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are smaller than, the median. if elements in the data increase arithmetically, when placed in some order, then the median and arithmetic average are equal. for example, consider the data sample . the average is , as is the median. however, when we consider a sample that cannot be arranged so as to increase arithmetically, such as , the median and arithmetic average can differ significantly. in this case, the arithmetic average is 6.2, while the median is 4. in general, the average value can vary significantly from most values in the sample, and can be larger or smaller than most of them. there are applications of this phenomenon in many fields. for example, since the 1980s, the median income in the united states has increased more slowly than the arithmetic average of income. generalizations weighted average a

weighted average, or weighted mean, is an average in which some data points count more heavily than others, in that they are given more weight in the calculation. for example, the arithmetic mean of and is , or equivalently . in contrast, a weighted mean in which the first number receives, for example, twice as much weight as the second (perhaps because it is assumed to appear twice as often in the general population from which these numbers were sampled) would be calculated as . here the weights, which necessarily sum to the value one, are and , the former being twice the latter. the arithmetic mean (sometimes called the "unweighted average" or "equally weighted average") can be interpreted as a special case of a weighted average in which all the weights are equal to each other (equal to in the above example,

and equal to in a situation with numbers being averaged). continuous probability distributions if a numerical property, and any sample of data from it, could take on any value from a continuous range, instead of, for example, just integers, then the probability of a number falling into some range of possible values can be described by integrating a continuous probability distribution across this range, even when the naive probability for a sample number taking one certain value from infinitely many is zero. the analog of a weighted average in this context, in which there are an infinite number of possibilities for the precise value of the variable in each range, is called the mean of the probability distribution. a most widely encountered probability distribution is called the normal distribution; it has the

property that all measures of its central tendency, including not just the mean but also the aforementioned median and the mode (the threem's), are equal to each other. this equality does not hold for other probability distributions, as illustrated for the log-normal distribution here. angles particular care must be taken when using cyclic data, such as phases or angles. naively taking the arithmetic mean of 1 and 359 yields a result of 180. this is incorrect for two reasons: firstly, angle measurements are only defined up to an additive constant of 360 (or 2, if measuring in radians). thus one could as easily call these 1 and 1, or 361 and 719, since each one of them gives a different average. secondly, in this situation, 0 (equivalently, 360) is geometrically a better average value: there is lower dispersion

about it (the points are both 1 from it, and 179 from 180, the putative average). in general application, such an oversight will lead to the average value artificially moving towards the middle of the numerical range. a solution to this problem is to use the optimization formulation (viz., define the mean as the central point: the point about which one has the lowest dispersion), and redefine the difference as a modular distance (i.e., the distance on the circle: so the modular distance between 1 and 359 is 2, not 358). symbols and encoding the arithmetic mean is often denoted by a bar, (a.k.a vinculum or macron), for example as in (read bar). some software (text processors, web browsers) may not display the x symbol properly. for example, the x symbol in html is actually a combination of two codes - the

base letter x plus a code for the line above (̄ or ). in some texts, such as pdfs, the x symbol may be replaced by a cent () symbol (unicode &#162), when copied to text processor such as microsoft word. see also frechet mean generalized mean geometric mean harmonic mean inequality of arithmetic and geometric means mode sample mean and covariance standard deviation standard error of the mean summary statistics references further reading external links calculations and comparisons between arithmetic mean and geometric mean of two numbers calculate the arithmetic mean of a series of numbers on fxsolver means

the american football conference (afc) is one of the two conferences of the national football league (nfl), the highest professional level of american football in the united states. this conference currently contains 16 teams organized into 4 divisions, as does its counterpart, the national football conference (nfc). both conferences were created as part of the 1970 merger between the national football league, and the american football league (afl). all ten of the afl teams, and three nfl teams, became members of the new afc, with the remaining thirteen nfl teams forming the nfc. a series of league expansions and division realignments have occurred since the merger, thus making the current total of 16 teams in each conference. the current afc champions are the cincinnati bengals, who defeated the kansas city chiefs

in the 2022 afc championship game for their third conference championship, and their first since 1988. teams like the nfc, the conference has 16 teams organized into four divisions each with four teams: east, north, south and west. season structure this chart of the 2021 season standings displays an application of the nfl scheduling formula. the bengals in 2021 (highlighted in green) finished in first place in the afc north. thus, in 2021, the bengals are scheduled to play two games against each of its division rivals (highlighted in light blue), one game against each team in the afc east and nfc south (highlighted in yellow), and one game each against the first-place finishers in the afc south, afc west (highlighted in orange), and nfc east (highlighted in pink). currently, the fourteen opponents each team

