140  DE.  T.  WILLIAMS’S  EESEAECHES  ON  THE  STEHCTUEE  AND  HOMOLOGY 
latter.  As  far  as  it  is  possible  at  present  to  determine,  however,  they  have  only  a single 
external  opening.  But  it  is  certain  there  exists  between  their  tubular  interior  and  the 
perigastric  cavity  some  oj^en  communication^  since  the  ova  and  sperm-cells  find  a path 
from  the  former  place  into  the  latter. 
The  segmental  organ  of  the  Nereid  group  must  be  looked  upon  as  only  a variation 
from  the  type  of  that  of  the  Terebellidse  and  Arenicolidae. 
So  obvious  is  the  unity  of  type  between  the  segmental  system  of  the  Planariea  and 
that  of  the  Trematoda,  that  no  controversy  can  for  a moment  he  thought  of. 
The  segmental  system  of  organs  in  the  Aphroditadae  is  now  first  demonstrated. 
Neither  its  nature  nor  its  homologue  has  ever  before  been  brought  within  the  reach  of 
demonstration. 
But  so  unquestionable  is  the  morphological  resemblance  between  it  and  the  repro- 
ductive organs  of  the  Echinidae  and  Asteriadae,  that  it  is  impossible  to  doubt  their 
morphological  affinity.  They  agree  in  two  points  of  structure:  both  have  a single 
external  orifice  or  attachment ; both  are  caecally  branched.  And  it  may  be  added,  that 
in  all,  the  germinal  products  are  retained  within  the  proper  membranous  limits  of  these 
organs  in  all  these  families ; at  no  time  do  they  escape  into  the  general  cavity  of  the 
body. 
Thus,  through  the  morphological  relations  of  the  “ segmental  organ,”  not  only  are  the 
orders  and  genera  of  the  Annelids  themselves  linked  into  one  consistent  chain  of  reci- 
procal relationships,  but  a new  point  of  comparison,  a new  bond  of  teleological  affinity 
has  been  discovered  between  great  divisions  of  annuloid  and  radiated  animals  which 
hitherto  have  been  held  as  irreconcilably  separated. 
Swansea,  October  1857. 
Explanation  of  the  Plates. 
PLATE  VI. 
Fig.  1.  A,  B,  a pair  of  the  “ordinary”  segmental  organs  oil^ais  serpentina-,  A a,  the 
attached  extremity  opening  externally,  the  arrow  shows  the  direction  of  the 
ciliary  current ; b,  the  trumpet-shaped  ciliated  internal  extremity,  which  floats 
freely  in  the  fluid  of  the  general  cavity  of  the  body.  The  current  excited  by 
the  ciha  (see  arrows)  sets  strongly  into  the  mouth  of  the  tube,  c is  a thick- 
walled  enlargement  of  the  tube,  which  aids  the  current  either  by  its  sucking  or 
propelling  power.  At  d the  tube  is  attached  to  the  roof  of  the  cavity,  or 
side  of  the  septum,  by  means  of  a bridle  of  threads.  From  a to  f the  tube  is 
single,  and  the  contained  current  is  single ; from  f to  e ii  is  complexly  folded 
upon  itself,  as  shown  at  i,  h and  ji.  B represents  the  corresponding  tube  in 
the  other  moiety  of  the  segment,  in  outline. 
