280  The  Natural  History  of  the  Liver-Fluke 
back-boned  animal  with  its  food,  then  grow  much  in  size, 
become  adult,  and  reproduce  itself  by  eggs. 
The  snail  or  slug  which  harbours  the  first,  second,  and 
following  generations  is  called  the  intermediate  host,  the  back- 
boned animal  being  the  ultimate  or  final  host,  and  there  is 
thus  a constant  alternation  of  one  generation  living  its  adult 
life  in  the  back-boned  animal,  and  of  one  or  more  generations 
in  the  snail  or  slug.  This  is  an  example  of  what  naturalists 
call  an  alternation  of  generations. 
In  the  case  of  the  liver-fluke  there  are  at  least  two  genera- 
tions which  live  the  whole  of  their  adult  lives  in  the  particular 
snail  which  has  now  been  clearly  proved  to  serve  as  the  inter- 
mediate host  of  the  fluke.  The  third  generation  arises  as  a 
germ  in  the  snail,  and  whilst  still  a minute  larva  passes  in  the 
manner  explained  below  into  the  sheep.  That  is  to  say,  sup- 
posing we  start  with  the  fluke  as  it  is  found  in  the  sheep, 
its  child  does  not  grow  up  into  an  animal  like  the  parent,  nor 
does  the  grandchild  do  so.  Both  child  and  grandchild  look 
very  different  from  the  true  liver-fluke,  and  live  in  the  snail ; 
but  the  great-grandchild,  forming  the  third  generation,  does,  as 
a rule,  grow  up  into  a true  liver-fluke  and  live  in  the  sheep. 
Wonderful  as  this  may  seem  to  any  one  who  has  not  studied 
the  marvels  of  animal  life,  the  fact  of  alternate  generation  has 
been  established  beyond  all  doubt,  not  merely  for  the  liver- 
fluke,  but  also  for  numerous  other  forms  of  life,  which,  how- 
ever, do  not  at  present  concern  us. 
The  Eggs  of  the  Liver-Fluke. — The  adult  fluke,  whilst  living 
in  the  bile-ducts  of  the  sheep  or  other  rotten  animal,  produces 
vast  numbers  of  eggs.  The  bile-ducts  (popularly  known  as 
the  pipes ) become  large,  and  their  walls  thick,  owing  to  the 
presence  of  the  parasites.  The  bile  contains  numbers  of  eggs 
too  small  to  be  separately  recognised  by  the  naked  eye,  but 
which  are  nevertheless  present  in  such  quantities  that  they 
give  it  a dark  colour.  In  some  of  the  small  ducts  where  the 
eggs  have  not  been  washed  away  by  the  bile,  they  may  be  present 
in  such  enormous  numbers  as  to  form  a stiff  dark-brown  mass, 
looking  like  fine  wet  sand,  and  completely  blocking  up  the 
inside  of  the  duct. 
The  eggs  are  very  minute  indeed,  being  only  an 
inch  in  length  ; but  they  may  be  rendered  visible  to  the  naked 
eye  by  placing  some  of  the  dark-brown  contents  of  the  bile- 
ducts  into  a glass-vessel  with  water  and  shaking  it  up.  On 
now  holding  the  vessel  up  to  the  light,  the  tiny  eggs  will 
be  seen  as  brownish  specks  suspended  in  the  water.  If  they 
are  very  numerous,  they  will  colour  the  water  a light  brown,  or 
may  even  give  it  a dark  coffee-colour.  They  are  a little  heavier 
