138 LIFE AND HER CHILDREN. 
there are others which, unable to get a living in 
the mud and sand, were forced to work their way 
into the bodies of snails, caterpillars, or grubs, and 
now make them their natural home. Unpleasant 
as it may be to think of these parasites, yet when 
we look at the question from their point of view, 
they are after all only doing their best to get 
a living, and they have many curious weapons to 
help them in doing it, nor do they always injure the 
animal upon which they live, unless they are in great 
numbers. 
Thus, for example, one of the flukes,* a minute 
flat -worm shaped like a tiny flounder, has a most 
strange succession of changes in its life. Firstly, 
The mother lives within the intestines of some 
water-bird, holding on firmly to her host by two 
rows of tiny hooks round her head, while her mouth 
is firmly applied like a sucker ; secondly, the eggs 
are thrown out and fall into the water or moist mud, 
and out of them comes, thirdly, the embryo or 
imperfect animal, surrounded with lashes ; but it 
does not long remain free, for out of it again comes 
a fourth form, a small bag-like animal, which at once 
seeks out a water-snail (Paludina) and clings to it. 
Nor are the transformations yet ended. Within this 
hanging sac, which is called the " nurse " of the fluke, 
there appear, fifthly, a number of little tailed ani- 
mals like tadpoles, and by and by the nurse bursts, 
and all these little creatures come swimming out 
once more free in the water. But the snail is not 
rid of them ; either upon her or upon some other 
snail like her, a number of these little creatures fix 
* Distoma militare. 
