Not only are flatworms easily missed, they are often misidentified when spotted. This leads some divers to think that they are Spanish Dancers, but that name properly applies to the giant red or orange sea slug, Hexabranchus sanguineus. They are only a few cell layers thick, while sea slugs tend to be quite chunky little things. Finally, because flatworms are so thin, they can survive without gills. They rely instead on diffusion of oxygen into their tissues directly from the sea water.
Marine flatworms belong to a major group of worms called the Platyhelminthes.
This group also includes such nasties as liver flukes and tapeworms. That is not to say that they are without importance. Although often overlooked, marine flatworms are significant members of coral reef ecosystems. Several are mimics of toxic nudibranch sea slugs, and yet others live symbiotically on a variety of reef invertebrates. Many flatworms are brightly coloured, in most cases as a warning.