What are they?

Phylum Platyhelminthes are soft-bodied dorsoventrally flattened animals that represent one of the simplest body plans and are one of the earliest animals to develop cephalization and a central nervous system. Most are marine animals that swim across coral reefs and the bottom of the ocean, but they are also found in freshwater and terrestrial environments. Their bodies allow oxygen and nutrients to diffuse directly through their tissues. In addition to free-living forms, they have also evolved parasitic lifestyles, living inside vertebrate and invertebrate hosts, playing significant ecological roles. Flatworms demonstrate incredible adaptivity despite their simple organ systems. They have regenerative abilities, along with unique reproductive strategies, and altogether their ecological diversity makes them an essential part of aquatic ecosystems.