Macrostomum
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 (e.g. Macrostomum tuba). They are usually transparent, and the smaller species appear rather round in cross-section than dorsoventrally flattened. Name The term "Macrostomum", meaning "big-mouthed", derives from the Greek μάκρος makros, "large", and στόμα, stoma, mouth. Relative to other turbellaria, Macrostomum species have indeed a long mouth, connected to the gut by a muscular pharynx which can expand, in certain species, to almost the width of the animal. Ecology Macrostomum species inhabit different aquatic or moist environments. Marine and brackish species are often interstitial (living in the space between grains of sediment), whereas freshwater species are also frequently found associated with aquatic plants. Many of these small worms feed on unicellular algae such as diatoms, others on zooplankton or smaller benthic invertebrates. Species This genus includes Macrostomum lignano, a new model organism for studies on different areas of biology, including development, bioadhesion, regeneration, stem cell biology, ageing, toxicology, genomics, and evolution. The following species are recognised in the genus Macrostomum: Macrostomum acus