faces over the 17-game regular season schedule are set using a pre-determined formula: each afc team plays the other teams in their respective division twice (home and away) during the regular season, in addition to eleven other games assigned to their schedule by the nfl: three games are assigned on the basis of a particular team's final divisional standing from the previous season, and the remaining eight games are split between the roster of two other nfl divisions. this assignment shifts each year and will follow a standard cycle. using the 2021 regular season schedule as an example, each team in the afc west plays against every team in the afc north and nfc east. in this way, non-divisional competition will be mostly among common opponents the exception being the three games assigned based on the team's

prior-season divisional standing. at the end of each season, the four division winners and three wild cards (non-division winners with best regular season record) in the afc qualify for the playoffs. the afc playoffs culminate in the afc championship game, with the winner receiving the lamar hunt trophy. the afc champion then plays the nfc champion in the super bowl. history both the afc and the nfc were created after the nfl merged with the american football league (afl) in 1970. the afl began play in 1960 with eight teams, and added two more expansion clubs (the miami dolphins in 1966 and the cincinnati bengals in 1968) before the merger. in order to equalize the number of teams in each conference, three nfl teams that predated the afl's launch (the cleveland browns, pittsburgh steelers, and the then-baltimore

colts) joined the ten former afl teams to form the afc. the two afl divisions afl east and afl west were more or less intact, while the nfl's century division, in which the browns and the steelers had played since 1967, was moved from the nfl to become the new afc central. upon the completion of the merger of the afl and nfl in 1970, the newly minted american football conference had already agreed upon their divisional setup along mostly geographical lines for the 1970 season; the national football conference, however, could not agree upon their setup, and one was chosen from a fishbowl on january 16, 1970. since the merger, five expansion teams have joined the afc and two have left, thus making the current total 16. when the seattle seahawks and the tampa bay buccaneers joined the league in 1976, they were

temporarily placed in the nfc and afc respectively. this arrangement lasted for one season only before the two teams switched conferences. the seahawks eventually returned to the nfc as a result of the 2002 realignment. the expansion jacksonville jaguars joined the afc in 1995. there have been five teams that have relocated at least once. in 1984, the baltimore colts relocated to indianapolis. in 1995, the cleveland browns had attempted to move to baltimore; the resulting dispute between cleveland and the team led to modell establishing the baltimore ravens with the players and personnel from the browns, while the browns were placed in suspended operations before they were reinstated by the nfl. the ravens were treated as an expansion team. in california, the oakland raiders relocated to los angeles in 1982,

back to oakland in 1995, and then to las vegas in 2020, while the san diego chargers returned to los angeles in 2017 after 56 years in san diego. the houston oilers moved to tennessee in 1997, where they were renamed the tennessee oilers. the team would change its name again, two years later, to the tennessee titans. the nfl would again expand in 2002, adding the houston texans to the afc. with the exception of the aforementioned relocations since that time, the divisional setup has remained static ever since. between 1995 and 2021, the afc has sent only 9 of its 16 teams to the super bowl: new england patriots (10 times), denver broncos (4 times), pittsburgh steelers (4 times), baltimore ravens (2 times), indianapolis colts (2 times), kansas city chiefs (2 times), cincinnati bengals (1 time), las vegas raiders

(1 time), tennessee titans (1 time). by contrast, the nfc has sent 13 of the 16 nfc teams during that same time frame with only the detroit lions, minnesota vikings, and washington commanders missing out on an appearance in the super bowl. 17 of the 19 afc champions from 2001 to 2019 have started one of just three quarterbacks - tom brady, peyton manning and ben roethlisberger - in the super bowl. the afc has started 7 quarterbacks in the last 20 super bowls, while the nfc has started 16. logo the merged league created a new logo for the afc that took elements of the old afl logo, specifically the "a" and the six stars surrounding it. the afc logo basically remained unchanged from 1970 to 2009. the 2010 nfl season introduced an updated afc logo, with the most notable revision being the removal of two stars

(leaving four representing the four divisions of the afc), and moving the stars inside the letter, similar to the nfc logo. television nbc aired the afc's sunday afternoon and playoff games from 1970 through the 1997 season. from 1998 to 2013, cbs was the primary broadcast rightsholder to the afc; in those years, all interconference games in which the afc team was the visiting team were broadcast on either nbc or cbs. since 2014, the cross-flex policy allows select afc games (that involve them playing an nfc team at home or intraconference games) to be moved from cbs to fox. since 1990, select afc playoff games have been seen on abc or espn. references national football league conference sports organizations established in 1970

