292 
The  Natural  History  of  the  Liver-Fluke 
therefore,  that  not  only  does  the  race  of  the  liver-fluke  multiply 
and  increase  abundantly  in  the  sheep  by  producing  myriads  of 
eggs,  but  that  there  is  a further  and  great  increase  of  the  forms 
within  the  snail.  If  only  the  greatest  degree  of  ordinary 
increase  were  reached,  a single  fluke  might  give  rise  to  more 
than  a hundred  million  descendants  in  the  next  generation,  of 
liver-flukes  proper,  inhabiting  the  sheep.  But,  fortunately  for 
farmers,  the  chances  are  enormously  against  any  such  disastrous 
increase. 
The  habits  of  Limnceus  truncatulus. — The  habits  of  the  snail 
Limnceus  truncatulus,  which  serves  as  an  intermediate  host  (see 
p.  280),  are  of  much  importance,  for  they  show  how  the  snail  is 
likelv  to  become  infected  with  the  larval  forms  of  the  liver- 
fluke,  and  also  how  the  cysts  are  distributed  in  places  where 
they  have  a good  chance  of  being  picked  up  by  the  sheep. 
Limnceus  truncatulus  (Figs.  19,  20,  p.  302)  is  a fresh-water  snail, 
with  a brown  spiral  shell ; it  is  very  common,  and  has  a very  wide 
distribution  over  the  world.  It  is  very  small,  its  shell  never 
reaching  more  than  half  an  inch  in  length,  whilst  it  is  usually 
much  smaller,  a common  variety  in  England  scarcely  ever 
measuring  so  much  as  a quarter  of  an  inch.  Owing  to  its  small 
size  it  often  escapes  notice,  or  is  taken  for  the  young  of  some 
other  kind  of  snail,  so  that  it  has  no  popular  name.  Two  or 
three  of  the  kinds  of  snails  most  closely  allied  to  L.  truncatulus 
(for  instance,  Limnceus pereger)  may  occasionally  crawl  out  of  the 
water  for  short  distances ; but  in  Limnceus  truncatulus  itself,  the 
habit  is  so  much  more  strongly  developed,  that  the  snail  should 
be  termed  amphibious.  Indeed  it  is  oftener  found  out  of  the 
water  than  in  it.  When  kept  in  an  aquarium  it  quits  the  water, 
and  as  often  as  it  is  put  back,  insists  on  crawling  out  again,  so 
long  as  the  necessary  strength  remains.  It  is  said  to  breed  upon 
the  mud  on  the  banks  of  ditches.  Although  so  common,  it  is 
often  very  difficult  to  find,  no  doubt  owing  to  its  minute  size 
and  its  habit  of  wandering  from  the  water. 
As  showing  how  much  this  snail  lives  out  of  the  water,  it 
may  be  interesting  to  give  my  own  observations.  I have  related 
in  a former  paper  that  I was  quite  unable  in  1881  to  get 
Limnceus  truncatulus  for  my  experiments,  nor  did  I succeed  in 
doing  so  until  July  of  the  following  year,  when  there  were  floods 
on  the  river  Isis  near  Oxford.  The  waters  brought  this  snail 
down  in  vast  multitudes,  probably  from  its  breeding  haunts  in 
marshy  places  up  the  river.  It  was  extremely  abundant,  so 
numerous  indeed,  that  a single  sweep  of  a small  hand-net 
repeatedly  gave  me  more  than  500  examples,  and  this  was  in  a 
ditch  where  the  year  before  I could  not  obtain  a single  L.  trun- 
catulus. All  along  the  margins  of  the  ditches  the  ground  was 
