LIFE'S SIMPLEST CHILDREN. 31 
slime manages to suck their bodies out of the shells. 
Still these hard spiky outside skeletons must be a 
great protection to them, and we find every kind of 
shape devised by these wonderful architects in the 
construction of their tiny houses, though these are so 
small as to look like a grain of sand when seen by 
the naked eye. Perhaps the most wonderful of all is 
the one shown at/, Fig. 8. It is broken open to show 
the three balls one within another, each kept in its 
place by rods of flint passing through the whole. 
This beautiful little shell looks just like the carved 
balls of the Chinese, yet, instead of being the work of 
intelligent man, it is built by a mere mass of slime. 
We have now learned to know the simplest of all 
animals ; how they live, and move, and the homes 
they build. All the forms are not quite equally 
simple, for some of the higher ones have a solid 
spot or nucleus in the middle of the slime, and some- 
times a small wateiy bubble, as in the Monad or the 
Bell-flower, which contracts and expands at intervals : 
and in these forms the outside of the slime is rather 
thicker than the inside, so that we might say that 
they are on the road to having a skin, while the 
shell-builders have a uniform slimy body. But both 
classes alike belong to that first and lowest branch 
of the children of life, called by scientific men the 
Protozoa (protos first, zoon animal) or first animals. 
The still water everywhere is swarming with them, 
though we may see and know nothing of them. Yet 
we owe them something ; for not only do the dead 
shells of many of them form our solid ground, but 
those now living purify our waters by feeding upon 
the living and dead matter in them. These tiny 
