or  THE  EEPEODUCTIVE  OEGANS  OE  THE  ANNELIDS. 
115 
The  tube  (fig.  9 d h,  Plate  VII.),  which  in  the  case  of  the  segmental  organ  of  the  Common 
Leech,  proceeds  from  one  limb  (n)  of  the  organ  and  terminates  in  a spherical  membranous 
vesicle  (I  m),  is  evidently  the  counterpart  of  the  ciliated  extremity  of  the  segmental 
organs  of  lims  and  Lumhricus.  Both  open  into  the  cavity  of  the  body.  In  the  Leech, 
however,  neither  the  vesicle,  nor  the  tube  into  which  it  is  prolonged,  is  ciliated ; but,  in 
lieu,  they  are  highly  contractile.  The  vesicle  is  the  respiratory  sacculus  of  Duges.  It 
does  not,  as  Duges  contends,  communicate  externally  by  an  orifice  in  the  integuments, 
but  internally  with  the  cavity  of  the  body  as  just  stated.  The  tube  which  supports 
this  vesicle  arises  from  that  limb  of  the  segmental  organ  which  contains  the  ova  in  the 
least  developed  condition  (fig.  9 a?,  n,  c).  It  is  probable,  therefore,  that  the  chylaqueous 
fiuid  which  this  tube  is  designed  to  discharge,  fiows  throughout  the  entire  extent  of  the 
organ  from  the  point  of  entrance  [n)  to  that  of  exit  (a),  before  it  escapes  externally.  In 
this  course  the  fiuid  may  afford  the  materials  of  nourishment  to  the  ova  (in  the  direction 
of  the  arrows)  as  they  travel  from  one  limb  to  the  other  of  the  looped  organ,  and  this  is 
the  more  probable,  as  the  proper  tunic  of  the  organ  is  very  scantily  supplied  with  blood- 
vessels. 
The  organ  itself  in  the  common  leech  (fig.  9)  may  be  described  as  a looped  tube,  the 
limbs  (a,  (f)  of  which,  returning  upon  themselves,  and  one  invaginating  the  other,  open 
externally  at  the  same  point.  These  organs  form  two  series,  one  on  either  side  of  the 
stomach.  They  are  external  to  the  stomach  and  to  the  testicular  series.  There  are 
therefore  (as  in  Nats  and  iMmhricus)  two  of  these  organs  in  each  ring  of  the  body. 
This  fact  is  of  morphological  value,  for  the  testes  bear  no  relation  either  in  number  or 
position  to  the  annuli  of  the  body.  The  organs  stand  vertically  in  situ,  the  ends  [a, 
being  attached  to  the  abdominal  surface,  and  the  fundus  {h)  floating  upwards  near  the 
roof  of  the  containing  chamber. 
In  order  to  the  further  examination  of  these  delicate  organs  they  must  now  be  cut 
out,  as  closely  to  the  abdominal  surface  as  possible,  by  means  of  small  curved  scissors. 
They  should  then  be  floated  in  water  on  a glass  slide,  covered  by  a thin  slip  and  placed 
under  the  microscope.  If  the  organs  have  been  successfully  dissected  out,  it  will  be  seen 
that  one  limb  (a^)  is  broader  and  more  transparent  than  the  other  [g  a).  In  the  former 
limb  (d^)  the  ova  {k)  are  merely  pellucid  cells,  consisting  exclusively  of  the  germinal 
vesicle  and  germinal  spot.  In  the  latter  limb  {g  a)  the  ova  [j)  have  acquired  their  full 
complement  of  vitellus.  At  this  stage  they  exhibit  a flattened  ovoid  form.  This  fact 
affords  a demonstration  as  to  the  course  taken  by  the  ova  in  their  passage  from  the  ovary 
if  pi’operly  so  called,  towards  the  exterior.  The  seat  of  the  true  ovary  is  marked 
by  the  plexus  of  vessels  [f).  By  the  ovary  is  meant  that  part  of  the  apparatus  in 
which  the  ova  are  formed,  that  part  which  is  endowed  with  the  peculiar  power  of  ovoge- 
nesis. In  relation  to  this  special  centre,  all  the  rest  of  the  organ  may  be  characterized 
density  of  the  integuments,  it  was  impossible  to  obtain  sucli  a view  of  the  internal  parts  as  would  justify 
any  conclusions  on  the  subject.  These  organs  in  Clepsina  are  stated  by  Letdig  to  be  ciliated.  No  cilia 
can  be  discovered  in  any  part  of  their  structure,  either  in  the  Common  Leech  or  in  the  Sea-leech. 
