Flatworms are members of the phylum Platyhelminthes.

They are completely unrelated to nudibranchs but are similar in colour, size and habitat. They 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

phylum - Platyhelminthes
   class     - Turbellaria
      order       - Polycladida

...much more information about flatworms