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.
Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively anoxic environments. They were first recognised and described in 1956.
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.
Monogeneans are a group of ectoparasitic flatworms commonly found on the skin, gills, or fins of fish. They have a direct lifecycle and do not require an intermediate host. Adults are hermaphrodites, meaning they have both male and female reproductive structures.
Acoelomorpha is a subphylum of very simple and small soft-bodied animals with planula-like features which live in marine or brackish waters. They usually live between grains of sediment, swimming as plankton, or crawling on other organisms, such as algae and corals. With the exception of two acoel freshwater species, all known Acoelomorphs are marine.
The Polycladida represents a highly diverse clade of free-living marine flatworms. They are known from the littoral to the sublittoral zone, and many species are common from coral reefs. Only a few species are found in freshwater habitats.
Pseudoceros is a genus of the flatworms Platyhelminthes.
Vetigastropoda is a major taxonomic group of sea snails, marine gastropod mollusks that form a very ancient lineage. Taxonomically the Vetigastropoda are sometimes treated as an order, although they are treated as an unranked clade in Bouchet and Rocroi, 2005.
The Macrostomida are a taxon of small basal free-living flatworms, which inhabit freshwater, brackish, and marine environments. The taxon name was coined by Josef Meixner (1924). There are currently about 260 named species in this taxon.
Macrostomum is a genus of flatworm with a worldwide distribution, with over a hundred species described to date. These hermaphroditic, free-living flatworms are usually small in size, with large species reaching up to 5 mm in body length. They are usually transparent, and the smaller species appear rather round in cross-section than dorsoventrally flattened.
Macrostomum lignano is a free-living, hermaphroditic flatworm. It is transparent and of small size, and is part of the intertidal sand meiofauna of the Adriatic Sea. Originally a model organism for research on developmental biology and the evolution of the bilaterian body plan, it has since expanded to other important fields of research such as sexual selection and sexual conflicts, ageing and the evolution of the bilaterian body plan, ecotoxicology, and, more recently, genomics.
The Lecithoepitheliata are an order of rhabditophoran flatworms. They are free-living worms, found in both freshwater and marine environments.
Catenulida is an order of flatworms in the classical classification, or a class of flatworms in a phylogenetic approach. They are relatively small free-living flatworms, inhabiting freshwater and marine environments. There are about 100 species described worldwide, but the simple anatomy makes species distinction problematic.
Neodermata is a clade of rhabditophoran flatworms containing the parasitic groups Trematoda, Monogenea and Cestoda.
Rhabditophora is a class of flatworms. It includes all parasitic flatworms and most free-living species that were previously grouped in the now obsolete class Turbellaria. Therefore, it contains most of the species in the phylum Platyhelminthes, excluding only the catenulids, to which they appear to be the sister group.
Proseriata is an order of free-living flatworms in the class Rhabditophora with over 400 species described worldwide.
Fecampiida is an order of flatworms in the class Rhabditophora. It is a considerably recent clade, erected after molecular studies.
Trepaxonemata is a subclass of the Platyhelminthes or flatworms. It includes all parasitic flatworms and several free-living species that were previously grouped in the now obsolete class Turbellaria. Therefore, it contains the majority of species in the phylum Platyhelminthes, excluding the Catenulida, and the Macrostomorpha.
Dalytyphloplanida is a suborder of rhabdocoel flatworms. It contains about 1000 species and has a cosmopolitan distribution in both marine and freshwater environments, with several groups having comensal or parasitic lifestyles.
Pseudoceros canadensis is a species of free-living, flatworm in the genus Pseudoceros, belonging to the family Pseudocerotidae.
