Flatworms are members of the phylum Platyhelminthes, they are known from all oceans.
They are a separate species, completely independent of slugs, but similar in color and size, and they often share the same habitat. Flatworms differ in that they are typically very thin and lack external gills.
There are four main classes of Platyheminthes, three of which are parasitic. The fourth class Turbellaria are mainly marine and free swimming.The brightly coloured members of the order Polycladia are the ones most usually noticed. They usually feed on soft coral or bryozoans