding  element 
might  be  considered  as  a sort  of  rudimentary  water-system,  by  those  who  believe  in  the 
existence  of  such  a system.  Its  function,  however,  seems  to  be  that  of  excretion. 
That  portion  of  it  denominated  the  cloaca  is  for  the  outlet  of  the  branchial  currents,  and 
at  the  same  time  for  the  discharge  of  the  faeces,  and  of  the  ova  or  spermatozoa,  according 
to  the  sex  of  the  indmdual.  But  the  other  portion  must  have  some  special  office  to 
perform ; and  as  its  walls  are  washed  by  the  blood,  it  would  seem  probable  that  it  is  for 
canning  out  of  the  system  the  deleterious  or  effete  nitrogenous  matters.  At  the  same 
time  it  seems  not  unlikely  that  this  extension  of  the  atrium  may  be  to  facilitate  the 
discharge  of  the  redundant  fluid,  which  ha\ing  entered  the  blood,  is  supposed  to  per- 
vade the  tissues  of  these  animals.  Thus  the  atrium  may  be  viewed  as  performing  the 
two  gi’eat  functions  of  a renal  apparatus. 
This  atrial  membrane  is  found  in  the  most  rudimentary  condition  in  the  Polyzoa,  for 
instance,  in  Fredericella  sultana.  In  this  species  the  membrane  lining  the  polyzoon,  or 
that  corresponding  to  the  mantle  or  outer  tunic,  is  invaginated  at  the  orifice  of  the  cell 
and  encloses  the  upper  portion  of  the  animal  with  a third  tunic.  When  the  polype  is 
exserted,  this  is  only  partially  everted.  It  must  be  through  the  agency  of  this  mem- 
brane that  the  effete,  nitrogenized  matter  is  eliminated,  as  well  as  the  superfluous  water, 
if  any  such  discharge  takes  place. 
The  Ascidians,  as  has  been  shown,  exhibit  the  next  step  of  development  in  the 
arrangement  of  this  membrane  into  the  form  of  an  atrium,  into  which  the  water  is 
admitted.  Then  come  the  Brachiopods,  in  which  the  atrium  is  largely  developed,  and 
considerably  modified ; but  now  the  external  communication  is  much  restricted.  The 
most  interesting  change,  however,  is  in  the  structure  of  the  excretory  channels  or 
oviducts,  which  have  assumed  a glandular  character.  Thus  one  portion  of  the  atrial 
membrane  has  become  specialized,  and  probably  performs  one  of  the  functions  of  a kid- 
