marine animal member includes animals dorsal nerve time called term animals live solitary individuals replicate colonies unit known marine filter water filled sac like body structure tubular openings known draw expel water feeding water expel filtered water adult permanently attached rocks hard surfaces ocean floor swim zone sea adults species commonly known sea sea pork sea sea earliest species appears fossil record early period despite simple appearance different adult form close relationship evidenced fact mobile stage possess rod resemble derives unique outer covering formed proteins acts species thin translucent gelatinous thick tough stiff taxonomy species exist world oceans living shallow water numerous group fewer species found depths greater solitary animals leading existence attached colonial supported stalk attached directly substrate rock shell coral root dock ship hull found range solid translucent colours resemble seeds barrels bottles largest sea grow metre tall established introduced second group emphasize affinity group doubt largely influence authors supported term usage invalid precedence grounds existed accordingly current formally correct trend abandon favour original invariably modern scientific works accepted valid common names different species sea colourful bodies supported slender sea named habit contracting bodies sharply water disturbed sea liver sea pork names resemblance dead colonies pieces meat classification closely related including invertebrates clade comprising called contain roughly described species traditionally divided classes members included result studies traditional classification accepted newer evidence suggests artificial group status fossil record undisputed fossils rare best known earliest identified species lower village near common possible found upper skeletons upper united_states enigmatic species found period sac like second new like genus northern russia results new study shown possible affinity organisms lived shallow coastal waters slightly million years ago believed oldest evidence lineage russian fossil tentatively identified fossils near preserved identification questioned fossils rare bodies decay soon death families microscopic present preserved occasionally found later rocks familiar mistaken studies multi taxon molecular study proposed sea descended hybrid ancestor study based quartet approach designed reveal horizontal gene transfer events anatomy body form colonies occur range forms degree individual organisms known integrate simplest systems individual animals widely separated linked horizontal connections called grow species growing closer sharing common base advanced colonies involve integration common structure surrounded separate single central organized larger systems hundreds star shaped units colony tiny numerous colonies form large mat like patches body structure far largest class body surrounded test derives varies thickness species tough resembling thin delicate transparent gelatinous composed proteins complex includes variety unique grow animal need periodically shed inside body wall mantle composed tissue muscle blood vessels nerves openings found body wall water flows interior ventral expelled large occupies interior body muscular tube linking opening rest gut groove known ventral surface net collects food particles wound dorsal lower end links loop gut terminates near walls bands slits known water escapes surrounding water filled cavity crossed rope like extend mantle provide support preventing collapsing hold organs main class characterised free swimming individuals filter net catch prey colonial hollow cylindrical structure outside inside species known found tropics species small long solitary opposite ends barrel shaped bodies swim jet propulsion species small long found surface waters warm cold seas jet propulsion form long chains new individuals class group retain characteristics adult state product extensive species resemble larvae amphibians tail right angles body retained animals long propelled tail external net known house completely surround efficient trapping particles physiology internal anatomy like early development time completed stages lost body members true develop clear body end development remain adults heart sac organs enclosed membrane called surrounded like begin life mobile stage resembles minority species retain general form life rapidly settle attach suitable surface later developing barrel like usually adult form lives complex developed heart system heart double shaped tube situated gut blood vessels simple tissue tubes blood types blood appear pale green respiratory oxygen transported dissolved exact details system unclear gut gills nervous system arranged series parallel happens animals minutes heart stops beating pumping fluid reverse direction blood unusual features species contains high concentrations transitional metal associated proteins blood cells known concentrate level million times surrounding stored form requires stability achieved containing acid later deposited outer surface presence thought deter predation unclear presence heavy metal low species concentrate iron serve similar function species produce organic compounds chemical defenses predators lack kidney like organs typical structures rely tissues rid waste simple system typical organ mass large clear walled occupy loop structure remnant primitive cells extract waste matter circulating blood accumulate inside crystals obvious means material adult hollow cerebral equivalent brain hollow structure known originate tube located nerves arise ends anterior end posterior end supply rest body