94  DE.  T.  WILLIAMS’S  EESEAECHES  ON  THE  STELCTLTIE  AND  HOMOLOG-T 
be  said  to  exist  in  most,  the  generative  products  being  merely  developed  from  some  part  of 
the  walls  of  the  perivisceral  cavity  into  which  they  eventually  freely  float,  making  their 
way  out  in  a manner  which  is  not  quite  understood  at  present ; probably  through  some 
temporary  or  permanent  apertures  at  the  bases  of  the  parapodia.” 
From  this  passage  it  is  obvious  that  by  the  most  recent  expounders  of  the  science  of 
comparative  anatomy,  it  is  formally  asserted  that  in  the  Annelida  “ special  reproductive 
organs  can  hardly  be  said  to  exist.”  It  will  afterwards  appear  that  the  facts  to  he 
related  in  this  memoir  conduct  the  anatomist  to  a widely  different  conclusion. 
A brief  and  special  historical  introduction  will  be  preflxed  to  each  department  of  the 
subject. 
By  this  arrangement  references  to  the  researches  of  other  observers  •s^dll  be  much 
facilitated. 
The  families  of  Annelids  selected  for  examination  will  be  placed  in  juxtaposition, 
more  on  account  of  an  afiinity  of  type  presented  by  the  “ segmental  organ  ” than  in 
deference  to  any  other  principle  of  classiflcation. 
The  Naidea,  Lumbricidea,  Hirudinea,  Terebellidse,  Nereidse,  &c.  will  on  this  ground 
be  taken  in  the  order  in  which  they  are  named. 
Naidea. — In  his  Keport*  on  the  Annelids,  the  author  made  the  following  statement 
with  reference  to  the  segmental  organ  in  the  genus  Nais:  “The  whole  reproductive 
system  is  limited  by  Duges  to  the  glandular  mass  which  is  so  readily  obsen  ed  about  the 
anterior  third  of  the  body,  whereas  in  reality  this  only  constitutes  one  segmental  unit, 
more  developed  only  than  those  which  are  repeated  in  every  ring  of  the  body.” 
Although  in  the  present  memoir  the  author  will  avail  himself  of  the  opportunity  to  cor- 
rect certain  inaccuracies  committed  in  his  “ Beport  ” with  respect  to  the  interpretation 
there  given  in  relation  to  the  segmental  organ  in  Nais,  he  will  only  at  present  observe, 
that  in  1852  he  drew  attention  to  the  fact  that  the  generative  structures  were  developed 
upon  one,  two  or  more  of  the  segmental  organs  common  to  almost  every  ring  of  the 
body. 
To  M.  Duges  the  segmental  organs,  as  distributed  throughout  the  two  posterior 
thu’ds  of  the  body,  were  unknown.  No  special  description  of  them  has  been  given  by 
any  subsequent  anatomist.  It  is  doubtful  whether  the  genus  Nais  is  included  by  Ley- 
dig  in  his  recent  memoir'l’,  “ Ueber  den  Bau  und  die  systematische  Stellung  der  Rader- 
thiere,”  while  incidentally  referring  to  the  family  of  the  Lumbricidea.  'V\Tiether  this 
be  the  case  or  not,  the  author  is  not  acquainted  with  any  special  account  of  this  organ 
in  this  group  of  Annelids. 
The  following  description  is  drawn  entirely  from  his  own  researches : — 
In  the  freshwater  Naides  the  segmental  organ  is  readily  examined.  A few  indi^iduals 
should  be  selected  and  placed  between  two  slips  of  glass,  slightly  compressed,  and  thus 
viewed  as  transparent  objects. 
* Transactions  of  the  British  Association,  1852. 
t Zeitschrift  fiir  Wissenschaftliche  Zoologie,  1855. 
