Turbellaria

Turbellaria is a traditional grouping of mostly free-living flatworms, including forms with ciliated bodies. It is paraphyletic because it excludes parasitic flatworm lineages descended from within it.

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Platyhelminthes: A phylum of bilaterally symmetrical, soft-bodied animals commonly known as flatworms. Turbellarians are the mostly free-living members traditionally grouped within this broader flatworm phylum.
Pharynx (flatworm): A muscular feeding tube that many flatworms extend through an opening to ingest food. Many turbellarians use an extendable pharynx to draw food into the digestive cavity.
Trematoda: A class of parasitic flatworms, commonly called flukes, with complex life cycles in many species. Trematodes were excluded from traditional Turbellaria despite their evolutionary relationship to free-living flatworms.
Planarian: A free-living flatworm, often freshwater, known for its sensory organs and regenerative ability. Planarians are familiar representatives of the traditional turbellarian grouping.
Catenulida: A lineage of small flatworms, many of which reproduce asexually by forming chains of individuals. This early-branching flatworm lineage was traditionally included among turbellarians.
Gastrovascular cavity: A digestive chamber with a single opening that distributes nutrients through the body. Its branching structure serves digestion and internal nutrient transport in many turbellarians.
Cestoda: A class of parasitic flatworms, including tapeworms, adapted to absorb nutrients through their body surface. Cestodes are another excluded parasitic lineage descended from within the broader flatworm radiation.
Dugesia: A genus of freshwater planarians used extensively in studies of regeneration and stem cells. Dugesia provides a widely studied example of turbellarian anatomy and regenerative biology.
Rhabditophora: A major flatworm lineage that includes most free-living flatworms and all major parasitic groups. Its inclusion of both free-living forms and parasites exposes the limits of traditional Turbellaria.
Planarian locomotion: Movement in planarians, driven mainly by cilia gliding over mucus and by muscular contractions. It illustrates the ciliary gliding characteristic of many small free-living flatworms.