or  THE  EBPEODUCTIVE  OEGANS  OF  THE  ANNELIDS. 
121 
It  now  remains  to  show  that  the  segmental  organ  of  Arenicola,  the  minute  anatomy 
of  which  has  been  just  described,  is  the  homologue  of  the  segmental  organ  of  Nain, 
lAimbncus  and  Hirudo. 
If  the  partition  {d,  fig.  11)  which  separates  the  two  rudimentary  limbs  (A,  B)  of  this 
organ  were  carried  a little  higher  in  the  direction  of  the  fundus  (c),  the  entire  organ 
would  be  converted  into  a tube,  which,  if  one  end  of  it  opened*  into  the  cavity  of  the 
body,  instead  of  communicating  externally,  would  correspond  precisely  with  the  ciliated 
tubes  of  Nais  and  Lumbricus.  But  sufficient  of  the  tubular  character  is  presented  by 
the  structure  of  this  organ  to  connect  it  indisputably  with  its  homologues  in  Nais,  I/win- 
bricus  and  Hirudo.  In  all  it  is  a looked  organ ; in  all  it  is  the  basis  of  the  reproductive 
system ; in  aU  there  passes  through  its  axis  a current  of  fluid,  the  design  of  which  is  to 
transport  the  generative  products,  either  into  the  cavity  of  the  body  or  externally.  The 
difierence  is  only  in  the  mode  in  which  the  tube  is  folded  upon  itself. 
It  is  here  of  importance  to  remark  that  this  is  first  Annelid  we  have  encountered, 
in  which  the  cavity  of  the  body  has  been  employed  as  a receptacle  for  the  generative 
products.  The  segmental  organ  being  still  the  true  generative  apparatus,  a change  of 
anatomical  arrangement  became  obviously  necessary,  in  order  to  adapt  it  to  the  novel 
condition  just  stated. 
Terebella. — The  segmental  organ  in  the  genus  Terebella  (fig.  12)  occurs  under  a form 
and  character  which  ally  it  in  the  most  direct  and  intimate  manner  with  that  of  Areni- 
cola.  T.  nebulosa  is  an  elegant  Annehd.  It  is  large  enough  to  admit  of  dissection.  In 
the  summer  months  the  blood-vascular  system  is  highly  developed,  the  branchiee  are 
brightly  florid,  and  the  segmental  organs  (when  the  worm  is  opened)  present  an  intensely 
vascular  appearance. 
Dissected  in  the  usual  mode,  removing  with  care  the  alimentary  canal,  it  will  be  seen 
that  a series  of  curved  elongated  saccuh  (of  which  fig.  12  are  two  specimens)  exist  on 
either  side  of  the  ventral  median  line.  They  extend  throughout  the  setiferous  portion 
of  the  body  (in  this  family  it  may  be  called  the  thoracic),  and  seem  to  be  as  numerous 
as  the  largest  setiferous  feet,  which  in  T.  nebulosa  are  sixteen  in  number. 
Although  the  identity  of  the  segmental  organ  of  Terebella  and  that  of  Arenicola  is  so 
evident  as  to  admit  of  no  doubt,  yet  there  are  between  them  several  differential  features. 
In  Terebella  the  organ  is  more  deeply  divided  by  the  partition  (fig.  12  A.  k)  between 
the  limbs  than  it  is  in  Arenicola,  a fact  which  approximates  it  more  nearly  to  a tube. 
The  vascular  appendage  [h,  B)  is  also  differently  placed.  In  Terebella  it  is  in  con- 
sexes  m Arenicola  and  Terehella  (Comptes  Eendus,  xvi.  1843);  but  long  before  this,  Staxxius  had  concluded 
that  the  sexes  were  separate  in  Arenicola,  from  the  fact  that  in  different  individuals  the  contents  of  the 
general  cavity  of  the  body  were  different  (Mullee,  Archiv,  1840).  Stannixjs  also  observes  that  the  parent 
sperm-ceUs  leave  the  segmental  organ  (his  testes)  before  the  formation  of  the  spermatozoa,  which  are  found 
only  in  the  cavity  of  the  body.  Keohx,  in  Alciopa,  confirms  this  fact  (Wiegmann’s  Archiv,  1845).  Put 
the  great  mistake  committed  by  Keohn,  Feey  and  Leuckaet  is,  that  they  assert  that  in  Arenicola,  Alciopa, 
Nereis,  Syllis,  Sabella  and  other  Annelids,  the  ova  and  the  spermatozoa  “ are  developed  free  in  the  general 
cavity ...!!” 