animal farm is a satirical allegorical novella by george orwell, first published in england on 17 august 1945. the book tells the story of a group of farm animals who rebel against their human farmer, hoping to create a society where the animals can be equal, free, and happy. ultimately, the rebellion is betrayed, and the farm ends up in a state as bad as it was before, under the dictatorship of a pig named napoleon. according to orwell, the fable reflects events leading up to the russian revolution of 1917 and then on into the stalinist era of the soviet union. orwell, a democratic socialist, was a critic of joseph stalin and hostile to moscow-directed stalinism, an attitude that was critically shaped by his experiences during the may days conflicts between the poum and stalinist forces during the spanish civil

war. in a letter to yvonne davet, orwell described animal farm as a satirical tale against stalin (""), and in his essay "why i write" (1946), wrote that animal farm was the first book in which he tried, with full consciousness of what he was doing, "to fuse political purpose and artistic purpose into one whole". the original title was animal farm: a fairy story, but u.s. publishers dropped the subtitle when it was published in 1946, and only one of the translations during orwell's lifetime, the telugu version, kept it. other titular variations include subtitles like "a satire" and "a contemporary satire". orwell suggested the title for the french translation, which abbreviates to ursa, the latin word for "bear", a symbol of russia. it also played on the french name of the soviet union, . orwell wrote the book

between november 1943 and february 1944, when the united kingdom was in its wartime alliance with the soviet union against nazi germany, and the british intelligentsia held stalin in high esteem, a phenomenon orwell hated. the manuscript was initially rejected by a number of british and american publishers, including one of orwell's own, victor gollancz, which delayed its publication. it became a great commercial success when it did appear partly because international relations were transformed as the wartime alliance gave way to the cold war. time magazine chose the book as one of the 100 best english-language novels (1923 to 2005); it also featured at number 31 on the modern library list of best 20th-century novels, and number 46 on the bbc's the big read poll. it won a retrospective hugo award in 1996 and

is included in the great books of the western world selection. plot summary the poorly-run manor farm near willingdon, england, is ripened for rebellion from its animal populace by neglect at the hands of the irresponsible and alcoholic farmer, mr. jones. one night, the exalted boar, old major, holds a conference, at which he calls for the overthrow of humans and teaches the animals a revolutionary song called "beasts of england". when old major dies, two young pigs, snowball and napoleon, assume command and stage a revolt, driving mr. jones off the farm and renaming the property "animal farm". they adopt the seven commandments of animalism, the most important of which is, "all animals are equal". the decree is painted in large letters on one side of the barn. snowball teaches the animals to read and write,

while napoleon educates young puppies on the principles of animalism. to commemorate the start of animal farm, snowball raises a green flag with a white hoof and horn. food is plentiful, and the farm runs smoothly. the pigs elevate themselves to positions of leadership and set aside special food items, ostensibly for their personal health. following an unsuccessful attempt by mr. jones and his associates to retake the farm (later dubbed the "battle of the cowshed"), snowball announces his plans to modernise the farm by building a windmill. napoleon disputes this idea, and matters come to head, which culminate in napoleon's dogs chasing snowball away and napoleon declaring himself supreme commander. napoleon enacts changes to the governance structure of the farm, replacing meetings with a committee of pigs who

will run the farm. through a young porker named squealer, napoleon claims credit for the windmill idea, claiming that snowball was only trying to win animals to his side. the animals work harder with the promise of easier lives with the windmill. when the animals find the windmill collapsed after a violent storm, napoleon and squealer persuade the animals that snowball is trying to sabotage their project, and begin to purge the farm of animals accused by napoleon of consorting with his old rival. when some animals recall the battle of the cowshed, napoleon (who was nowhere to be found during the battle) gradually smears snowball to the point of saying he is a collaborator of mr. jones, even dismissing the fact that snowball was given an award of courage while falsely representing himself as the main hero of the

battle. "beasts of england" is replaced with "animal farm", while an anthem glorifying napoleon, who appears to be adopting the lifestyle of a man ("comrade napoleon"), is composed and sung. napoleon then conducts a second purge, during which many animals who are alleged to be helping snowball in plots are executed by napoleon's dogs, which troubles the rest of the animals. despite their hardships, the animals are easily placated by napoleon's retort that they are better off than they were under mr. jones, as well as by the sheep's continual bleating of four legs good, two legs bad. mr. frederick, a neighbouring farmer, attacks the farm, using blasting powder to blow up the restored windmill. although the animals win the battle, they do so at great cost, as many, including boxer the workhorse, are wounded. although

he recovers from this, boxer eventually collapses while working on the windmill (being almost 12 years old at that point). he is taken away in a knacker's van, and a donkey called benjamin alerts the animals of this, but squealer quickly waves off their alarm by persuading the animals that the van had been purchased from the knacker by an animal hospital and that the previous owner's signboard had not been repainted. squealer subsequently reports boxer's death and honours him with a festival the following day. (however, napoleon had in fact engineered the sale of boxer to the knacker, allowing him and his inner circle to acquire money to buy whisky for themselves.) years pass, the windmill is rebuilt, and another windmill is constructed, which makes the farm a good amount of income. however, the ideals that

