or  THE  EEPRODTJCTIVE  OEGANS  OF  THE  ANNELIDS. 
95 
Nais  serpentina  may  be  taken  as  the  type  of  the  genus.  In  this  species  the  segmental 
organ  is  very  readily  studied  in  consequence  of  the  softness  and  transparence  of  the  inte- 
guments, Two  of  these  organs  are  present  in  every  ring  of  the  body,  one  on  either  side 
of  the  intestine.  (Fig.  1 A,  B,  Plate  VI.  indicates  the  system  of  one  segment ; at  A the 
organ  is  shown  in  its  perfect  state,  at  B in  outhne.)  Each  organ  (A  & B)  commences  (or 
if  the  course  of  the  contained  current  be  considered,  ends)  in  a single  external  orifice  [a). 
For  some  distance  from  this  attached  end  (to  (e))  the  organ  bears  a single  tube,  the  figure 
being  tape-  or  band-shaped,  and  the  walls  thick.  From  this  point  (e)  to  (f),  a point  not 
far  from  the  free  extremity,  the  band  carries  a double  current,  one  tending  towards  and 
the  other  out  from  the  general  ca^dty.  This  is  only  apparent ; it  arises  from  the  agglu- 
tination of  two  portions  of  the  cihated  tube.  If  this  part  were  examined  alone,  it  might 
be  concluded  that  the  entire  organ  was  double,  bearing  a double  current,  one  going  in 
and  the  other  setting  out  from  the  general  cavity.  This  apparent  double  current  in  the 
mid-portions  of  the  organ  proves  a source  of  extreme  perplexity  to  the  observer.  By 
careful  observation  it  may  be  distinctly  seen  that  both  at  the  attached  portion  (e)  and 
the  free  floating  portion  (f)  the  band  is  small  and  the  current  single.  It  will  now  be 
understood  that  the  double  currents,  setting  in  opposite  directions,  which  are  seen  when 
the  middle  third  of  the  tube  (^,  g,  h)  is  in  focus,  result  from  the  folding  of  the  tube  upon 
itself.  Insignificant  as  this  point  may  seem,  it  involves  in  truth  the  important  questions 
whether  the  segmental  organ  be  single  or  double,  whether  it  carries  one  current  or  two, 
whether  in  fact  it  consists  of  a simple  siphon  leading  out  of  the  general  cavity,  or  of  a 
complex  apparatus  adapted  at  once  to  excrete  and  to  renew  the  chylaqueous  fluid.  The 
solution  of  this  problem  involves  great  care  and  patience. 
At  its  free  internal  extremity  (A,  ^),  this  organ  is  held  in  a determinate  position  by 
means  of  bridles  of  delicate  threads  fixed  at  {d).  This  arrangement  is  one  of  great  in- 
terest, since  the  free  open  end  (A,  b)  is  thus  held  in  a fixed  position,  one  in  which  it  can 
never  become  obstructed.  By  the  action  of  the  large  and  vigorous  cilia  (A,  ^),  a strong- 
current  of  fluid  is  drawn  into  this  open  mouth  (see  arrows,  fig.  I,  A).  At  a short  distance 
from  this  extremity,  which  is  itself  trumpet-shaped,  the  tube  undergoes  a remarkably 
bulbous  enlargement  (c).  Here  the  interior  bore  or  channel  is  also  dilated.  The  walls 
are  thick  and  muscular,  and  capable  of  suddenly  closing  and  again  of  expanding ; a 
rhythmic  movement,  which  no  doubt  is  intimately  connected  with  the  poAver  of  this  organ 
to  discharge  the  chylaqueous  fluid.  This  contractile  property  resides  indeed  in  the  parietes 
of  the  entire  organ.  It  is  a highly  irritable  structure.  At  one  moment  it  may  be  seen 
relaxed,  outstretched  and  floating  in  the  cavitary  fluid,  at  the  next  coiled  or  shrunk  up 
into  a minute  ball;  while  in  this  latter  state  of  contraction,  the  current  of  the  cilia- 
driven  fluid  coursing  in  the  interior  is  entirely  stopped.  As  soon  as  the  tube  relaxes,  it 
goes  on  again,  and  so  on.  Thus  the  normal  action  of  the  tube  is  dependent  upon  the 
cilia  by  which  the  bore  is  lined,  the  irregular  action  being  due  to  an  irregular  contrac- 
tile moA'ement.  From  the  form  and  structure  of  the  trumpet-shaped  extremity,  from 
the  setting  of  the  ciliary  currents  at  the  mouth  and  at  the  base  of  the  tube,  it  seems 
0 2 
