compose large animals known extant species recognized largest marine comprising named marine organisms numerous live freshwater terrestrial habitats highly diverse size structure behaviour habitat typically divided taxonomic classes entirely extinct squid octopus advanced invertebrates giant squid squid largest known species far numerous terms classified species account total scientific study called universal features defining modern mantle significant cavity breathing presence structure nervous system things express great morphological diversity textbooks base descriptions hypothetical ancestral image single like shell proteins reinforced calcium mantle covering upper surface underside animal consists single muscular foot tends small main body cavity blood systems mainly open feeding system consists tongue complex digestive system microscopic muscle powered hairs called play important roles paired nerve brain species eyes sensors detect chemicals vibrations touch simplest type reproductive system relies external complex variations occur produce eggs emerge larvae complex larvae miniature adults good evidence exists appearance cephalopods period million years ago evolutionary history emergence ancestral known living fossil forms subjects vigorous debate scientists important food source modern humans risk food poisoning accumulate certain conditions countries regulations reduce risk centuries source important luxury goods notably mother pearl purple sea silk shells money societies species represent hazards human activities bite blue octopus fatal octopus causes month species large tropical cone shells kill sophisticated easily produced important tools research known fever transmitted humans water hosts affects million people agricultural accidental deliberate introduction species new environments seriously damaged ecosystems etymology words derived french originated latin soft adaptation aristotle soft things applied denote division animal kingdom containing members groups having supposed somewhat resemble known groups relation little abandoned definition universal features body structure mantle significant cavity breathing organization nervous system abundant metallic element calcium developed varied range body structures difficult find defining characteristics apply modern groups general characteristic symmetrical following present modern dorsal body wall mantle plates shells overlaps body spare room form mantle cavity open mantle cavity pairs main nerve characteristics commonly appear textbooks significant exceptions diversity estimates accepted described living species vary maximum species estimated probable total number living fresh water terrestrial comprise total classes living estimated number described living species estimated named species include named marine organisms second numbers living animal species far ahead living species total estimated fossil species total number species existed preserved times greater number alive today varied forms animal include cephalopods lesser known similarly distinctive majority species live oceans zone form significant freshwater fauna terrestrial ecosystems extremely diverse tropical temperate regions found latitudes known species squid advanced invertebrates giant squid recently observed alive adult form largest invertebrates recently caught specimen squid long weighing overtaken freshwater terrestrial appear exceptionally vulnerable extinction estimates numbers vary widely partly regions thoroughly surveyed shortage specialists identify animals area species iucn red list threatened species included nearly endangered comparison great majority species marine appeared red list recorded year consisting entirely species great range diversity textbooks start subject anatomy describing called hypothetical hypothetical ancestral illustrate common features found species reproduce like sea star depiction similar modern suggest resemble early symmetrical single like shell shell mantle covering upper surface underside consists single muscular foot mass soft region contains body organs mantle mantle cavity mantle cavity fold mantle significant space lined exposed according habitat sea fresh water air cavity rear earliest position varies group group pair chemical sensors incoming lane pair gills exit openings reproductive organs mantle cavity soft body lies enlarged mantle cavity shell mantle edge shell absent number taxonomic groups consists mainly protein hardened calcium layer cases use construct hard parts questionable exception shells composed mainly lay eggs hard shell use traces construct shell consists layers outer layer organic matter middle layer inner layer consisting foot underside consists muscular foot adapted different purposes different classes foot carries pair act balance sensors aid movement forms having shell foot acts animal hard surface vertical muscles clamp shell vertical muscles pull foot exposed soft parts shell foot adapted sediment cephalopods jet propulsion arms derived foot system systems mainly open reduced fairly small spaces heart main body cavity blood fluid internal organs spaces act efficient skeleton blood contains respiratory pigment oxygen carrier heart consists pairs receive blood gills pump pumps main fairly short opens heart function system waste products blood dumping pair little rear connected extracts usable materials additional waste products tubes discharge mantle cavity pair gills gill generally gills like feathers shape species gills divide mantle cavity water enters near exits near kinds drives water current mantle cavity help gills clean detect chemicals possibly sediment entering mantle cavity gills stop beating ceased gill incoming blood vessel connected outgoing heart eating members family use function muscular mouths bearing rows teeth replaced rear wear primarily functions bacteria algae rocks associated supporting organ unique equivalent animal mouths contain food sticks beating tiny hairs drive stomach forms long string called food string tapered rear end stomach projecting slightly backward pointing cone rotated acts winding string string reaches stomach makes sticky particles particles group send smaller particles mainly minerals eventually larger ones mainly food sent stomach exit process means perfect periodically circular muscles entrance pinch piece preventing growing large mantle cavity swept outgoing lane current created gills usually simpler digestive systems head largely disappeared mouth equipped mouth collect nervous system pairs main nerve organized number paired serving internal organs pedal ones serving foot pairs corresponding sides body linked relatively large nerves gut cerebral located pedal control foot cerebral surround nerve ring nerve collar ring obvious important pairs cerebral pedal largest important functioning principal center thinking eyes edges shells connect pair looped nerves provide ability distinguish light shadow reproduction simplest reproductive system relies external complex variations produce eggs emerge larvae complex larvae miniature adults sit small cavity surrounds heart shed sperm extract mantle cavity use system remain sex lives rely external use internal functioning sexes methods require complex reproductive systems basic feeds floating food particles bands equator sweep food mouth uses drive stomach uses expel remains new tissue grows bands interior pushed apart animal grows stage succeeded stage band nearest develops veil pair bearing eventually adult form usual state cephalopods differ exhibiting direct development form adult ecology feeding grazing algae filter grazing feeding strategies predominant feed microscopic algae sea floor feed plants plant surface