106  DE.  T.  WILLIAMS’S  EESEAECHES  ON  THE  STEHCTHEE  AND  HOMOLOGY 
certain  conditions  of  growth  some  extraordinary  function  devolves  upon  the  tube  itself  1 
else  why  this  exceptional  condition  of  the  vessels '?  On  this  point  the  author  does  not 
at  present  wish  to  pledge  himself  to  any  particular  doctrine,  not  even  to  the  opinion 
which  he  ventured  to  express  in  his  Keport  in  1851,  or  the  opposite.  All  he  is  desirous 
to  state  is  that  this  periodic  afflux  of  an  extraordinary  quantity  of  blood  to  the  region 
of  the  ordinary  segmental  organs,  in  the  Earth-worm,  seems  to  indicate  that  under  the 
constraining  influence  of  certain  exceptional  conditions  they  may  experience  a generative 
activity.  On  no  occasion,  however,  has  he  been  able  to  prove  this  by  the  discovery  of 
the  presence  of  the  real  ova  and  young  of  the  worm  itself.  But  in  support  of  this  view 
the  question  may  be  asked,  if  the  “ ordinary”  segmental  organ  were  designed  merely 
and  exclusively  to  drain  off,  in  an  wnselecting  mechanical  manner,  a given  quantity  in  a 
given  time,  of  the  chylaqueous  fluid,  does  it  not  seem  incomprehensible  that  so  highly 
organized,  such  a completely  constructed  apparatus  should  have  been  pro'^ided  to  exe- 
cute a purpose  so  simple  % This  latter  is  undoubtedly  its  purpose  in  Nais,  and  accord- 
ingly the  organ  is  reduced  to  the  characters  of  a simple  non-vascular  tube.  It  cannot 
be  the  exphatory  half  of  a respiratory  apparatus,  as  supposed  by  Letdig  ; such  a sup- 
position is  opposed  to  all  that  is  known  of  the  mechanism  of  resphation  in  the  inver- 
tebrated  animals.  If  it  be  designed  to  fulfil  the  offlce  of  an  excretory  gland,  it  may  be 
said  that  by  its  open  internal  mouth  it  may  eliminate,  in  body  and  du'ectly,  the  chyl- 
aqueous fluid,  while,  from  its  peculiar  connexion  with  the  vascular  system,  it  could 
only  remove  the  contents  of  the  latter  by  the  normal  cell-agency  method  of  membranous 
secretion. 
Accepted  in  this  latter  sense,  they  may  be  said  to  discharge  a renal  function.  In 
relation  to  the  fluid  contents  of  the  vascular  system,  these  organs  may  be  said  to  be 
capable  of  exercising  a selective  influence  in  vu’tue  of  some  property  resident  in  the 
membranous  walls  of  the  vessels  and  tube.  But  such  selective  property  cannot  be  said 
to  reside  in  the  cilia  with  which  the  trumpet-shaped  extremity  of  the  tube  is  armed. 
They  act  on  the  cavitary  fluid  simply  mechanically.  If,  therefore,  the  segmental  organ 
stands  in  the  relation  of  a kidney  to  the  blood  proper,  it  is  scarcely  possible  that  it  can 
do  so  in  relation  to  the  chylaqueous  fluid,  unless  we  admit  the  maxim  that  the  simpler 
the  fluid,  the  simpler  is  the  anatomical  construction  of  the  solid  organs  designed  to  react 
upon  it. 
These  botryoidal  appendages  to  the  vessels  of  the  segmental  organ  do  not  appear  to 
exist  in  Nais,  or  if  they  do,  they  are  so  small  as  to  be  invisible  even  under  a high  power ; 
if  they  do  not,  it  is  a point  of  difference  between  the  structure  of  the  part  in  Nais  and 
Lumhriciis. 
In  parting  with  this  subject,  the  reader’s  attention  is  particularly  di’auui  to  the  analogy 
which  exists  between  the  botryoidal  or  pear-shaped  caecal  appendages  which  project 
laterally  from  certain  of  the  vessels  of  the  segmental  organ  of  Inmibricus  and  those 
vascular  tufts  and  pouches  (hereafter  to  be  described)  of  the  segmental  organs  of 
Arenicola,  Terebella^  and  those  of  the  Nereid  family. 
