Roundworms are almost always parasitic and can live anywhere in you from intestines to brains. One of the most commonly heard of roundworms is trichinella. This worm comes from undercooked pig and bear meat. It causes a very painful condition known as trichinosis. This is a brain worm, commonly found in deer.
Flatworms are a step behind the roundworms.
They must be flat in order to breathe and distribute the nutrients from their meals through their bodies. This is the first group to have bilateral symmetry. The flatworms include turbellaria, trematoda, and cestoda. The only group out of these three that is not a parasite are the tubellarians. These are little cross-eyed worms that live independently.