Flatworms (Platyhelminthes, Greek "platy"': flat; "helminth": worm) are a phylum of invertebrates. They are relatively simple animals. They have soft bodies.
A troublesome terrestrial example is the New Zealand flatworm, Arthurdendyus triangulatus. It is an invasive species which colonized large areas of Ireland and Scotland. It was brought there by accident in the 1960s. Since then, it has destroyed most of the indigenousearthworms.