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.
Traditional classifications divide the Platyhelminthes into four groups: Turbellaria and the wholly parasitic Trematoda, Monogenea and Cestoda.[4] 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.[5]
As bilaterians, platyhelminthes are triploblastic,[4] but have no internal body cavity (are acoelomate), and lack specialized circulatory and respiratory organs,[4][5] so gas exchange is by simple diffusion. This limits the thickness of the body, so they are either microscopic or are flat and ribbon- or leaf-shaped,[6] and vulnerable to fluid loss.[4] The body is filled with mesenchyme, a connective tissue that can regenerate injured tissues and permits asexual reproduction.[4][7] The nervous system is concentrated at the head end.[4][5]
These have about 4,500 species,[5] are mostly free-living, and range from 1 mm (0.039 in) to 600 mm (24 in) in length. Most are predators or scavengers, and terrestrial species are mostly nocturnal and live in shaded humid locations such as leaf litter or rotting wood. However some are symbiotes of other animals such as crustaceans, and some are parasites. Free-living turbellarians are mostly black, brown or gray, but some larger ones are brightly colored.[4]
Turbellarians have no cuticle (external layer of organic but non-cellular material). In a few species the skin is a syncitium, a collection of cells with multiple nuclei and a single shared external membrane. However the skins of most species consist of a single layer of cells, each of which generally has multiple cilia (small mobile "hairs"), although in some large species the upper surface has no cilia. These skins are also covered with microvilli between the cilia. They have many glands, usually submerged in the muscle layers below the skin and connect to the surface by pores through which they secrete mucus, adhesives and other substances.[5]