THE OUTCASTS OF ANIMAL LIFE. 139 
themselves, and each one boring into her foot, drops 
off its tail, and forming a transparent bag round its 
body, begins to grow a crown of booklets. In this 
state it remains till the snail, gobbled up by some 
water-bird, passes into its stomach, and there the 
gastric juice, digesting the snail, dissolves the bag, 
and at last the fluke becomes a perfect animal again, 
fixing itself by hooks and suckers in the same kind 
of home from which its mother came. 
And now consider what a number of chances 
occur to this animal during its short life, any of which 
may destroy it. Their eggs are not placed in a fit 
spot by a careful mother, but fall wherever the bird 
may chance to drop them, and twice in their lives 
they have to find a snail in which alone they can live 
and grow. Many fail, and clinging to stones or 
weeds, die for want of their home. And even if they 
succeed in these first attempts, the last step of all is 
entirely out of their control, for unless they are car- 
ried down the throat of the water-bird, they can 
never grow and lay eggs. But they exist in such 
myriads that this is of no consequence to the race. 
You can scarcely cut open any snail without finding 
some of these curious creatures within it, different 
species living in different snails ; and in most cases 
the worm must pass into another animal to become 
complete. The liver-fluke of the sheep for example, 
which causes the " rot " when too abundant, lives its 
early life in a snail, which is licked up by the sheep 
as they eat the damp grass. 
The bladder-worm, however, which gets into the 
brain of the sheep, and causes it to hang its head, 
belongs to another and perhaps more dangerous 
