and  the  Prevention  of  Rot. 
289 
upon  which  they  feed.  A redia  differs  from  a sporocyst,  its 
parent,  in  several  points.  The  most  important  difference  is  that 
the  former  has  a mouth  and  an  intestine  (Fig.  10,  in)  shaped 
like  a bottle,  whereas  the  sporocyst  has  none.  Its  habits  are 
more  active ; towards  its  hind  end  are  two  short  projections 
(Fig.  10,  r),  which  serve  as  legs  of  a very  simple  kind,  and 
prevent  the  animal  from  slipping  backwards  as  it  moves  through 
the  body  of  its  host. 
If  an  infected  snail  which  has  a clean  and  transparent  shell 
be  chosen,  the  redia?  may  be  watched  by  the  aid  of  the 
microscope  as  they  move  inside  the  living  snail. 
In  forcing  their  way  through  the  body  of  the  snail  the  para- 
sites necessarily  do  a great  deal  of  harm  ; so  much  indeed,  that 
in  my  experiments  in  the  laboratory  I found  very  few  snails  live 
many  weeks  after  infection.  In  fact,  the  fluke-disease  is,  in  the 
laboratory  at  least,  more  destructive  to  the  snail  than  it  is  to 
the  sheep. 
Within  the  redia  are  found  germs,  the  third  generation,  which 
at  first  are  very  like  those  produced  in  the  sporocyst.  But  later 
on  we  see  that  each  germ  becomes  an  animal  shaped  like  a 
tadpole,  with  a flat  oval  body  and  long  slender  tail.  This 
animal  is  usually  called  a cercaria,  the  name  simply  meaning 
that  the  animal  has  a tail. 
An  adult  redia,  such  as  is  shown  in  Fig.  10,  may  grow  to  the 
length  of  one-sixteenth  of  an  inch.  It  contains  about  a score  of 
germs  in  all  the  different  stages  of  growth  ; there  are  usually 
two  or  three  so  far  advanced  that  they  have  become  tadpole- 
shaped and  are  ready  to  come  forth.  They  escape  from  the 
parent  by  a special  opening  ( v . Fig.  10),  and  then  crawl  or 
wriggle  their  way  out  of  the  snail. 
Now  these  tadpole-like  animals  are  destined  to  enter  the  . 
sheep  and  there  become  true  liver-flukes,  and  I shall  have  to 
show  how  they  get  inside  it. 
When  the  snails  infested  with  the  above-described  forms  of  the 
liver-fluke  are  kept  in  an  aquarium,  the  cercariae  or  tadpole-shaped 
larva?  may  occasionally  be  found  swimming  about  in  the  water 
in  a very  lively  manner.  Fig.  11  fp.  290)  shows  a free  cercaria  as 
it  appears  when  swimming.  It  has  a flat  oval  body  about  of 
an  inch  in  length,  with  a tail  more  than  twice  as  long.  There 
are  two  suckers  ( y and  y)  corresponding  to  those  of  the  adult 
fluke,  and  at  the  sides  of  the  body  are  many  granules  arranged 
in  rounded  masses  ( cy ). 
The  free-swimming  life  of  the  cercaria  never  seems  to  last  long, 
for  on  coming  in  contact  with  the  sides  of  the  aquarium  or  the 
water-plants  contained  in  it,  the  little  animal  soon  comes  to  rest, 
draws  its  body  up  into  a round  ball,  and  exudes  from  the  whole 
YOL.  XIX. — S.  S.  U 
