OF  THE  EEPEODUCTIVE  OEGANS  OP  THE  ANNELIDS, 
99 
lopment,  while  all  the  rest  should  remain  in  abeyance  and  in  the  condition  of  mere 
“ excretionary  tubes ; ” or  why  the  ciliated  tube  of  one  side  (always  the  same  in  every 
individual)  should  be  changed  into  the  female  system,  and  that  on  the  other  into  the 
male.  Such  is  the  nature  of  the  hermaphroditism  of  these  Annelids,  It  is  symmetrical ; 
the  centres  of  the  opposite  sexes  are  separate  and  distinct ; the  organs  upon  which  they 
are  respectively  developed  are  homologically  identical.  The  vascular  and  nervous  systems 
have,  however,  a special  arrangement  in  each. 
This  centralized  position  of  the  sexual  structures  is  peculiar  to  the  genera  Nais  and 
Lumhricus.  In  every  other  Annelid,  Planaria  forming  no  exception,  every  segmental 
organ  in  the  body  is  enlisted  in  the  office  of  reproduction. 
There  is  nevertheless  some  peculiar  power  resident  in  the  ordinary  segmental  organs. 
Towards  the  end  of  the  summer  these  worms  [Naides)  divide  themselves  by  fission, 
generally  into  two  parts.  The  normal  reproductive  masses  are  always  in  the  anterior 
half.  The  tail-half  at  this  time  has  no  sexual  system-,  but  in  process  of  time  two  of  the 
cihated  tubes  will  swell,  enlarge  in  size,  and  at  the  same  time  become  more  complex 
in  sti’ucture.  It  is  evidently  the  effoet  in  the  tail-half  to  reconstruct  itself  into  a per- 
fect individual.  That  it  does  become  a perfect  individual  the  author  has  never  been 
able  to  prove  by  his  own  observations.  The  fact  is  here  adduced  to  prove  that  there 
does  reside  in  the  segmental  organ  a power  which  enables  it  (the  conditions  being  favour- 
able) to  transform  itself  into  an  ovary  or  a testis. 
The  ciliated  tubes  in  various  species  of  Nais  have  of  late  years  been  observed  and 
described  by  several  anatomists,  e.  g.  by  Henle,  Udekem,  Siebold,  Leydig,  Gegenbauee 
and  others.  No  one  has  hitherto,  however,  attempted  to  connect  them  with  the  repro- 
ductive system. 
Lumbricidea. 
Anatomists  have  long  been  aware  of  the  existence  of  the  “ ciliated  tubes”  of  the  Lum- 
bricini.  By  Moeeen* * * §  they  were  described  as  the  “ Vesicules  Aeriennes,”  and  as  Tracheae 
by  Willis  and  LEof.  DugesJ  states  that  they  are  “toujours  remplis  d’un  liquide 
aqueux.”  Henle  supports  this  view.  Siebold^  professes  a similar  opinion.  This 
distinguished  author  observes:  “The  structure  of  the  respiratory  system  is  not  less 
difficult  to  be  understood.  In  all  genera  (of  Lumbricini)  there  are,  at  the  commence- 
ment and  on  each  side  of  the  intestine,  very  tortuous  canals,  which  open  upon  the  ven- 
tral surface  by  a nari'ow  orifice  near  the  median  line.”  Then,  describing  the  cilia,  he 
states : “ In  Limibricus  these  aquiferous  canals  are  surrounded  by  a very  distinct  vascular 
network,  which  has  a botryoidal  aspect  ft’om  its  numerous  pedunculated  vesicular  dila- 
tations, which  are  filled  with  blood : ” and  then  Siebold  observes : “ These  canals,  thus 
* Moeeex,  ‘ De  Liimbrici  terrestris  Hist.  Nat.’  p.  149. 
t Leo,  ‘ De  Structura  Lumbrici  terrestris,  dissert,  inaug.’  in  4to,  p.  25,  1820. 
X Consult  a paper  by  Hexle,  “Ueber  Enchytrseus,”  and  ‘ Archiv  fiir  Anat.  und  Fhys.,  von  Muller,’ 
1837,  pi.  6.  figs.  7,  8. 
§ Siebold  and  Staxnius,  ‘ Anat.  of  Invertebrata,’  translated  by  Burnett,  p.  171. 
