ow a certain path?

The narrow canals in the Sponge are lined with lashes, or tiny hairs, so
very small that you can just see them through a microscope. Now the
secret of the wonderful water-current is a secret no longer. As long as
the Sponge lives, these little lashes are always moving, always lashing
the water along in one direction. They cause it to follow its proper
course, through and through the Sponge, and out again into the sea. On
its way it loses the tiny scraps of food which it contains, and carries
away any waste stuff out of the Sponge.

You will have noticed that there are various kinds of Sponges in the
market; some are large and flat, others small and cup-shaped; some are
soft, and others rather hard. They are all somewhat horny and elastic.
This "spongy" material is the skeleton of the Sponge animal, cleaned and
dried for your use. Some kinds of Sponge would tear your skin if you
tried to use them, for they have a hard skeleton. It is made of lime,
and sometimes of flint, which the Sponge obtains from its food. Of
course we use only those sponge-skeletons which are soft; but the
cheaper kinds do often contain little flinty needles.

The best washing-sponges live in warm seas, attached to the rocks on the
sea-bed. Divers go down and obtain them; or else they are dredged up,
cleaned, dried, and sorted, and then sent to the market. Some Sponges,
called Slime Sponges, have no skeleton, being merely a living mass of
slime.

Coral is also the hard skeleton of a little animal, known as the Coral
Polyp. The rest of the polyp's body is soft jelly, which many fish
regard as good food. The Sea Anemone--another jelly-animal--is first
cousin to the Coral Polyp. And we may call the Jellyfish second cousin
to these two, for it is in the same big division of the animal kingdom.

The pretty red Coral, then, is really the hard part of a little
jelly-animal. This animal is much like a Sea-anemone, with a hard
skeleton of lime. Coral, as you know, looks like a solid rock; it is
really made of needles of lime, fastened together into a solid mass by
the little Coral Polyp.

Now, many of the Coral animals have the strange habit of budding. The
buds become perfect polyps, and then they, too, begin to bud. In this
way, those marvellous _coral-reefs_ and _coral-islands_ have been made.
Branch by branch, layer by layer, the hard Coral is built up by myriads
of the small, soft-bodied creatures. This kind of polyp can live o