CESTODE  LIFE-HISTORIES

Tai-nia  scrrata,  one  of  the  commonest  tape-worms  of  the  dog,  affords
an  cxrrllent  example  of  the  typical  cestode  life-history.  The  further
development  of  the  egg  takes  place  if,  and  only  if,  it  be  swallowed  by  a
rabbit.  In  this  event  there  hatches  out  within  the  alimentary  canal
.1  small  rounded  larva,  provided  with  six  sharp  blades  by  means  of
which  it  cuts  its  way  into  the  wall  of  the  alimentary  canal.  Eventually
!  the  blood  and  is  carried  off  in  the  blood-stream  towards  the
liver.  It  apparently  usually  leaves  the  blood-vessel  within  that  organ
and  then  may  migrate  for  some  distance  before  it  finds  a  suitable  spot,
Mi«h  a>  the  linin-  of  the  body-cavity,  in  which  to  settle  down.
1 1  now  ^rows  into  the  cysticercus  or  bladder-worm — a  semi-transparent

tped  vesicle  about  the  size  of  a  pea  and  full  of  clear  fluid.
i.-h  the  wall  of  the  vesicle  can  be  seen  shimmering  an  elongated

whitish  body  which  projects  inwards  from  one  pole.     In  a  well-infected

! 'ladder-worms  may  be  seen  scattered  about  in

it  the  bo.ly-cavity.    If  a  fresh  bladder- worm  be  slightly  squeezed

between  the  fin-ers  the  whitish  structure  within  it  shoots  out,  and  it,


v  TAENIA  175

is  now  seen  to  be  a  typical  scolex,  with  suckers  and  hooks,  which  had
been  inverted  into  the  interior  of  the  cysticercus.
If  a  live  bladder-worm  is  swallowed  by  a  dog  the  scolex  becomes
everted  within  its  alimentary  canal  and  attaches  itself  to  the  lining  by
means  of  its  hooks  and  suckers.  The  vesicle  is  digested  off  and  the
scolex  proceeds  to  grow  in  length  and  bud  off  the  chain  of  proglottides
so  that  it  assumes  the  characters  of  the  typical  tape-worm.
