23 
This worm is 
very common in 
Europe and not 
uncommon in the 
United States. 
From an agricul- 
tural standpoint it 
is the most impor- 
tant of the flukes, 
but from a stand- 
point of human 
medicine it is much 
less important 
than several other 
forms, notabl}^ 
Fa r a g o n i m u s , 
ho reins ^ and 
SchlstOSO))Kl, 
Life history. — 
As an example of 
the life histoiy of 
flukes, Ave ma}" take 
that of Fasciola 
h e i> at I c a , w hich 
ma}^ be summa- 
rized as follows: 
(a) The, adult herma- 
phroditic v'orin (figs. 
19-20), the characters 
of which are given on 
p. 22, fertilizes itself 
or a cross fertilization 
of two individuals 
takes place in the bili- 
ary passages of the 
liver, and produces a 
large number ( esti- 
mated at 37,000 to 
45,000) of eggs. 
(5) Eggs (figs. 21 
and 22). — Each egg is 
composed of the fol- 
lowing parts: (1) A 
true germ cell, which 
originates in the ovary 
and is destined to give 
rise to the future em- 
bryo; (2) a number of 
vitelline or yolk cells, 
which are formed in a 
