The first flatworms, or platyhelminthes, probably evolved from something like a hydra about 550 million years ago. Like all other living things at this time, these early flatworms all lived in the ocean. A flatworm has a body shaped roughly like a bag, like the earlier hydras.

Flatworms were the first animals that had bilateral symmetry. Inside, flatworms have a nervous system with most of the senses concentrated in the head end, so that when it moves forward, the head, can detect danger in time to escape it. On top of the head, they have simple eyes that can just sense light.

To get food, a flatworm takes in food through its mouth, which is also in its head, and then it excretes out through its mouth again. Flatworms have no circulatory, respiratory, or skeletal system. Because they're so flat, most of their cells are on the surface, so they could get enough oxygen and food directly from the water around them.

Some flatworms live in the oceans, and some live in fresh water, while others live inside people and animals, both in the ocean and on land. Flatworms that live inside people are tapeworms and flukes. Tapeworms have lived inside people and animals for so long that they don't even have a digestive system anymore. They just get their food predigested by the animals they're living in.