Of the worms, the Platyhelminthes ("flat worms") are considered the prototypes, having developed such innovations as bilateral symmetry, a head, tail, and three germinal tissue layers (stinging celled animals, comb-jellies... only have two). These simple soft-bodied animals use their skin to breath through and only have one body opening, the mouth serving also as an anus. 

    The Flatworm Phylum includes two groups of well-known parasitic species of aquatic animals; the Tapeworms (Class Cestoda) and Flukes (Class Trematoda). These are important species to humans directly and indirectly through their negative interactions with food and ornamental animal life, including fishes. There are many species of both Cestodes and trematodes that live on/in fishes and their gill cavities. Fortunately, due to complex life histories and careful quarantine, dips/baths, most aquarists don't come in contact with these parasitic forms. 

    The free-living (i.e. non-parasitic) species of Flatworms, the Class Turbellaria are more of note... True, some of them do trend toward being pests when their numbers get out of control... and can be problematical should you provoke a poisonous variety to the point of toxin release... most are benign organisms that are best ignored. Most of us have had contact at High School science classes with the tiny freshwater genus Planaria. These are the remarkable worms that you cut in two and witnessed regeneration with.

Class Turbellaria: Mostly free-living Flatworms; about 3,000 described species. 

Acoel Flatworms: Simple, "gut-less", small worms... including the ones that "reproduce like rabbits" in reef tanks... best left alone... or selectively siphoned off (mainly) soft corals, out of reef systems.

Polyclad Flatworms: Referring to their multi-branched digestive system. Similar to Nudibranchs, but lacking the latter's "naked gills"; flatworms are "flatter", and often much faster moving. 

Genus Maiazoon:

To: Part 2, Part 3, Part 4, Part 5, Part 6