138  DE,  T.  WILLIAMS’S  EESEAECHES  OH  THE  STEUCTL’HE  AHD  HOMOLOGY 
The  segmental  organs  of  the 
Lumbricina 
Naidea 
Hirudinei 
Clepsina 
Nephelis 
Terebellidse 
Arenicolidse 
Nereidee 
Chloraemea  J 
Planariea  } 
Aphroditadae 
Are  immediately  homologous  urith  those  of  the 
’Hydrozoal  . 
. . [•Yoeieiiterata. 
-!  Actinozoa  J 
Eotifera. 
J Synaptiadse'. 
j Holotburiadse. 
rSipunculidse. 
lEchiuridse. 
{Trematoda. 
I Echinadae. 
lAsteriadee. 
The  author  has  drawn  up  the  above  table  of  homologies  from  an  extensive  and  careful 
series  of  dissections  and  practical  comparisons.  He  is  deeply  imbued  with  the  conHction, 
that  the  parallelism  which  it  purports  to  sketch  will  acquire  greater  and  greater  im- 
portance as  special  anatomical  investigations  extend. 
If  the  generative  system  of  Liicernaria  be  compared  with  that  of  Actinia,  a close  and 
striking  resemblance  in  structure  and  disposition  wdll  be  at  once  perceived,  notwith- 
standing that  in  iMcernaria  the  organ  is  branched  and  multiplied  by  lateral  csecal 
tubuli,  hitherto  undescribed.  In  both  there  is  a coiled  cihated  tube ; in  both  there  is 
an  appended  mass  of  ova  or  sperm-cells,  according  to  the  sex.  So  intimate  is  the  simi- 
larity of  form  between  the  complex  tubuli  which  are  attached  to  the  mesenteric  septa 
in  Actinia,  and  the  ciliated  or  segmental  organs  of  the  Lumbricina,  that  the  idea  of 
their  typical  identity  at  once  arises  in  the  mind.  If  it  be  conceded  that  these  two 
forms  are  the  homologues  of  each  other,  it  follows  that  the  ciliated  tubes  of  Lumhricus 
and  Nais  are  in  truth  the  homologues  of  the  radiated  reproductive  system  of  Liicernaria, 
and,  through  this  zoophyte,  of  that  of  the  entire  group  of  the  Hydrozoa*.  And  this 
conclusion  (startling  as  it  may  now  appear),  as  minute  anatomy  proceeds  in  her  com-se  of 
discovery,  will  certainly  come  to  be  universally  admitted. 
The  mode  in  which  the  ovarian  and  sperm-masses  are  connected  with  the  convoluted 
chords  in  the  generative  organs  of  Actinia,  atfords  a strong  proof  in  favoiu’  of  the  Hew 
* The  segmental  organ  in  the  hydroid  polypes  sinks  into  a condition  of  rudimentary  abeyance.  Eut  that 
they  do  exist  along  the  lines  which  indicate  the  positions  of  the  vertical  septa  of  the  higher  grades  of  polypes, 
I am  persuaded.  The  time  will  come  when  it  will  be  necessary  to  review  the  entire  doctrine  of  generation 
in  connexion  with  the  system  of  the  segmental  organs.  My  observations  have  strongly  suggested  to  my 
mind  the  belief  that  the  gemmae  in  the  hydroid  polypes  only  arise  along  those  vertical  lines  of  the  body 
of  the  parent,  beneath  which  there  lie  rudimentary  segmental  organs,  and  that  the  gemma  most  probably 
proceeds  from  an  ovum,  and  not  from  any  indifferent  portion  of  the  parent-structure. 
