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The  Natural  History  of  the  Liver-Fluke 
prevent  the  rot,  but  will  have  the  further  advantage  of  greatly 
improving  the  herbage. 
Where  it  is  not  practicable  to  drain  the  land  at  once,  either 
salt  or  lime  may  be  scattered  over  the  surface.  Both  these 
substances  will  destroy  the  embryos  of  the  fluke,  and  at  a later 
period  the  cysts  when  attached  to  the  grass.  And,  still  further, 
they  will  destroy  the  snails  which  serve  as  hosts  to  the  inter- 
mediate forms  of  the  liver-fluke.  The  freedom  from  rot  of  sheep 
feeding  on  salt-marshes  is  well-known,  and  is  now  shown  to  be 
due  to  the  poisonous  action  of  the  salt  on  the  embryos,  sporocyst, 
redia,  cercaria,  and  cyst,  and  to  its  similar  action  on  Limnceus 
truncatulus  itself.* 
Even  a weak  solution  of  salt  in  water  (containing  only 
f per  cent,  of  salt)  proves  fatal  to  this  snail.  Dressings  of  salt 
have  the  advantage  over  lime  in  not  spoiling  the  grass  for 
immediate  use,  whereas  lime  will  do  so.  It  may,  however,  at 
times  be  better  for  the  land  itself  that  lime  should  be  applied. 
It  will  be  for  the  farmer  to  decide  which  dressing  should  be 
used. 
It  is  naturally  of  much  importance  that  the  salt,  or  lime, 
should  be  distributed  at  the  right  time  of  the  year,  when  fluke- 
germs  and  snails  are  present  in  the  greatest  numbers.  The  snail 
buries  itself  in  mud,  or  soil,  in  the  winter-time,  and  owing  to 
the  cold  no  embryos  are  hatched  out  at  that  time.  If  the  weather 
be  warm  in  April,  it  is  possible  that  a few  may  be  hatched  out 
towards  the  end  of  the  month,  but  they  will  not  be  numerous. 
In  May,  however,  greater  numbers  will  be  hatched,  and  still 
more  in  June  and  July.  These  two  months  are  the  time  of 
the  year  when  the  snails  are  most  liable  to  be  infected.  As 
more  eggs  are  distributed  through  the  whole  of  the  summer 
by  fluked  animals,  it  is  clear,  of  course,  that  the  production  of 
embryos,  though  in  less  numbers,  will  continue  from  August 
until  the  time  when  the  development  is  checked  by  autumnal 
cold. 
Even  under  the  most  favourable  circumstances  it  will  be  seven 
weeks  before  the  cercariae  are  formed  within  the  snail  so  as  to 
be  ready  for  transference  to  the  sheep.  Animals  may  take  the 
rot  as  early  as  June,  but  this  is  not  of  frequent  occurrence.  In 
July,  however,  the  germs  or  cysts  on  the  grass  become  more 
numerous,  though  still  comparatively  scarce.  In  August  more 
will  be  found,  but  it  is  in  the  months  of  September  and  October 
that  they  are  most  numerous.  These  are  the  most  dangerous 
months.  The  Northumberland  farmers  hold  that  the  end  ol 
* It  may  be  worth  noting  that  water-cress  should  be  steeped  in  salt  and  water 
before  it  is  eaten,  so  ns  to  guard  against  any  possible  infection  from  adhering 
cysts  of  the  liver-fluke. 
