OF  THE  EEPEODUCTIVE  OE&ANS  OF  THE  AOT^ELIDS, 
119 
One  of  them  thus  viewed  is  shown  at  fig.  11.  It  consists  of  a simple  sac,  or  bag,  the 
distal  fundus  of  which  (c)  is  caecal.  The  interior  cavity  (m)  throughout  the  upper  three- 
fourths,  from  d to  c,  is  a single  undivided  space.  The  attached  extremity  is  formed 
into  two  very  distinctly  marked  channels  or  tubes  (A  h),  the  interior  of  which  is  lined 
by  ciha  which  beat  in  opposite  directions.  In  the  glandular  tube  {a  A)  the  ciliary 
current  sets  towards  the  cavity  of  the  organ  (m) ; in  the  simple  tube  [b  B)  it  sets  out 
from  this  cavity  (as  indicated  by  the  arrows).  Although  this  organ  has  long  been 
known  to  anatomists,  this  peculiar  looped  formation  has  never  been  recognized.  The 
lining  membrane  is  everywhere  covered  by  ciliated  epithelium.  In  the  fundus  the 
current  leads  up  on  one  side,  and  dovm  towards  the  outgoing  limb  on  the  other.  The 
two  hmbs  (A  b)  by  which  the  organ  is  tied  to  the  wall  of  the  perigastric  chamber  are 
not  similarly  formed.  The  ingoing  limb  exhibits  a more  glandular  character ; its  walls 
are  considerably  thicker  and  more  richly  supplied  with  blood  than  those  of  the  other. 
About  the  middle  of  its  com’se  it  enlarges  into  a round  gland-like  body  (^),  the  axis  of 
which  is  perforated  by  the  tube.  This  gland  is  subdivided  into  parallel  lobuli,  the 
vessels  being  specially  and  peculiarly  distributed.  It  is  therefore  not  a simple  bulging 
of  the  tubular  leg  of  the  organ.  It  is  a special  structure,  destined  to  perform  a special 
office,  which  will  be  afterwards  explained.  The  exact  mode  and  the  place,  in  which 
this  ingomg  hmb  of  the  organ  begins,  is  a point  of  essential  importance  to  determine. 
As  long  as  the  author’s  investigations  were  confined  to  full-grown  specimens,  necessita- 
ting the  part  to  be  cut  out  in  order  to  be  examined,  he  found  it  impossible  to  solve  this 
point.  It  was  only  after  having  discovered  young  Arenicolce  in  a suitable  condition,  that 
he  was  enabled  to  prove  by  direct  demonstration  that  it  commenced  in  an  external 
orifice  (A*)  on  the  abdominal  surface  of  the  animal.  If  this  limb  opened  directly  into 
the  general  cavity,  it  would  be  the  exact  equivalent  of  the  ciliated  trumpet-shaped 
extremity  of  the  segmental  organ  of  'Nais  and  Lumbricus.  The  ciliary  current  in  this 
limb  (A*,  A)  would  then  consist  of  the  cavitary  fluid ; but  as  this  limb  opens  externally, 
the  ingoing  current  which  it  serves  to  convey  can  consist  only  of  sea-water. 
It  is  of  great  interest  to  inquire,  What  can  be  the  purpose  of  this  inflowing  stream 
of  the  exterior  sea-water  % Is  it  intended  to  replenish  the  fluid  contained  in  the  cavity 
of  the  body  1 If  so,  this  organ  in  Arenicola  performs  an  office  which  is  precisely  the 
reverse  of  that  which  devolves  upon  it  in  Nais  and  Lnimbricus.  But  it  is  certain  that 
the  fluid  thus  introduced  into  the  cavity  of  the  organ  (m)  is  driven  out  again,  in  whole 
or  in  great  part,  by  outwardly-acting  cilia  through  the  limb  {b  B).  That  this  second 
or  outgoing  tube  also  (y^ens  externally  (B*),  can  be  distinctly  seen  by  the  eye  (in  young 
specimens  properly  arranged  for  the  examination).  The  water-current,  therefore, 
which  is  driven  into  the  organ  at  the  limb  (A),  is  driven  out  again  almost  entirely 
at  the  hmb  (B):  none,  if  any  part  at  all,  penetrates  into  the  cavity  of  the  body. 
Now  there  is  no  doubt  upon  the  author’s  mind,  although  he  has  never  been  able 
actually  to  demonstrate  the  fact,  that  this  outgoing  leg  [b)  of  the  looped  organ  (A,  B) 
not  only  communicates  directly  with  the  exterior,  as  at  B*,  but  also,  by  means  of  a 
R 2 
