284 
The  Natural  History  of  the  Liver-Fluke 
surface  of  the  animal,  with  the  exception  of  the  projection  just 
mentioned,  is  covered  with  the  long  hair-like  paddles  or  cilia, 
by  means  of  which  it  swims,  with  the  boring-tool  drawn  in, 
swiftly  and  restlessly  through  the  water.  It  is  exceedingly 
interesting  to  watch  it  speeding  through  the  water  ; it  is  won- 
derfully active  ; sometimes  it  goes  rapidly  forwards,  revolving 
upon  itself  like  a top,  just  twisting  a little  from  side  to  side, 
as  if  searching  for  something.  At  other  times,  by  curving  its 
body,  it  sweeps  round  in  circles,  or,  curving  itself  yet  more, 
spins  round  and  round  the  same  spot,  without  moving  from  it. 
Towards  the  front  end  of  the  embryo  are  two  black  marks 
(Fig.  3,  h ) ; they  have  the  shape  of  two  half-moons  placed  back 
to  back.  These  are  very  simple  eyes,  and  though  they  cannot 
give  the  embryo  the  power  of  seeing  very  distinctly,  yet  they 
render  it  very  sensitive  to  light,  and  are  no  doubt  of  very  much 
help  to  it  in  searching  for  its  new  home. 
When  the  embryo,  in  moving  through  the  water,  comes  in 
contact  W'ith  any  object,  it  pauses  for  a moment,  and  feels  about, 
as  if  trying  to  discover  its  nature  ; and  if  not  satisfied,  it  darts  off 
hastily  again.  But  if  the  object  be  the  particular  kind  of  water- 
snail  known  as  Limnseus  truncatulus,  it  at  once  tries  to  bore 
into  it.  This  snail  is  a very  common  one ; but  it  is  so  small 
that  it  usually  escapes  notice,  and  hence  it  has  no  common  or 
popular  name.  More  will  be  said  about  it  further  on  (p.  292). 
Under  ordinary  circumstances,  the  boring-tool  in  the  head  of  the 
embryo  (f  Fig.  3)  is  short  and  blunt,  but  as  soon  as  the  animal 
begins  to  bore,  it  becomes  longer  and  more  pointed.  The 
embryo  spins  round  and  round  on  itself,  just  as  the  handle  of 
a gimlet  is  turned,  the  hair-like  paddles  working  vigorously, 
and  pressing  the  little  embryo  against  the  surface  of  the  snail. 
The  pressure  is  increased  by  the  body  of  the  embryo  being 
alternately  drawn  together  and  then  suddenly  stretched  out.  As 
the  boring-tool  sinks  further  into  the  substance  of  the  snail  it 
becomes  longer  and  longer,  until  at  length  it  reaches  five  times 
its  original  length  (see  Fig.  4),  and  the  substance  of  the  snail 
is  forced  apart  as  if  by  a wedge  ; and  a gap  is  thus  made 
through  which  the  embryo  squeezes  its  way  into  the  snail. 
Figure  4 shows  the  embryo  whilst  in  the  act  of  boring  into 
a snail ; only  a very  small  portion  of  the  snail’s  body  has  been 
drawn. 
It  may  be  interesting  for  me  to  state  briefly  how  the  further 
development  of  the  embryo  within  the  snail  was  traced.  Very 
large  quantities  of  eggs  were  collected  from  the  livers  of  rotten 
sheep,  and  hatched  out  in  water.  The  embryos  were  then 
transferred  to  vessels  of  water  containing  many  different  kinds 
of  snails,  and  their  behaviour  watched.  When  it  was  discovered 
