OF  THE  EEPEODUCTIVE  OEGANS  OF  THE  ANNELIDS. 
137 
segmental  organs,  which  are  now  fused  into  two.  The  external  orifice  is  single,  and  the 
organ  is  branched  and  unciliated. 
9.  In  the  Aphroditadse  the  segmental  organ  forms  a bilateral  series,  each  organ 
having  a single  orifice  and  branched,  the  sexes  being  separate. 
Homology  of  the  Segmental  and  Reproductive  Organs  of  the  Annelida. 
The  history  of  this  system  of  organs  has  now  been  traced  uninterruptedly  throughout 
every  important  division  of  the  class  of  Annelids.  Its  continuity  as  an  organic  system 
within  these  hmits  has,  the  author  trusts,  been  fully  and  completely  demonstrated.  But 
the  homological  relations  of  this  system  do  not  terminate  at  the  limits  which  define  the 
Annelids  proper  into  an  independent  class;  they  are  traceable  with  clearness  and 
certainty  into  other  and  apparently  very  differently  organized  classes  of  animals.  They 
will  serve  to  connect  together  large  groups  of  inferior  forms  between  which,  as  hitherto 
beheved,  no  zoological  affinity  existed.  In  this  sense  the  inquiry  into  the  morphological 
relations  of  the  segmental  organs  of  the  Annelids  assumes  a character  of  unusual  im- 
portance. A few  detached  conjectures  upon  this  subject  some  years  ago  were  thrown 
out  by  Leydig: — “ Das  ‘ arabeskenformige  Organ’  der  Nephehs  und  das  ‘ rosettenformige 
Wimperorgan’  der  Clepsine,  worauf  auch  bereits  Gegenbauer  angespielt  hat,  nichts 
wei teres  sind,  als  die  Endstiicke  der  Respirationskanale  dieser  Hirudineen.  Von  gleicher 
Bedeutung  halte  ich  die  eigenthiimlichen  ‘ pantoffel-  und  fullhornf ormigen  Organe  ’ der 
Synapta  digitata,  welche  Joh.  Muller  (Archiv  fiir  Anat.  u.  Physiol.  1852),  aufgefunden 
und  deren  feine  Cilien  ebenfalls  nach  einwarts  schlagen*.” 
In  a very  recent  paper  “ fiber  Hydatina  Sentaf,”  Leydig  speaks  of  the  ciliated  tubes 
as  “ Respirationsorgane.” 
From  these  passages  it  is  evident  that  this  comparison  between  the  ciliated,  organs  of 
Clepsina  and  Nephelis  and  those  of  Synapta  digitata,  was  suggested  to  the  mind  of 
Leydig  simply  by  the  fact,  that  in  each  instance  the  organ  terminated  internally  by  an 
expanded  ciliated  umbrella-hke  extremity.  His  comparison  ends  where  this  arbitrary 
and  unimportant  point  of  resemblance  ceases.  He  originates  no  morphological  principle. 
The  resemblance  which  he  suggests  is  a mere  accidental  observation. 
The  author  will  now  proceed  to  develope  the  views,  with  the  importance  of  which  he 
is  deeply  impressed,  which  he  has  been  inductively  led  to  adopt  from  a clear  and  con- 
vincing recognition  of  an  essential  unity  of  design  in  the  structure  and  uses  of  the 
segmental  system  of  organs  throughout  the  classes  to  which  his  observations  relate.  The 
subject  may  be  conveniently  distributed  under  the  following  arrangement  of  heads : — 
* Zeitsch.  f.  W.  Zool.  1854. 
t Mullee,  Archiv  f.  Anat.  u.  Phys.,  1857. 
