132  DE.  T.  WILLIAMS’S  ELSE  ARCHES  THE  STEHCTHEE  AAT)  HOMOLOGY 
happy  accident,  indi\idual  specimens  are  met  with,  in  which  the  oi'«-bearing  segmental 
organs  can  be  most  clearly  and  distinctly  seen.  In  the  other  genera  of  the  Xemertidee 
dissection  is  necessary. 
The  segmental  organs  of  the  Nemertidee  coincide  in  shape,  place  and  structure  with 
the  ovarian  or  female  series  of  the  Hirudin  ei  in  these  respects.  In  the  female  they  are 
utriculo-ovarian,  they  retain  within  then’  own  limits  the  ova,  they  observe  the  Annelidan 
type  of  the  horseshoe  or  looped  figm’e,  and  they  are  co-numerous  with  the  true  segments 
of  the  body. 
The  male  system,  in  every  respect,  is  the  precise  correlate  of  the  female.  There  is  no 
analogy  between  the  male  organs  of  the  Nemertidee  and  those  of  the  Hirudinei.  In  the 
latter  a special  segment  is  devoted  to  the  male  apparatus,  and  the  individuals  are  heiana- 
phrodite ; in  the  former,  the  sexes  are  seated  on  separate  individuals ; the  entire  series  of 
segmental  organs  in  each  sex  respectively  being  converted  into  ovaiia  and  testes.  As 
the  vascular  system  is  little  developed,  it  sends  no  special  branches  to  the  segmental 
organs. 
The  discovery  of  the  real  generative  parts  in  the  Nemertidse  enables  the  natmulist  to 
determine  their  true  affinities  in  the  class.  It  is  evident  that  to  place  them  under  a 
common  title  of  Turbellaria  in  contact  with  the  Planariea,  is  to  ignore  altogether  the 
existence  of  the  organs  of  reproduction,  for  in  this  respect  they  are  separated  from  the 
Planariea  by  a very  wide  interval.  The  latter  are  monoecious,  the  former  dioecious. 
Nor  is  there  the  slightest  analogy  in  the  plan  of  structure  upon  which  the  ahmentary 
system  is  formed. 
These  two  groups  are  joined  together  by  only  one  common  character,  and  that  the 
most  superficial,  namely,  by  the  ciliated  integuments.  The  author  proposes,  therefore, 
to  separate  the  Nemertina  or  the  cestoid  Annelids  from  the  true  Trubellaria,  sjTiony- 
mous  vvdth  the  order  Planariea,  under  which  order  he  includes  only  the  tribes  Cryptoccela, 
Bendrocoela  and  Ithabdoccela.  These  latter  are  united  by  a striking  similarity  of  struc- 
ture ; all  are  hermaphrodite,  in  all  the  reproductive  organs  are  formed  on  one  common 
type. 
Although  the  organization  of  the  Planariea  has  long  been  known  to  anatomists,  there 
remain  to  be  noticed  several  points  of  morphological  novelty.  No  attempt  has  ev'er  been 
made  to  connect  the  generative  viscera  of  this  family  with  those  of  the  typical  Annelids. 
In  the  characters  of  these  organs,  the  Planariea,  on  a superficial  view,  appear  to  be  irre- 
concilably separated  from  all  the  normal  Annelids.  A deeper  insight  into  the  homology 
of  these  parts  will,  however,  most  certainly  enable  the  philosophic  anatomist  to  recog- 
nize in  them  none  other  than  a modification  of  the  typical  segmental  organ  which,  under 
many  variations  of  outward  form,  has  now  been  traced  almost  throughout  the  entire  class 
of  the  Annelida. 
The  generative  organs  of  the  Planariea  consist  apparently  of  two  parts,  viz.  of  a cen- 
tral male  sacculus  (fig.  25,  h)  with  its  dependent  cseca,  and  of  a central  female  saccuhis 
(c)  with  its  complex  system  of  appended  canals  [d  d and  f f)-  Thus  it  seems  that 
