118  DE.  T.  WILLIAMS’S  EESEAECHES  ON  THE  STEUCTIJEE  AND  HOMOLOGY 
viscus,  but  by  the  skilful  adaptation  of  a normal  organ.  By  this  simple  morphological 
view  an  intelhgible  principle  is  suggested,  by  the  light  of  which  the  most  anomalous 
organs  are  reduced  to  a rule  of  consistent  homology.  The  testes  are  arranged  on  the 
same  plan  in  Hirudo  officinalis^  H.  mcca,  H.  hranchiata^  and  Albione.  In  all  they  take 
their  origin  from  two  segmental  organs,  the  pair  belonging  to  the  same  ring.  According 
to  this  interpretation  the  testes  of  the  Hirudinei  are  as  naturally  referable  to  the  seg- 
mental organ  as  those  of  Im/mlricus  or  'Nais.  In  the  latter  the  developed  glands  keep 
within  the  limits  of  the  annuli  from  which  they  originate.  In  the  former  they  extend 
over  a considerable  portion  of  the  body.  Both  are  traceable  to  the  same  roots. 
Terehella*. — The  segmental  organ  in  Arenicola  will  be  taken  as  the  type  of  the 
structure  and  function  of  this  viscus  in  the  family  of  the  Terebellidae.  With  these  organs 
in  the  common  Sand-lug  every  comparative  anatomist,  in  a general  sense,  is  familiar  ; 
on  laying  open  the  cavity  of  the  body  they  appear  at  once  under  the  character  of 
vascular  pouches  or  sacculi.  By  Cuvier,  Miljste-Edwards,  and  every  other  anatomist, 
they  have  been  correctly  described  as  the  true  organs  of  reproduction.  To  this  con- 
clusion all  have  been  alike  driven,  since  no  other  organs  in  the  body  could  be  discovered 
to  which  such  functions  could  reasonably  be  imputed. 
This  is  literally  all  the  information  that  we  possess  at  the  present  time  with  respect 
to  the  generative  system  of  this  and  the  allied  genera  of  Annehds. 
It  may  well  be  asked  how  far  does  this  information  carry  the  philosophic  anatomist '? 
It  is  in  truth  of  little  value.  It  conveys  no  homological  idea.  It  imphes  no  general 
principle.  It  does  not  connect  the  reproductive  system  of  this  genus  with  that  of  any 
other  Annelid.  It  establishes  no  system. 
It  cannot  be  disputed  that  organs  so  important  as  the  generative,  must  in  the  very 
nature  of  things  be  subjected  to  the  governance  of  a morphological  law  of  more  or  less 
extended  application.  Abrupt  transitions  and  exceptions  are  impossible. 
There  is  no  difficulty  in  proving  that  the  reproductive  system  of  Arenicola  is  a (hrect 
continuation  of  that  which  has  akeady  been  described  in  the  Lumbricidae,  Naidae,  and 
Hirudinei.  The  morphologist  thus  rises,  step  by  step,  accumulating  proof  upon  proof 
of  the  unity  of  the  segmental  organ  in  the  several  families  of  Annelids  as  they  ai‘e  being 
successively  examined. 
In  Arenicola  piscatorum  (common  Sand-lug)  there  are  six  lateral  pouches  which,  in 
healthy  specimens,  during  the  months  of  July  and  August  are  in  a condition  of  extreme 
vascularity.  They  are  quite  visible  to  the  naked  eye,  by  theh  bright  red  colom’.  The 
minute  structure  of  these  pouches  can  be  studied  by  carefully  dissecting  them  from  their 
attachments  to  the  abdominal  wall  of  the  cavity  and  placing  them  under  the  microscope. 
* Under  this  head  the  genus  Arenicola  is  included,  on  the  ground  of  the  close  anatomical  resemblance 
which  exists  between  its  segmental  organs  and  those  of  Terehella.  The  former  is  classified  as  a Dorsibran- 
chiate,  and  the  latter  as  a Cephalobranchiate  Annelid,  terms  apparently  descriptive  of  wide  differences. 
Beneath  this  exterior  difference  there  lie  however  the  strongest  proofs  of  intimate  relationship  of  structure 
in  some  of  the  most  important  organs  of  the  body. 