employ strategy plant large sit smaller plants eaten larger counterparts filter feed suspended matter food particle water typically passing water gills filter cephalopods primarily predatory takes secondary role jaws food acquisition uses usual fashion diet includes sea suck algae row pierce cell walls feed feed extensive list unusual feeding habits available appendix graham classification opinions vary number classes example table shows living classes extinct ones unlikely form clade older works combine class commonly recognized classes known fossils classification higher taxa groups remains problematic phylogenetic study suggests form clade additionally suggests sister taxon relationship exists evolution fossil record good evidence exists appearance cephalopods period million years ago evolutionary history emergence ancestral group known living fossil forms vigorously debated debate occurs early fossils million years ago described like unwilling probable debate million years ago centers feeding apparatus type similar worms opposes idea written earlier million years ago fragments genuinely like appears concept ancestral appear million years ago early rocks siberia china thought early like shells shelled earliest fossils millimeters long specimens centimeters long found like shapes tiny specimens suggested juveniles larger ones adults analyses concluded earliest scientists convinced early fossils clear signs identifies modern twists internal organs lies head fossils million years ago long thought discoveries detailed fossils showed shell built grains mineral dioxide divided series compartments fossil shelled cephalopods living classification uncertain late fossil thought earliest clearly fossil shell strand tissue uses remove water compartments vacated grows visible fossil shells early cephalopods instead swimming shells contained ballast deposits thought underside stripes thought upper surface cephalopods external shells extinct end cretaceous period million years ago shell squid octopus abundant today early fossils regarded modern looking appeared period million years ago group major reef builders cretaceous extinct cretaceous extinction event remain abundant diverse class extinct animals shell authors suggest deserve comment position tree life phylogeny phylogeny evolutionary family tree controversial subject addition debates closely related debates arise relationships classes living fact groups traditionally classified distinct related generally regarded members group defined having larvae case living feeding structure called members worms seven marine right phylogeny presented relationships members family tree uncertain difficult identify features inherited common ancestor example uncertain ancestral composed repeating units suggest origin like worm scientists disagree colleagues concluded repetition gills foot muscles later developments concluded ancestral foot shell particular branch family tree shell thought evolved small spines difficult reconcile origins shell appears originated coating eventually cuticle forced development sophisticated respiratory apparatus form gills eventually cuticle genetic machinery skeletons shell certainly reinforced mineral evolutionary relationships debated widely supported morphological analyses tend recover clade receives support molecular analyses results lead unexpected instance scattering groups analysis morphological molecular comparisons concluded particular separate unrelated remaining classes traditional excluded analysis recovered traditional groups showed demonstrating available data contaminated current molecular data insufficient phylogeny methods determine confidence prone risky place emphasis areas different studies agree eliminating unlikely relationships latest studies add new internal relationships bringing hypothesis question human interaction source food humans important luxury goods notably mother pearl purple sea silk chemical compounds shells form currency societies number species bite sting humans agricultural uses humans especially important food source advent modern humans resulted commonly eaten include china accounted global catch tonnes long tons short tons europe france remained industry leader countries regulate handling mainly minimize poison risk accumulate animals shells produce shells lined valuable best natural produced marine pearl live tropical subtropical waters pacific_ocean natural form small foreign object gets stuck mantle shell methods insert seeds seed method uses grains ground shell freshwater purpose endangered freshwater species southeastern usa pearl industry important areas significant sums money spent monitoring health farmed luxury high status products purple ink shells weight silver fourth century according discovery large numbers shells crete suggests pioneered extraction imperial purple period 20th 18th centuries centuries sea silk fine rare valuable fabric produced long silky threads particularly attach sea bed writing persian wars circa stated hereditary governors armenia received insignia roman emperor given apparently ruling classes allowed wear shells including kind money shell money societies generally differed important ways standardized government backed money familiar industrial societies shell commercial transactions mainly social status displays important occasions weddings commercial transactions functioned money value differed place place result difficulties transport vulnerable inflation efficient transport behavior appeared monitor health aquatic environments fresh water marine environments population status structure physiology behaviour level contamination elements compounds indicate state contamination status ecosystem particularly useful representative environment sampled placed typical project today worldwide bites risk food poisoning accumulate occurs certain conditions countries regulations aim minimize risk blue octopus bites fatal bite cause unpleasant symptoms species large tropical cone shells kill sophisticated cone important tools research promise sources new handled alive species sting bite species present risk human handling animal perspective deaths number deaths species pose significant threat humans blue genus live australia new_guinea bite humans severely provoked venom kills human victims tropical species octopus causes severe month treated correctly bite octopus cause lasts longer month weakness week treated species cone sting handled species small pose risk humans feed marine invertebrates case larger species fish venom based huge array fast acting slower afford require time energy produced compared snakes spiders painful reported fatalities reported fatalities larger species cone capture kill fish likely seriously dangerous humans effects individual cone shell victims nervous systems precise useful tools research small size molecules makes easy traditional belief giant trap leg person causing drowning myth parasites known fever transmitted humans water hosts affects million people species agricultural addition accidental deliberate introduction species new territory resulted damage natural ecosystems disease caused worm second malaria devastating parasitic disease tropical countries estimated million people countries infected disease million africa parasite known species humans parasite species freshwater intermediate hosts species particularly certain crop introduced new environments local ecosystems pest giant african introduced parts asia islands 1990s species reached west_indies attempts control introducing predatory proved disastrous predator ignored went native species instead despite viral disease unrelated
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