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An octopus ( or ; plural: octopuses, octopi, or octopodes; see below) is a cephalopod mollusc of the order Octopoda. It has two eyes and four pairs of arms and, like other cephalopods, it is bilaterally symmetric. An octopus has a beak, with its mouth at the center point of the arms. An octopus has no internal or external skeleton (although some species have a vestigial remnant of a shell inside their mantles) allowing it to squeeze through tight places.〔(【引用サイトリンク】 title=Facts About Octopuses )〕 Octopuses are among the most intelligent and behaviorally diverse of all invertebrates. Octopuses inhabit diverse regions of the ocean, including coral reefs, pelagic waters, and the ocean floor. They have numerous strategies for defending themselves against predators, including the expulsion of ink, the use of camouflage and deimatic displays, their ability to jet quickly through the water, and their ability to hide. An octopus trails its eight arms behind it as it swims. All octopuses are venomous, but only one group, the blue-ringed octopus, is known to be deadly to humans.〔(Unimelb.edu.au ), Tentacles of venom: new study reveals all octopuses are venomous, University of Melbourne, Media Release, Wednesday 15 April 2009〕 Around 300 species are recognized, which is over one-third of the total number of known cephalopod species. The term 'octopus' may also be used to refer specifically to the genus ''Octopus''. == Etymology and pluralization == The scientific Latin term ''octopus'' was derived from Ancient Greek ''ὀκτώπους'' (oktōpous), which literally translates to "eight-foot" (''ὀκτώ'' "eight" + ''πούς'' "foot").〔(【引用サイトリンク】title=Online Etymology Dictionary )〕〔(【引用サイトリンク】title=Octopus | Define Octopus at Dictionary.com )〕〔(【引用サイトリンク】title=Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'': ὀκτώπους )〕 Related to the word "octopus" are the term "Octopoda" (the taxonomic order of cephalopod molluscs that comprises the octopuses) and the adjective "octopoid".〔 The standard pluralized form of "octopus" in the English language is "octopuses" , although the Ancient Greek plural "octopodes" , has also been used historically. The alternative plural "octopi" — which misguidedly assumes it is a Latin "-us"-word — is considered grammatically incorrect.〔Peters, Pam (2004). ''The Cambridge Guide to English Usage''. Cambridge: Cambridge University Press. ISBN 0-521-62181-X, p. 388.〕〔''Fowler's Modern English Usage'' states that the only acceptable plural in English is "octopuses", that "octopi" is misconceived, and "octopodes" pedantic.〕〔(''Chambers 21st Century Dictionary'' ) (retrieved 19 October 2007)〕 It is nevertheless used enough to make it notable, and was formally acknowledged by the descriptivist ''Merriam-Webster 11th Collegiate Dictionary'' and ''Webster's New World College Dictionary''. The ''Oxford English Dictionary'' (2008 Draft Revision)〔(OED.com ) (subscription required). Retrieved 1 February 2010.〕 lists "octopuses", "octopi", and "octopodes", in that order, labelling "octopodes" as rare and noting that "octopi" derives from the misapprehension that ''octōpus'' comes from Latin. In contrast, ''New Oxford American Dictionary'' (3rd Edition 2010) lists "octopuses" as the only acceptable pluralization, with a usage note indicating "octopodes" as being still occasionally used but "octopi" as being incorrect. == Biology == Octopuses are characterized by their eight arms, usually bearing suction cups. The arms of octopuses are often distinguished from the pair of feeding tentacles found in squid and cuttlefish.〔Norman, M. 2000. ''Cephalopods: A World Guide''. ConchBooks, Hackenheim. p. 15. ISBN 978-3-925919-32-9 "There is some confusion around the terms ''arms'' versus ''tentacles''. The numerous limbs of nautiluses are called tentacles. The ring of eight limbs around the mouth in cuttlefish, squids and octopuses are called ''arms''. Cuttlefish and squid also have a pair of specialized limbs attached between the bases of the third and fourth arm pairs (). These are known as ''feeding tentacles'' and are used to shoot out and grab prey."〕 Both types of limb are muscular hydrostats. Unlike most other cephalopods, the majority of octopuses – those in the suborder most commonly known, Incirrina – have almost entirely soft bodies with no internal skeleton. They have neither a protective outer shell like the nautilus, nor any vestige of an internal shell or bones, like cuttlefish or squid. The beak, similar in shape to a parrot's beak, and made of chitin, is the only hard part of their bodies. This enables them to squeeze through very narrow slits between underwater rocks, which is very helpful when they are fleeing from moray eels or other predatory fish. The octopuses in the less-familiar Cirrina suborder have two fins and an internal shell, generally reducing their ability to squeeze into small spaces. These cirrate species are often free-swimming and live in deep-water habitats, while incirrate octopus species are found in reefs and other shallower seafloor habitats. Octopuses have a relatively short life expectancy, with some species living for as little as six months. Larger species, such as the giant pacific octopus, may live for up to five years under suitable circumstances. However, reproduction is a cause of death: males can live for only a few months after mating, and females die shortly after their eggs hatch. They neglect to eat during the (roughly) one-month period spent taking care of their unhatched eggs, eventually dying of starvation. In a scientific experiment, the removal of both optic glands after spawning was found to result in the cessation of broodiness, the resumption of feeding, increased growth, and greatly extended lifespans. Octopuses have three hearts. Two branchial hearts pump blood through each of the two gills, while the third is a systemic heart that pumps blood through the body. Octopus blood contains the copper-rich protein hemocyanin for transporting oxygen. Although less efficient under normal conditions than the iron-rich hemoglobin of vertebrates, in cold conditions with low oxygen pressure, hemocyanin oxygen transportation is more efficient than hemoglobin oxygen transportation. The hemocyanin is dissolved in the plasma instead of being carried within red blood cells, and gives the blood a bluish color. The octopus draws water into its mantle cavity, where it passes through its gills. As molluscs, their gills are finely divided and vascularized outgrowths of either the outer or the inner body surface. 