and  the  Prevention  of  Rot. 
291 
however,  be  remembered  that  whilst  the  perfect  animals  emerge 
from  the  chrysalides  of  insects  of  their  own  accord  as  soon  as 
Fig.  12. 
A cercaria  in  the  act  of  forming  its  cyst  or  enve- 
lope. Magnified  160  times.  (Original. — A.  P.  T.) 
Fig.  13. 
Three  cysts  of  the  Liver-fluke  attached  to  a portion  of 
grass-stalk.  Each  cyst  contains  a young  fluke,  which 
remains  quiet  until  the  cysts  are  swallowed  with  the 
grass  by  a sheep  when  grazing.  Magnified  10  times. 
(Original. — W.  H.  J.) 
they  are  ready  to  do  so,  and  live  a free-winged  existence,  the 
animals  will  only  come  fortrh  from  the  cysts  of  the  liver-fluke 
when  the  grass  or  plant  to  which  they  were  attached  has  been 
swallowed  by  some  sheep  or  other  suitable  animal.  If  the  cysts 
are  not  so  swallowed  within  a few  weeks,  the  young  flukes  which 
they  contain  will  perish. 
The  number  of  cercariae  descended  from  a single  fluke-egg 
cannot  be  less  than  200,  and  is  sometimes  much  larger.  For 
the  rediae  forming  the  second  generation,  instead  of  producing 
cercariae,  may,  and  perhaps  as  a rule  do,  give  rise  to  another 
generation  of  daughter-rediae  resembling  themselves,  and  these 
then  produce  cercariae.  When  this  is  the  case,  a single  fluke-egg 
may  give  rise  to  more  than  a thousand  cercariae.  It  will  be  seen, 
u 2 
