See Flatworm key to families
A phylum of soft-bodied, generally flattened, worm-like, unsegmented animals, with a wide range of body forms. The phylum is divided into three classes: Turbellaria, Trematoda and Cestoda. The majority of aquatic flatworms are turbellarians. Trematodes (flukes) and cestodes (tapeworms) are parasitic but some trematodes have free-living dispersive stages. The lower-level Key to Flatworms and Allied Forms covers free-living turbellarians at family-group level, ectoparasitic trematodes (fish flukes, etc.) and the final aquatic stage (cercaria) of endoparasitic trematodes of some terrestrial vertebrates (family Fasciolidae: sheep liver fluke, etc.) at order-level.
Most but not all flatworms are markedly flattened dorso-ventrally. The known Australian aquatic species range in size from under 1mm to about 10mm when fully extended, but most species contract strongly when disturbed and size is not a good identificatory character. Body-shape characters can also be problematic in preserved specimens unless care has been taken to kill and fix the animal in a relaxed posture.
Identification to phylum can be based on overall body shape, the possession of a ventral or terminal mouth and a blind gut without anus, an absence of appendages (other than tentacles in some families) and absence of a long eversible proboscis.
Species which move with a leech-like motion might readily be mistaken for, and in some cases were originally described as, leeches (Annelida: Oligochaeta: Hirudinea). Confusion with leeches is especially possible for the commensal or ectoparasitic families Temnocephalidae and Procerodidae. Some temnocephalids also may be confused with Hydra (Cnidaria). These flatworms, which are commensal on crayfish and other animals, attach by a rear adhesive disk and wave tentacles to catch food. The most obvious distinguishing feature is that Hydra have a simple gut cavity while Temnocephalidae possess a muscular pharynx.
Tardigrades (Phylum: Tardigrada) may appear similar to some small flatworms but are instantly distinguished by their short, stumpy legs with claws.
Annelid worms from families Aeolosomatidae (Class: Aphanononeura) and Histriobdellidae (Class: Polychaeta) might be mistaken for flatworms. These annelids are small (about 1mm long) worms with reduced segmentation, a sub-terminal mouth, a posterior anus and an internal coelomic cavity as in other annelid worms. Aeolosomatids are free-living, moving by ciliated areas, sometimes paired, in the head region. Histriobdellids are ectoparasites in the gill chambers of freshwater decapods (Crustacea). They have five very short tentacles on a rounded head and 5-6 bundles of cirri (setae) each side, one on the head, 4-5 on the body. The tail region is developed into two short 'limbs'.
Nemertea (ribbon worms) are almost exactly like flatworms except for a long, extensible, anterior proboscis held in a sack-like rhynchocoel extending along the greater part of the body dorsally.