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The  Natural  History  of  the  Liver-Fluke 
the  autumn  of  1879,  and  when  it  was  killed  on  the  18th  of 
December,  1880,  its  liver  was  found  to  be  still  full  of  flukes. 
Summary  of  the  Life-History. — The  adult  fluke  in  the  liver  of 
the  sheep  produces  enormous  numbers  of  eggs,  which  are  dis- 
tributed with  the  droppings  of  the  sheep.  If  these  eggs  have 
moisture  and  a suitable  degree  of  warmth,  they  continue  to  live, 
and  in  each  is  formed  an  embryo.  The  embryo  leaves  the  egg 
and  swims  in  search  of  the  particular  snail,  Limnceus  trun- 
catulus , within  which  its  future  life  and  growth  takes  place. 
The  embryo  bores  into  the  snail,  and  then  grows  into  the  form 
which  is  called  a sporocyst.  The  sporocyst  gives  rise  to  the 
second  generation.  This  generation  is  known  as  redia.  The 
rediae  in  turn  produce  the  third  generation,  which  has  the  form 
of  a tadpole,  and  is  called  cercaria.  The  cercariae  quit  the  snail, 
and  enclose  themselves  in  envelopes  or  cysts,  which  are  attached 
to  the  grass.  When  the  grass  to  which  the  cysts  adhere  is  eaten 
by  the  sheep,  or  other  suitable  host,  the  young  liver-fluke  comes 
out  of  the  cyst  and  takes  up  its  abode  in  the  liver  of  its  host, 
and  the  fatal  circle  is  thus  completed. 
It  will  be  seen,  therefore,  that  the  fluke-disease  is  a disease 
which  alternates  between  a particular  snail  and  the  sheep.  A 
sheep  cannot  take  the  infection  directly  from  another  sheep,  nor 
can  one  snail  take  it  directly  from  another  snail.  The  sheep,  by 
spreading  the  eggs  of  the  fluke,  gives  the  infection  to  the  snail, 
and  the  snail  in  turn,  by  harbouring  and  distributing  the  cer- 
cariae, conveys  the  infection  to  the  sheep. 
Part  III. — The  Prevention  of  Rot. 
Now  that  we  have  followed  the  whole  of  the  life-history  of  the 
parasite,  and  know  where  it  is  to  be  found  and  to  be  fought,  at 
all  the  stages  of  its  varied  existence,  we  are  in  a position  to 
discuss  the  important  practical  bearings  of  the  question.  We 
may  therefore  proceed  with  the  consideration  of  the  preventive 
measures,  which,  if  carefully  carried  out,  will,  it  is  believed, 
enable  us  to  reduce  the  losses  to  a comparatively  insignificant 
amount ; and  eventually,  if  all  farmers  will  combine  their 
efforts,  totally  stamp  out  the  disease,  so  that  it  may  become 
altogether  a thing  of  the  past. 
Let  me  first  state  the  conditions  necessary  for  the  existence 
of  liver-rot,  as  proved  by  the  foregoing  research,  and  discuss 
the  measures  which  will  prevent  the  conditions  being  satisfied, 
and  therefore  prevent  the  disease  itself ; and  lastly  give  a 
summary  of  the  preventive  measures. 
The  conditions  necessary  for  the  existence  of  liver-rot  in  any 
given  locality  are  as  follows : — 
