Turbellaria

Turbellaria Temporal range: Middle Triassic–0[1] PreꞒ Ꞓ O S D C P T J K Pg N Possible Ordovician record[2]
A marine species Pseudobiceros bedfordi (Bedford's Flatworm), a member of the Polycladida
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Platyhelminthes
Class:	Turbellaria Ehrenberg, 1831
Subgroups
See text.

The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes (flatworms), and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from 1 mm (0.039 in) to large freshwater forms more than 500 mm (20 in) long^[3] or terrestrial species like Bipalium kewense which can reach 600 mm (24 in) in length. All the larger forms are flat with ribbon-like or leaf-like shapes, since their lack of respiratory and circulatory systems means that they have to rely on diffusion for internal transport of metabolites. However, many of the smaller forms are round in cross section. Most are predators, and all live in water or in moist terrestrial environments. Most forms reproduce sexually and with few exceptions all are simultaneous hermaphrodites.

The Acoelomorpha and the genus Xenoturbella were formerly included in the Turbellaria, but are no longer regarded as Platyhelminthes. All the exclusively parasitic Platyhelminthes form a monophyletic group Neodermata, and it is agreed that these are descended from one small sub-group within the free-living Platyhelminthes. Hence the "Turbellaria" as traditionally defined are paraphyletic.