抄文引用元・出典: フリー百科事典『 An octopus ( or ; plural: octopuses, octopi, or octopodes; see below) is a cephalopod mollusc of the order Octopoda. It has two eyes and four pairs of arms and, like other cephalopods, it is bilaterally symmetric. An octopus has a beak, with its mouth at the center point of the arms. An octopus has no internal or external skeleton (although some species have a vestigial remnant of a shell inside their mantles) allowing it to squeeze through tight places.(【引用サイトリンク】 title=Facts About Octopuses ) Octopuses are among the most intelligent and behaviorally diverse of all invertebrates.Octopuses inhabit diverse regions of the ocean, including coral reefs, pelagic waters, and the ocean floor. They have numerous strategies for defending themselves against predators, including the expulsion of ink, the use of camouflage and deimatic displays, their ability to jet quickly through the water, and their ability to hide. An octopus trails its eight arms behind it as it swims. All octopuses are venomous, but only one group, the blue-ringed octopus, is known to be deadly to humans.(Unimelb.edu.au ), Tentacles of venom: new study reveals all octopuses are venomous, University of Melbourne, Media Release, Wednesday 15 April 2009Around 300 species are recognized, which is over one-third of the total number of known cephalopod species. The term 'octopus' may also be used to refer specifically to the genus ''Octopus''.== Etymology and pluralization ==The scientific Latin term ''octopus'' was derived from Ancient Greek ''ὀκτώπους'' (oktōpous), which literally translates to "eight-foot" (''ὀκτώ'' "eight" + ''πούς'' "foot").(【引用サイトリンク】title=Online Etymology Dictionary )(【引用サイトリンク】title=Octopus &">ウィキペディア（Wikipedia）』 ■An octopus ( or ; plural: octopuses, octopi, or octopodes; see below) is a cephalopod mollusc of the order Octopoda. It has two eyes and four pairs of arms and, like other cephalopods, it is bilaterally symmetric. An octopus has a beak, with its mouth at the center point of the arms. An octopus has no internal or external skeleton (although some species have a vestigial remnant of a shell inside their mantles) allowing it to squeeze through tight places.(【引用サイトリンク】 title=Facts About Octopuses ) Octopuses are among the most intelligent and behaviorally diverse of all invertebrates.Octopuses inhabit diverse regions of the ocean, including coral reefs, pelagic waters, and the ocean floor. They have numerous strategies for defending themselves against predators, including the expulsion of ink, the use of camouflage and deimatic displays, their ability to jet quickly through the water, and their ability to hide. An octopus trails its eight arms behind it as it swims. All octopuses are venomous, but only one group, the blue-ringed octopus, is known to be deadly to humans.(Unimelb.edu.au ), Tentacles of venom: new study reveals all octopuses are venomous, University of Melbourne, Media Release, Wednesday 15 April 2009Around 300 species are recognized, which is over one-third of the total number of known cephalopod species. The term 'octopus' may also be used to refer specifically to the genus ''Octopus''.== Etymology and pluralization ==The scientific Latin term ''octopus'' was derived from Ancient Greek ''ὀκτώπους'' (oktōpous), which literally translates to "eight-foot" (''ὀκτώ'' "eight" + ''πούς'' "foot").(【引用サイトリンク】title=Online Etymology Dictionary )(【引用サイトリンク】title=Octopus &">ウィキペディアで「NOTE: This article uses the common pluralization "octopuses". This is correct and intentional; please do not change it. Other variants are discussed in the Etymology and pluralization section.-->An octopus ( or ; plural: octopuses, octopi, or octopodes; see below) is a cephalopod mollusc of the order Octopoda. It has two eyes and four pairs of arms and, like other cephalopods, it is bilaterally symmetric. An octopus has a beak, with its mouth at the center point of the arms. An octopus has no internal or external skeleton (although some species have a vestigial remnant of a shell inside their mantles) allowing it to squeeze through tight places.(【引用サイトリンク】 title=Facts About Octopuses ) Octopuses are among the most intelligent and behaviorally diverse of all invertebrates.Octopuses inhabit diverse regions of the ocean, including coral reefs, pelagic waters, and the ocean floor. They have numerous strategies for defending themselves against predators, including the expulsion of ink, the use of camouflage and deimatic displays, their ability to jet quickly through the water, and their ability to hide. An octopus trails its eight arms behind it as it swims. All octopuses are venomous, but only one group, the blue-ringed octopus, is known to be deadly to humans.(Unimelb.edu.au ), Tentacles of venom: new study reveals all octopuses are venomous, University of Melbourne, Media Release, Wednesday 15 April 2009Around 300 species are recognized, which is over one-third of the total number of known cephalopod species. The term 'octopus' may also be used to refer specifically to the genus ''Octopus''.== Etymology and pluralization ==The scientific Latin term ''octopus'' was derived from Ancient Greek ''ὀκτώπους'' (oktōpous), which literally translates to "eight-foot" (''ὀκτώ'' "eight" + ''πούς'' "foot").(【引用サイトリンク】title=Online Etymology Dictionary )(【引用サイトリンク】title=Octopus &」の詳細全文を読む スポンサード リンク
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