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.
Trematoda is a class within the phylum Platyhelminthes. It includes two groups of parasitic flatworms, known as flukes.
Riftia pachyptila, commonly known as the giant tube worm, is a marine invertebrate in the phylum Annelida related to tube worms commonly found in the intertidal and pelagic zones. R. pachyptila lives on the floor of the Pacific Ocean near hydrothermal vents, and can tolerate extremely high hydrogen sulfide levels. These worms can reach a length of 3 m, and their tubular bodies have a diameter of 4 cm (1.6 in). Ambient temperature in their natural environment ranges from 2 to 30°C.
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.
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.
Acoela, or the acoels, is an order of small and simple invertebrates in the subphylum Acoelomorpha of phylum Xenacoelomorpha, a deep branching bilaterian group of animals, which resemble flatworms. Historically they were treated as an order of turbellarian flatworms.
Geoplanidae is a family of flatworms known commonly as land planarians or land flatworms.
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.
Xenacoelomorpha is a small phylum of bilaterian invertebrate animals, consisting of two sister groups: xenoturbellids and acoelomorphs. This new phylum was named in February 2011 and suggested based on morphological synapomorphies, which was then confirmed by phylogenomic analyses of molecular data.
Paracatenula is a genus of millimeter sized free-living marine gutless catenulid flatworms.
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.
Rhabdocoela is an order of flatworms in the class Rhabditophora with about 1700 species described worldwide. Most of rhabdocoels are free-living organisms, but some live symbiotically with other animals.
Proseriata is an order of free-living flatworms in the class Rhabditophora with over 400 species described worldwide.
Bothrioplana is a genus of freshwater flatworms, the sole genus in the family Bothrioplanidae and order Bothrioplanida.
Kalyptorhynchia is a suborder of rhabdocoel flatworms. It contains almost 600 species and has a cosmopolitan distribution.
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.
Kentrophoros is a genus of ciliates in the class Karyorelictea. Ciliates in this genus lack a distinct oral apparatus and depend primarily on symbiotic bacteria for their nutrition.
Stilbonematinae is a subfamily of the nematode worm family Desmodoridae that is notable for its symbiosis with sulfur-oxidizing bacteria.
Astomonema is a genus of nematode worms in the family Siphonolaimidae. They lack a mouth or conventional digestive tract, but contain symbiotic sulfur-oxidizing bacteria that serve as their primary food source. They live in the marine interstitial habitat.
Pseudoceros canadensis is a species of free-living, flatworm in the genus Pseudoceros, belonging to the family Pseudocerotidae.
