Like all other animals, the Sponge animal must eat, and its way of doing so is rather strange.  If you look at any ordinary washing-sponge, you notice a great many very small openings and some larger ones amongst them.  It is through the smaller holes, or pores, that the Sponge gets its supply of food.  When it is alive, and in its own home, there is a current of water always passing through its and the Sponge depends on the food which the water brings.  Now, if you could watch this water-current, you would see that it rushes into some of the holes, and out of others; it has a certain path to follow.  It enters the small pores, or openings, of the Sponge, and goes along narrow canals, and is then led into larger ones.  Finally, it rushes out again through those large openings we noticed.  We may compare it with traffic coming into a city by many narrow streets, then passing into broader roads, and at last out again by big main roads.

[Illustration:  CUPS AND SOLID SPONGES]

[Illustration:  Photo:  A. F. Dauncey.  SEA FURZE]

How does the Sponge animal cause this current; and how is it made to follow 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.