ALTHOUGH flatworms are usually not more than half an inch long, they are very easy to find. Almost any rock or stick you lift out of the water is apt to have one or two of them sliding over its surface.

Planaria is one name of the fresh-water flatworm. It is a relative of tapeworms and liver flukes, but it is not parasitic as they are.

The forward end of the flatworm's body is shaped somewhat like a triangle. As the flatworm moves around the sides of a glass jar, you can see a tiny, ear-like flap at each side of the head end. These are sometimes called "false ears." You will also see two little dots between these false ears. These dots are sensitive to light, but they are not the complicated organs of sight which we usually mean when we say "eyes."

A surprising fact about the flatworm is that his mouth is near the middle of his body instead of at the forward end. Food is drawn in through a tube, or pharynx, which the flatworm pushes out of the central part of his body when he feeds.

If you hold a glass of water so that sunlight shines through the flatworm, you can see a pale stripe near the end of his body. This is where the pharynx is carried after it has been drawn back into the body.

You will also be able to see the brownish tube in which the flatworm's food is carried through the body and digested. One branch of the tube extends toward the forward end. Another extends back at each side of the pharynx, so that there are three branches in all. Each of these main tubes has many smaller divisions branching from it.

Perhaps you will see the flatworm swim. There are many very tiny cilia on the surface of its body. Through a magnifying lens these cilia look like short hairs or threads. Hundreds of them, moving rapidly, shove the worm through the water in much the way that oars move a boat.

When a flatworm is cut in two pieces, each half can grow and develop into a new worm. As it has no blood, it will not bleed to death when it is cut. Any part of the branched tube can digest food, so neither part will starve while it is developing into a whole animal.

Sometimes fresh-water flatworms divide into two worms by splitting lengthwise; sometimes by splitting crosswise. At other times they lay eggs from which young flatworms develop. The eggs are protected by a cocoon, or shell, each cocoon covering a number of eggs.

It is not difficult to keep flatworms in your aquarium. These animals are carnivorous and will eat bits of crushed meat. The aquarium should not be set in direct sunlight, and the water must be kept fresh and cool. If you do not feed them, flatworms will absorb their own bodies and may continue to live for a month or more without any other food.