384 
MR.  R.  OWEN  ON  THE  ANATOMY  OF  DISTOMA  CLAVATUM, 
stoma  clavatum.  The  surface  of  the  cavity  in  Distoma  clavatum,  of  which  the  small  pos- 
terior orifice  is  the  outlet,  singularly  contrasts  with  that  of  the  receptacles  on  either 
side  ;  being  of  a  yellowish  white  colour,  and  smooth.  The  office  of  this  posterior  sac 
being  most  probably  excretory,  it  may  be  regarded  as  exhibiting  a  rudimentary  condi- 
tion of  the  respiratory  system. 
This  large  species  oi  Distoma  exhibits,  according  to  Mr.  Menzies,  well-marked  loco- 
motive powers.  "  In  moving,"  he  observes,  "  it  fastens  itself  alternately  by  the  ven- 
tral aperture  and  its  mouth,  raising  its  slender  neck  between  them  into  an  arched  form, 
like  a  leech,  and  in  this  manner  drags  its  body  along  with  a  slow  motion.  It  is  of  a 
whitish  colour,  somewhat  pellucid,  discharging  at  its  mouth  a  black-coloured  fluid, 
which  can  easily  be  perceived  through  its  body'."  The  superaddition  of  an  excretive 
organ  is  in  just  accordance  with  the  muscular  energies  above  observed  ;  and  the  ana- 
logy to  the  Leech  is  further  shown  in  the  form  of  the  cells,  which  at  the  posterior  part 
of  the  body  communicate  with  and  form  part  of  the  digestive  apparatus  ;  especially  of 
the  last  two  cavities,  which  very  much  resemble  the  last  pair  of  gastric  caca  occu- 
pying a  similar  position  in  the  Leech. 
PLATE  XLI. 
Figg.  17  to  20.  Distoma  clavatum. 
Fig.  17.  Yentral  smface  oi  Distoma  clavatum. 
a.  Anterior  pore,  where  the  true  mouth  is  situated.  .  ,  ^ 
6.  Posterior  depression,  suctorial  disc,  or  holder. 
c.  The  genital  orifice. 
d.  The  orifice  of  the  posterior  sac. 
18.  Distoma  clavatum,  dissected  from  the  dorsal  aspect. 
e.  The  cuticular  wrinkled  integument. 
/.  The  muscular  integument. 
g.  The  anal  sac,  laid  open :  a  bristle  is  placed  through  the  orifice. 
h.  h.  Cavities  external  to  the  anal  sac,  with  a  black  minutely  wrinkled  surface. 
i.  Masses  of  ova. 
k.  The  oviduct. 
I.  The  testis. 
m.  Vessels. 
19.  Posterior  view,  showing  the  muscular  discs  surrounding  the  oral,  a',  and  pre- 
hensile, b',  fosses,  and  the  termination  of  the  genital  tube,  c'. 
m.  The  vascular  circle  surrounding  the  oral  disc. 
20.  The  oviduct,  k,  and  testis,  I. 
'  Loc.  cit.,  p.  188. 
