commonly referred group microscopic worm like animals widely distributed abundant freshwater marine environments benthic live layer tiny organisms found beds water bodies majority live particles sediment submerged surfaces species terrestrial live land film water surrounding grains soil divided orders marine species marine freshwater nearly species described simple body plan head region brain sensory organs trunk simple gut reproductive organs anchor substrate feed organic particles muscular marine species producing eggs develop directly miniature adults freshwater species producing eggs species viviparous mature great days etymology taxonomy comes greek meaning stomach meaning hair coined russian zoologist common apparently arises relationship unclear morphology suggests close hand genetic studies place close relatives species described contains single class divided orders report wholly marine rare poorly known species known fresh water comprises marine freshwater species anatomy vary size body length symmetrical transparent shaped bowling pin shaped body arched flattened anterior end clearly defined head contains sense organs brain found mouth ventral surface head body trunk contains gut reproductive organs posterior end body projections cement serve double system agent connection additional anterior end sides body body wall consists cuticle longitudinal circular bands muscle primitive species cell single feature shared ventral surface animal arranged rows patches transverse bands cuticle locally forms scales hooks spines body cavity interior animal filled poorly differentiated tissue shaped cells containing surround gut function skeleton mouth anterior end opens elongated muscular triangular shaped lined cells opens cylindrical lined digestive cells located ventral surface close posterior body species pores opening ventral surface contain allow excess water feeding system consists single pair open separate pores lateral underside animal usually body pairs opening body waste probably body wall believed function mainly unusually form flame cells instead cells consist skirt surrounding series rods turn central cells termed connect single outlet cell passes material typical small animals respiratory organs nervous system relatively simple brain consists connected lead pair nerve run body longitudinal muscle bands primary sensory organs body surface function pits head simple act distribution habitat cosmopolitan distribution inhabit spaces particles marine freshwater environments surfaces aquatic plants submerged objects surface film water surrounding soil particles land found pools mud thrive presence hydrogen pools dry survive periods eggs species capable forming harsh conditions marine sediments known reach individuals cm2 making common sediment freshwater reach density individuals cm2 fifth abundant group invertebrates sediment behaviour ecology marine freshwater environments form benthic community dead living organic material bacteria small mouths muscular action eaten small like microscopic animals primarily powered movement occurs different methods different members group typically proceeds smooth manner body propelled forward rhythmic action ventral surface genus movement proceeds long muscle activated spines forced body contrast typically multiple forward action similar response threat head trunk rapidly pulled backwards movement reversed muscular action important animal turns sideways individuals reproduction lifespan reproduction reproductive behaviour little studied probably represents ancestral lineage primitive simultaneous possessing male female sex organs generally single pair anterior portion contains sperm producing cells posterior portion producing sperm packaged released male open temporarily underside animal roughly thirds way body organ tail collects sperm transfers partner seminal female details process behaviour involved vary species range different reproductive organs male individual uses organ transfer sperm partner internal eggs released body wall repairs case development embryo cell destined specific animal body species viviparous species reproduce entirely species male portions reproductive system non functional cases entirely absent eggs diameter large comparison animals size species capable laying eggs remain dormant times low temperatures species able produce regular eggs hatch days environmental conditions favourable eggs undergo direct development hatch miniature versions adult young typically reach sexual maturity days laboratory lived days producing eggs days life demonstrate species having genetically fixed number cells adults cell division end development growth solely cell classification divided orders number families
Gastrotrich