animals known ribbon worms worms alternative names included centimetres long specimen estimated metres slim usually millimeters wide relatively short wide bodies patterns yellow orange red green coloration stomach run little body tip tail mouth little gut cavity runs ends little short rear body species lies inactive turns inside emerge mouth capture animal prey venom muscle pulls attack ends species bodies filter feed ends attach host brain ring positioned near end pair ventral nerve connect brain run length body heads species number pigment cup detect light form image skin lateral vessels joined ends form loop filled fluid heart flow fluid depends muscles vessels body wall filter waste products flame cells embedded lateral fluid vessels remove network pipes outside slowly external surfaces trail larger species use muscular waves crawl swim ventral live open ocean rest find hiding places dozen species inhabit freshwater mainly tropics dozen species live land cool damp places feeding crustaceans species species live inside mantle cavity species devastated commercial fishing crabs predators species sold fish bait species sexes separate freshwater species numerous temporary build temporary sperm emitted sperm ready eggs generally externally species shed water protect eggs ways egg divides spiral grows development fate cell usually predicted predecessors process division embryos taxa develop directly form juveniles like adult smaller form larvae long axis juvenile form developing juvenile gut lies body usually eats remains emerges bodies species fragment readily parts near tail grow bodies suggested fossil species confirmed traditional taxonomy divides classes unarmed little dagger orders armed dagger orders accepted given rise group order closely related order include traditional taxonomy says closely related relatively primitive regarded members large super includes history wrote marine worm apparently metres long width child arm touch hand swell wrote sea long worm wrote formal description animal called current total species described grouped genera named greek sea daughters alternative names included separate closest relatives appear description body structure major unusual animals typical body slim proportion length smallest millimeters long centimetres exceed metre longest animal found metres long specimen usually millimeters wide bodies stretch lot times resting length species reduce length increase width disturbed relatively short wide bodies example centimetres long wide smaller approximately cylindrical larger species flattened visible patterns combinations yellow orange red green layer body cuticle consists containing form cell layer rests basement membrane layers muscles circular longitudinal combinations muscle types vary different classes associated differences movement ventral muscles animals especially larger species inside concentric tubes layers kind tissue species tissue gelatinous mouth ventral little body stomach run little body tip tail gut separated gut cavity runs ends little short rear body class little mouth body class mouth share lined feeding body wall sits inactive muscles wall fluid pressure makes jump inside attack animal prey canal called muscle stretch times inactive length class unarmed exits separate mouth prey sticky toxic attack soon range branched described mass sticky animal draws prey mouth class armed exits common mouth typical member class animal stabs prey times digestive prey partial tissues mouth attached distance end extends expose active containing replace active animal grows active lost instead pad bears tiny animals separate mouth unlike attack contacting prey absorb organic food solution skins long slim bodies advantage suspension feeding found specialized use attach system lack specialized gills occurs surface body long flattened like animals thick body walls use fluid circulation substances bodies system consists peripheral vessels blood invertebrates contained main body cavity fluid moves substances functions fluid skeleton vessels fluid round body provides local circulation vessels system simplest type system lateral vessels joined ends form loop species additional long wise cross wise vessels heart pumping vessels flow fluid depends vessels body wall muscles species circulation intermittent fluid flows long wise vessels fluid vessels usually species contains cells yellow orange green red red type contain carry oxygen function unknown use organs called waste products especially products cellular flame cells filter embedded lateral fluid vessels flame cells remove collecting exit freshwater flame cells marines thousands reason difficult non marine environments nervous system senses central nervous system consists brain paired ventral nerve connect brain run length body brain ring masses nerve cells positioned round near end brains invertebrates species pair nerve species additional paired species dorsal cord species lie skin deeper inside muscle layers central nervous system red pink contains stores oxygen peak activity animal experiences example oxygen free sediments species paired cerebral organs openings outside species organs heads slits head grooves head associated paired cerebral organs thought cerebral organs aiding small pits appear sensors head species number pigment cup detect light form image hundreds tiny species live grains sand sense balance feeds follow prey trails find trail movement slowly use external surfaces trail produced head larger species use muscular waves crawl aquatic species swim ventral species means muscular powerful muscles species active extending sticking object pulling animal object reproduction life cycle larger species break stimulated fragments grow individuals species fragment routinely parts near tail grow bodies reproduce sexually species sexes separate freshwater forms numerous temporary forming row body temporary sperm emitted built sperm ready eggs generally externally species shed water lay tube protect gelatinous strings deep sea species internal viviparous growing embryos female body egg divides spiral grows development fate cell usually predicted predecessors process division embryos taxa develop directly form juveniles like adult smaller form larvae long axis juvenile stage short lived juvenile swimming time eating prey larger microscopic particles members order family form capture algae describes like cap ear flaps pulled gut lies body mouth ear flaps small number discs form developing gut form juvenile fully formed juvenile bursts body usually eats catastrophic species reported having life span months ecological significance marine animals sediments shells stones algae animals live deep open oceans gelatinous bodies build semi permanent lined produce like tubes mainly tropics species appear freshwater dozen species live land cool damp places example rotting logs terrestrial native australia found british isles azores lava tube cave island hawaii build allows avoid transported able build populations quickly new areas terrestrial genus found species widely frankfurt azores feeding crustaceans kill size fish living dead insects known prey terrestrial species generally good distance smell selective prey species live inside mantle cavity feed micro organisms filtered host near san_francisco consumed total egg production host crab considered significant factor collapse crab fishery coastal devastated beds predators include feeding fish sea birds invertebrates including crabs skins deter predators crabs clean eating american south african called respective sold fish bait taxonomy class unarmed includes animals mouth underneath brain order comprises marine species body wall outer circular inner length wise muscles addition circular inner length wise muscles extra muscle layers needed comprises species majority marine freshwater body wall muscles disposed layers alternately circular length wise starting layer order includes strongest swimmers genera branched class armed order mouth located underneath ahead brain main nerve run inside body wall muscles includes seven species live mantle large freshwater hosts filter feed hosts steal food short wide bodies posterior like comprises species live benthic sea water freshwater land feed armed includes species single central use capturing prey includes benthic species pads bear tiny evolutionary history fossil record expect mineral calcium found completely soft bodied expect fossils extremely rare middle fossil classed based resemblance unusual deep sea swimming accept classification fossils evidence suggested age animals northern central illinois fossil preserves outline worm evidence certainty represents doubt meaning includes descendants ancestor member trait shared ancestor descendants groups include located fox barnes treat regard basal contains ancestors recent unarmed represent evolutionary grade comprising armed find haven given rise fox barnes treat clade separate believe active implies synonyms branch tree tiny relationships english language writings conventionally treated primitive closely related pre analyses emphasised shared features multiple cell rod shaped bodies frontal organs body organization cells cells found taxa different structure microscopic scale frontal organs vary lot structure similar structures appear small marine larvae different structure position flame cells usually walls fluid vessels served drains exit small number tubes skin flame cells scattered body rigorous comparisons according recent analyses development embryos outer layer internal organs built derived cells labelled inner derived cell derived cells derived cell produce space gut mainly primarily developed muscles embedded tissue structure similar found larger similar structure body wall muscles embedded tissue widespread animals early cell divisions spiral pattern including ribbon worms members include half extant animal groups animals feed members embryos develop larvae example based molecular phylogeny compares sections organisms dna analyses molecular phylogeny confident members closely related non members relationships members unclear outside members animals include include regarded basal
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