Each one is moving along, with its edges partly opening and shutting.  It is plain that this waving motion causes the creatures to move through the water.  Also, they can rise to the surface, or fall to the depths, and do not collide with one another.  So the Jelly-fish is not at all helpless.

At night Jelly-fishes sometimes look very beautiful.  Each one shines in the water, with a soft yet strong light, like fairy lamps afloat in the sea.

They are of all sizes.  Some you could put in a small wineglass, others measure nearly two feet across.  Evidently the Jelly-fish grows, and, in order to live and grow, it must eat; but what does it eat, and how does it obtain its food?

[Illustration:  MEDUSA.]

Before noticing the wonderful way in which this animal finds its dinner, let us look at its body.  In any large Jelly-fish you can see marks which run from the centre of the body, and another mark round the edge of the “umbrella.”  These are really tubes.  They all join with a hollow space inside the body, which is the creature’s stomach.  The mouth-tube opens under the body, as can be seen by turning the Jelly-fish on its back, and moving the lobes of jelly aside.  All the food goes up this tube-mouth, and so into the stomach of the animal.  The whole creature is little more than so many cells of sea-water, the walls of the cells being a very thin, transparent kind of skin.

Perhaps the strangest thing about it is the way in which it catches prey.  Jelly-fish feed on all kinds of tiny sea animals, such as baby fish, and the young of crabs, shrimps, and prawns.  These small creatures form part of the usual dinner of many a hungry dweller in the sea, and the Jelly-fish takes a share of them.

[Illustration:  A MEDUSOID.]

From the edge of the “umbrella” there hangs a fringe of long, delicate hairs, rather like spiders’ threads.  These are fishing lines, yet much more deadly.  They trail through the water, stretching far from the main part of the Jelly-fish; and any small creature unlucky enough to touch them is doomed.

Down each one of these threads there are minute cells, hundreds and hundreds to every thread; and in each cell there is a dart, coiled up like the spring of a watch.  The tip of the dart is barbed like a fishhook.  Now the cells are so made that they fly open when touched.  The dart then leaps out and buries itself in the skin of the animal which touched the thread.  Not only that, but the darts are poisoned, and soon kill the small creatures which they pierce.

You see now how this innocent-looking Jelly-fish gets its food.  As it swims along, the threads touch the tiny living things in the sea, the darts pierce them and poison them.  Of course these stinging darts are very, very small, much too small for our eyes to see.

Sometimes there are numbers of large brownish Jelly-fish in the sea, or washed up on the shore.  If you are paddling or swimming, keep well away from them.  Their poison darts are able to pierce through thin skin, and may cause you illness and great pain.  Remember that the threads are very long; after you have passed the main body of the animal, you may still be in danger from the trailing threads.