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Class of flatworms

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 to large freshwater forms more than long or terrestrial species like Bipalium kewense which can reach 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.

Traditional classifications divide the Platyhelminthes into four groups: Turbellaria and the wholly parasitic Trematoda, Monogenea and Cestoda. In this classification the Turbellaria include the Acoelomorpha (Acoela and Nemertodermatida). The name "Turbellaria" refers to the "whirlpools" of microscopic particles created close to the skins of aquatic species by the movement of their cilia.

As bilaterians, platyhelminthes are triploblastic, and vulnerable to fluid loss. The nervous system is concentrated at the head end.

Most other turbellarians are carnivorous, either preying on small invertebrates or protozoans, or scavenging on dead animals. A few feed on larger animals, including oysters and barnacles, while some, such as Bdelloura, are commensal on the gills of horseshoe crabs. These turbellarians usually have an eversible pharynx, in other words, one that can be extended by being turned inside out, and the mouths of different species can be anywhere along the underside. Most turbellarians have pigment cup ocelli ("little eyes"), one pair in most species, but two or even three pairs in some. A few large species have many eyes in clusters over the brain, mounted on tentacles, or spaced uniformly round the edge of the body. The ocelli can only distinguish the direction from which light is coming and enable the animals to avoid it. In turbellarians there are one or more pairs of both testes and ovaries. Sperm ducts run from the testes, through bulb like seminal vesicles, to the muscular penis. In many species, this basic plan is considerably complicated by the addition of accessory glands or other structures. The penis lies inside a cavity, and can be everted through an opening on the posterior underside of the animal. It often, although not always, possesses a sharp stylet. Unusually among animals, in most species, the sperm cells have two tails, rather than one.) are the sister group to all other bilaterians, including the rest of the Platyhelminthes. The Platyhelminthes is a clade consisting of two monophyletic groups, Catenulida and Rhabditophora. It is also generally agreed that the Neodermata are a relatively small sub group a few levels down in the "family tree" of the Rhabditophora.