The flatworms, flat worms, Platyhelminthes, or platyhelminths are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Unlike other bilaterians, they are acoelomates, and have no specialized circulatory and respiratory organs, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both ingestion and egestion ; as a result, the food cannot be processed continuously.
A planarian is one of the many flatworms of the traditional class Turbellaria. It usually describes free-living flatworms of the order Tricladida (triclads), although this common name is also used for a wide number of free-living platyhelminthes. Planaria are common to many parts of the world, living in both saltwater and freshwater ponds and rivers. Some species are terrestrial and are found under logs, in or on the soil, and on plants in humid areas.
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 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.
Dugesia is a genus of dugesiid triclads that contains some common representatives of the class Turbellaria. These common flatworms are found in freshwater habitats of Africa, Eurasia, and Australia. Dugesia is best known to non-specialists because of its regeneration capacities.
Geoplanidae is a family of flatworms known commonly as land planarians or land flatworms.
Bipalium is a genus of large predatory land planarians. They are often loosely called "hammerhead worms" or "broadhead planarians" because of the distinctive shape of their head region. Land planarians are unique in that they possess a "creeping sole", a highly ciliated region on the ventral epidermis that helps them to creep over the substrate. Several species are considered invasive to the United States, Canada, and Europe. Some studies have begun the investigation of the evolutionary ecology of these invasive planarians.
Schmidtea mediterranea is a freshwater triclad that lives in southern Europe and Tunisia. It is a model for regeneration, stem cells and development of tissues such as the brain and germline.
Dugesiidae is a family of freshwater planarians distributed worldwide. The type genus is Dugesia Girard, 1850.
Schmidtea is a genus of freshwater triclads. Species of the genus Schmidtea are widely used in regeneration and developmental studies.
Dendrocoelidae is a family of freshwater tricladida flatworms that has a holarctic distribution.
Dugesia sicula is a species of dugesiid triclad that lives in freshwater bodies of the Mediterranean Basin, where it is widely distributed. It has been reported from Sicily, Elba and Mallorca, Eivissa, Sardinia, Algeria, Tunisia, Morocco and Crete.
Kenkiidae is a family of freshwater triclads. Their species can be found sporadically in caves, groundwater, and deep lakes in Central Asia, Far East and North America.
Girardia tigrina, known as the brown planarian or the immigrant triclad flatworm, is a species of dugesiid native to the Americas. It has been accidentally introduced into Europe and Japan.
Schmidtea lugubris is a species of freshwater flatworm, a dugesiid triclad found in Europe.
Weissius capaciductus is a species of dugesiid triclad found in Australia. It is the only species in the monotypic genus Weissius.
Dimarcusidae is a family of triclads found mostly in freshwater habitats of caves, although at least one species, Rhodax evelinae, occurs in surface waters. Currently the family contains only seven species distributed in five genera, although the total number of species is thought to be much higher.
Neppia is a genus of dugesiid triclad that is found in South America, Subantarctic region, Africa, Tasmania and New Zealand.
Bdellocephala is a genus of freshwater triclad that inhabits different regions of Eurasia.
Erpobdella octoculata is a freshwater leech in the Erpobdellidae family. This species can be found in Europe, the Mediterranean, and the Middle East.
Dendrocoelum lacteum, the milk-white planarian, is a freshwater planarian found in lakes and running waters in Europe, being the most widespread freshwater planarian in this continent.
