136  DE.  T.  WILLIAMS’S  EESEAECHES  ON  THE  STEUCTUEE  AND  HOMOLOGY 
This  completes  the  anatomical  history  and  morphological  relations  of  the  segmental 
system  of  organs  in  the  class  Annelida. 
It  now  remains  to  speak  of  the  leading  points  thus  established.  The  demonstration 
has  been  long  and  difficult.  The  general  results  are  satisfactory  in  the  highest  degree. 
It  may  be  convenient  to  condense  them  into  the  shape  of  a few  recapitulatory  state- 
ments. 
1.  In  the  Lumbricina  (including  the  Naides)  only  a few  of  the  segmental  organs  are 
concerned  in  the  office  of  reproduction.  The  others  are  diverted  to  a subsidiary  pur- 
pose, viz.  to  eliminate  the  chylaqueous  fluid.  This  may  possibly  be  the  case  in  Clepmia 
and  Nephelis. 
2.  In  the  Hirudinei  the  great  majority  are  dedicated  to  the  development  of  the  femi- 
nine or  ovario-utricular  system,  and  only  two,  or  four  (according  to  the  species),  to  the 
male  apparatus.  It  is  not  yet  clearly  proved  whether  in  this  family  the  collateral 
function  of  discharging  the  cavitary  fluid  is  or  is  not  accomplished  by  this  organ.  The 
Hirudinei,  the  Lumbricina,  and  Planaridee  amongst  the  Annelids  are  hermaphrodite. 
No  other  Annelid  falls  under  this  designation. 
3.  In  the  Terebellidse  and  Arenicolidse,  the  segmental  organs  are  definite  in  number, 
but  bearing  no  reference  in  their  number,  although  in  position,  either  to  the  segments 
of  the  body  or  to  the  branchise.  They  communicate  with  the  caHty  of  the  body,  Avhich 
stands  to  the  ova  in  the  relation  of  a vitellarium.  This  is  the  07ily  group  in  the  entire 
class  of  Annelids  in  which  the  perigastric  chamber  is  made  a place  of  sojoui-n  for  the 
reproductive  elements. 
4.  At  the  Sabellidse  and  Serpulidse  commences  the  Nereid  type.  The  segmental 
organ  becomes  now  a simple  looped  vascular  tube,  both  ends  of  which  open  externally, 
and  through  the  interior  of  which  is  driven,  by  ciliary  force,  a current  of  the  exter^ial 
ele^nent  (water),  and  to  which  is  affixed  a gland-like  appendage,  in  which  in  the  female 
the  ova,  in  the  male  the  sperm-cells,  are  retained  after  these  products  escape  from  the 
true  testes  and  ovaria. 
5.  This  description  applies  in  every  sense  to  the  segmental  and  reproductive  system  of 
the  entire  Nereid  group.  In  this  and  the  former  division  every  segmental  organ  in  the 
body,  both  in  the  male  and  female,  is  engaged  in  the  reproductive  function.  There  ai’e 
no  non-generative  organs  as  in  the  Lumbricinse.  The  distinctive  characteristic  of  this 
and  the  former  group  is  that  the  generative  products,  although in  the  segmental 
organs,  are  not  retained  in  them,  nor  are  ushered  into  the  perigastric  cavity.  They 
sojourn  in  appended  pouches  as  already  defined. 
6.  This  definition  applies  also  to  the  Chlorsemea. 
7.  The  segmental  organs  of  the  Nemertidae  or  Cestoid  Annelids  conform  in  structure 
with  the  type  of  the  lateral  ovarian  pouches  of  the  Hirudinei,  differing  from  the  latter 
in  having  the  sexes  on  separate  individuals. 
8.  In  the  Planariea  the  type  of  the  organ  changes.  There  are  only  four  original 
