Any worm that lives in a marine environment is consiered a marine worm. Marine worms are found in several different phyla, including the Platyhelminthes, Nematoda, Annelida, Chaetognatha, Hemichordata, etc. Some of them are segmented and some of them are not. These worms are also called polychaetes,which means "many bristles". Have segments with bristles; they occur in bundles.