Turbellaria are a group of generally small (<1 - 60cm) and free-living members of the flatworm phylum Platyhelminthes. The latter is nowadays mostly considered obsolete (paraphyletic), so the Acoela and Nemertodermata are excluded from the class into a phylum of their own (Acoelomorpha).

Their locomotion depends on cilia. Most turbellaria are carnivorous, and actively scavenge for food. Most species are marine and freshwater dwellers, but some have evolved to dwell on humid land, as well. Planarians and other flatworms lack organs specialized for gas exchange and circulation. Their flat body places cells close to the water, and the detailed branching of the gastrovascular cavity distributes food throughout the body. Nitrogenous waste, in the form of ammonia, diffuses directly from the cells into its surroundings. Flatworms also have a simple excretory apparatus that functions in maintaining an osmotic balance between the flatworm and the surrounding water. This system is composed of ciliated cells ("flame cells") that waft fluid through branched ducts open to the outside. This allows them to invade freshwater and moist terrestrial environments.

The planarian Dugesia is a common representative of class Turbellaria.

Summary of features for Class Turbellaria (L. turbellae = a combination, aria = like):

  • no mitosis in somatic cells; new cells made by totipotent stem cells
  • free-living (non-parasitic) or symbiotic flatworms
  • dorsoventrally flattened triploblasts with a blind gut
  • well-defined nervous system with sense organs (ocelli and ciliary chemoreceptors)
  • ganglia concentrated into a brain at one end of the body (cephalization)
  • a simple life cycle
  • most are marine animals, but quite a few live in freshwater environments, and a fair few live in terrestrial ecosystems but are restricted to very humid areas
  • monoecious (simultaneous hermaphrodites)

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