organs gut body wall sense organs sensory cells unusual animals produce large fraction structures form production animals unusual researchers denied presence outside plants time accepted occurs mammals unique scale application production possibly having functional announced presence test substance similar called recognized alternative substance feeding nearly suspension capturing particles sea water bodies typical digestive processes similar systems water drawn body action lining gill slits obtain food average needs process body volume water second drawn net lining continuously net sticky threads holes diameter trap particles including bacteria net rolled dorsal trapped particles drawn gut shaped contents stomach enlarged region lowest bend digestive adds food moved absorption takes place remains formed strings opens dorsal cavity near caught constant stream water carries waste exterior animal current way upstream draw contaminated water live soft sediments species sit wait predators trapping tiny small invertebrates muscular surround certain tropical species family green algae unique excess products assumed available host life cycle single near gut body wall solitary species sperm eggs shed sea larvae especially colonial species sperm released water drawn individuals incoming water current takes place eggs early developmental stages forms appear like primitive rod resemble small swim tail simple eye balancing organ sufficiently developed species finds suitable rock place form capable feeding rudimentary digestive system dispersal mechanism physical changes occur body significant reduction cerebral controls movement equivalent human brain comes common saying sea eats brain classes adults remain swimming drifting open sea larvae undergo similar higher lower degree colonial forms increase size colony new individuals share colonies grow new near posterior end colony sexual reproduction starts internally egg develops directly intervening form form detached single unit new colony complex life cycle includes different functions sexually members colony known eggs sperm individual viviparous developing embryo feeds yolk sac released sea free swimming like undergoes water column known nurse develops tail produced known function arranged lateral rows transport function arranged single central row link nurse develop mature separate live independently start cycle served purpose phase allows rapidly conditions favourable complex generations solitary life history phase producing chain tens hundreds individual length chain aggregate portion aggregate individuals known remain attached swimming feeding growing larger egg internally sperm colony egg develops brood sac inside connection circulating blood nurse fills body released start independent life reproduce sexually resembles trunk fully developed undergoes tail shift tail moves position ventral orientation twists relative trunk consists small fixed number cells grows cell division development rapid takes seven hours develop house building juvenile starting feed development exhibit fate cells set early reduced cell numbers rapidly evolving contrast cell determination relatively late development cell evolution relatively slow promotion crossing class releases sperm eggs surrounding simultaneously self sterile studies mechanism self self non self recognition molecules play key role process interaction sperm coat egg appears self non self recognition similar self systems flowering plants self promotes crossing provides advantage generation genetic avoidance depression class colonial member group able reproduce sexually colony eggs days peak sperm self avoided cross favored avoided self possible self eggs develop substantially higher frequency cross eggs significantly lower percentage larvae derived self eggs growth colonies derived significantly lower findings suggest self gives rise depression associated developmental likely caused expression model class organism lifetime employs original reproductive strategy entire female line contained single giant cell termed maintained laboratory culture growing interest model organism phylogenetic position closest sister group invasive species past decades notably genera invading coastal waters countries carpet taken km2 area bank northeast coast north_america covering stones stationary objects dense mat appeared thriving sound hood canal pacific_northwest invasive usually arrive organisms hulls ships introduced larvae ballast water possible means introduction shells brought marine cultivation current research indicates previously thought indigenous europe americas fact invaders invasions occurred centuries ago areas proving major threat operations use humans medical uses contain host potentially useful chemical compounds including effective types cancer effective types cancer effective types cancer able correct cellular series generations similar process possible humans mechanisms underlying phenomenon lead insights potential cells tissues compromised human organs food species consumed food world japan korea sea main species eaten cultivated palm tons produced mass mortality events occurred farmed sea soft tons produced uses use source researched body wall broken converted parts animal protein rich converted fish feed large scale possible economics attractive predators removal sea profound ecological impacts sea based production compete food production cultivation land based crops projects model organisms developmental studies species mitochondrial nuclear nuclear appears smallest species study gene regulation evolution development
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