and  the  Prevention  of  Rot. 
285 
that  the  embryos  bored  into  the  particular  kind  of  snail  called 
Limnceus  truncatulus,  large  numbers  of  this  kind  were  got. 
Embrvos  of  the  liver-fluke  were  allowed  to  bore  their  way  into 
them,  and  then  they  were  dissected 
at  definite  intervals — some,  three,  six,  ^ 
or  twelve  hours  later,  others  on  each 
successive  day,  until  the  whole  history 
was  traced.  In  this  way  the  changes 
passed  through  by  the  animals  found 
in  the  snail  could  be  followed  step 
bv  step,  and  the  relation  of  the  dif- 
ferent generations  proved  beyond  any 
possibility  of  doubt. 
The  embryo  of  the  liver-fluke  will 
not  bore  into  all  snails  alike  ; the 
only  other  kind  of  snail  into  which 
I have  found  it  enter  at  all  is  the 
one  called  Limnceus  yereger.  But  it 
is  only  into  the  very  youngest  and 
smallest  snails  of  this  kind  that  the 
embryo  will  force  its  way,  and  even 
these  do  not  afford  a suitable  place 
for  its  further  growth,  for  every  em- 
bryo that  succeeds  in  entering  perishes 
at  an  early  stage. 
The  embryo  seems  to  know  by  in- 
stinct the  Tight  kind  of  snail.  f hus  Embryo  of  Liver-fluke  boring  into  a 
T Uvo  nlnrwl  larcrp  nnmlipn  of  snail-  0ntr  a Tery  S[naU  Part  of 
1 na\e  placed  iaroe  niunuera  OI  the  snail,  has  been  drawn.  Magnified 
freshly-hatched  embryos  in  a small  370  times.  (OrigmaL— a.  p.  r.) 
vessel  of  water  containing  Limnceus 
truncatulus  and  several  other  kinds  of  fresh-water  snails  ; and  on 
subsequently  examining  them  all,  have  found  the  specimens  of 
Limnceus  truncatulus  to  contain  as  many  as  fifty  or  more  fluke- 
embryos  each,  whilst  the  other  kinds  of  snails  were  entirely  free 
from  them. 
The  natural  place  for  the  further  growth  of  the  embryo  is 
either  in  or  near  the  lung  of  the  snail.  Once  safely  lodged 
in  the  suitable  position,  the  embryo  undergoes  a great  change 
of  form.  It  is  now  settling  down  to  a sluggish  parasitic  life. 
It  no  longer  needs  the  paddles  to  propel  it  through  the  water, 
and  they  are  therefore  thrown  off.  It  no  longer  needs  eyes  to 
help  it  to  find  its  way  about,  for  it  scarcely  stirs  in  the  snail, 
and  so  they  get  indistinct,  and  may  at  length  be  altogether  lost. 
The  form  of  the  body  is  also  changed,  and,  instead  of  being 
like  a sugar-loaf  in  shape,  it  becomes  oval. 
Figure  5 shows  the  animal  whilst  the  change  is  taking  place. 
