Platyhelminths, commonly known as flatworms, are a phylum of invertebrate animals characterized by their flattened body shape and bilateral symmetry. They are primarily aquatic, although some species inhabit terrestrial environments, and they can be free-living or parasitic. This group includes well-known organisms such as planarians, tapeworms, and flukes. Flatworms lack a true coelom and have a simple body structure, with no specialized respiratory or circulatory systems.