290 
The  Natural  History  of  the  Liver-Fluke 
Fig.  11. 
of  its  surface  a gummy  substance  containing  many  granules 
(see  Fig.  12),  which  are  the  same  as  those  already  described  as 
forming  masses  at  the  sides  of  the 
body  of  the  free  cercaria.  The 
tail  meanwhile  is  wagged  violently, 
and  before  long  a more  vigorous 
movement  throws  it  off  altogether. 
The  gummy  substance  hardens  on 
exposure,  and  the  body  of  the 
cercaria  is  thus  enclosed  in  an 
envelope  which  protects  it  from 
injury. 
These  envelopes  are  termed 
cysts  (the  word  cyst  meaning  a 
bladder).  They  are  snowy-white, 
and  numbers  of  them  may  be 
found  adhering  to  the  walls  of 
the  aquarium  in  which  the  in- 
fected snails  are  kept,  or  attached 
to  the  dark-green  leaves  of  the 
water-plants. 
If  the  snails  are  crawling  on 
the  margin  of  a ditch  or  over  a 
damp  field,  the  cercariae  on  leaving 
the  snail  at  once  proceed  to  form 
their  envelopes  or  cysts  at  the 
bottom  of  the  grass,  and  so  attach 
themselves  to  the  stalks,  or  leaves, 
near  the  roots. 
Fig.  12  shows  a cercaria  in 
the  act  of  forming  its  cyst,  and 
Fig.  13,  which  is  less  highly  mag- 
nified, shows  three  fully-formed 
cysts  adhering  to  a portion  of  a 
grass-stalk.  As  even  the  latter 
figure  is  magnified  to  ten  times 
the  actual  size  of  the  object,  the 
minute  size  of  the  cysts  will  be 
easily  appreciated.  They  are  really 
only  one-hundredth  of  an  inch 
across. 
These  cysts  are  the  form  in  which  the  liver-fluke  is  swallowed 
by  the  sheep,  and  the  animals  within  them  are  young  flukes. 
They  remain  adherent  to  the  grass  until  the  time  comes  when 
they  are  picked  up  by  the  sheep  feeding  on  the  ground.  They 
may  be  compared  to  the  pupa;  or  chrysalides  of  insects.  It  must, 
A free  cercaria;  (third  generation)  of  the 
Liver-fluke  as  it  is  seen  when  swimming 
in  the  water  : y,  i/1,  the  two  suckers  cor- 
responding to  the  suckers  of  the  adult 
fluke ; in,  intestine  ; cy.  the  rounded 
masses  at  the  sides  of  the  body  which 
provide  the  material  for  the  formation 
of  the  cyst  or  envelope.  Magnified  160 
times.  (Original. — A.  P.  T.) 
