Our mystery critter is a worm. Worms have simple bodies. Muscles inside combined with bristles outside help them move. Oxygen passes through a worm's skin, which contains special cells that detect light. Like snails, worms have both male and female organs. They also have five "hearts" and plenty of "guts." They push their throat out of their mouth to snatch food. The meal moves from a worm's esophagus to the crop (a storage compartment) to the gizzard, which grinds it all up. Next, it's on to the intestine, where juices break everything down. Tunneling and burrowing underground, earthworms stir the soil, letting air in. They eat dirt, leaves, and other organic matter and deposit their castings in the soil. Rich in nutrients, worm castings make soil very fertile—great for growing plants. Be kind to these wiggly wonders. If you see a worm on a sidewalk or driveway, move him/her (remember they're both) to a patch of cool, damp grass.

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