snowball discussed, including stalls with electric lighting, heating, and running water, are forgotten, with napoleon advocating that the happiest animals live simple lives. snowball has been forgotten, alongside boxer, with "the exception of the few who knew him". many of the animals who participated in the rebellion are dead or old. mr. jones is also dead, saying he "died in an inebriates' home in another part of the country". the pigs start to resemble humans, as they walk upright, carry whips, drink alcohol, and wear clothes. the seven commandments are abridged to just one phrase: "all animals are equal, but some animals are more equal than others." the maxim "four legs good, two legs bad" is similarly changed to "four legs good, two legs better." other changes include the hoof and horn flag being replaced

with a plain green banner and old major's skull, which was previously put on display, being reburied. napoleon holds a dinner party for the pigs and local farmers, with whom he celebrates a new alliance. he abolishes the practice of the revolutionary traditions and restores the name "the manor farm". the men and pigs start playing cards, flattering and praising each other while cheating at the game. both napoleon and mr. pilkington, one of the farmers, play the ace of spades at the same time and both sides begin fighting loudly over who cheated first. when the animals outside look at the pigs and men, they can no longer distinguish between the two. characters pigs old major an aged prize middle white boar provides the inspiration that fuels the rebellion. he is also called willingdon beauty when showing.

he is an allegorical combination of karl marx, one of the creators of communism, and vladimir lenin, the communist leader of the russian revolution and the early soviet nation, in that he draws up the principles of the revolution. his skull being put on revered public display recalls lenin, whose embalmed body was left in indefinite repose. by the end of the book, the skull is reburied. napoleon "a large, rather fierce-looking berkshire boar, the only berkshire on the farm, not much of a talker, but with a reputation for getting his own way". an allegory of joseph stalin, napoleon is the leader of animal farm. snowball napoleon's rival and original head of the farm after jones' overthrow. his life parallels that of leon trotsky, but may also combine elements from lenin. squealer a small, white, fat porker

who serves as napoleon's second-in-command and minister of propaganda, holding a position similar to that of vyacheslav molotov. minimus a poetic pig who writes the second and third national anthems of animal farm after the singing of "beasts of england" is banned. literary theorist john rodden compares him to the poet vladimir mayakovsky. the piglets hinted to be the children of napoleon and are the first generation of animals subjugated to his idea of animal inequality. the young pigs four pigs who complain about napoleon's takeover of the farm but are quickly silenced and later executed, the first animals killed in napoleon's farm purge. probably based on the great purge of grigory zinoviev, lev kamenev, nikolai bukharin, and alexei rykov. pinkeye a minor pig who is mentioned only once; he is the taste

tester that samples napoleon's food to make sure it is not poisoned, in response to rumours about an assassination attempt on napoleon. humans mr. jones a heavy drinker who is the original owner of manor farm, a farm in disrepair with farmhands who often loaf on the job. he is an allegory of russian tsar nicholas ii, who abdicated following the february revolution of 1917 and was murdered, along with the rest of his family, by the bolsheviks on 17 july 1918. the animals revolt after jones goes on a drinking binge, returns hungover the following day and neglects them completely. jones is married, but his wife plays no active role in the book. she seems to live with her husband's drunkenness, going to bed while he stays up drinking till late into the night. in her only other appearance, she hastily throws a

few things into a travel bag and flees when she sees that the animals are revolting. towards the end of the book, one of the farm sows wears her old sunday dress. mr. frederick the tough owner of pinchfield farm, a small but well-kept neighbouring farm, who briefly enters into an alliance with napoleon. animal farm shares land boundaries with pinchfield on one side and foxwood on another, making animal farm a "buffer zone" between the two bickering farmers. the animals of animal farm are terrified of frederick, as rumours abound of him abusing his animals and entertaining himself with cockfighting. napoleon enters into an alliance with frederick in order to sell surplus timber that pilkington also sought, but is enraged to learn frederick paid him in counterfeit money. shortly after the swindling, frederick

and his men invade animal farm, killing many animals and destroying the windmill. the brief alliance and subsequent invasion may allude to the molotovribbentrop pact and operation barbarossa. mr. pilkington the easy-going but crafty and well-to-do owner of foxwood farm, a large neighbouring farm overgrown with weeds. pilkington is wealthier than frederick and owns more land, but his farm is in need of care as opposed to frederick's smaller but more efficiently run farm. although on bad terms with frederick, pilkington is also concerned about the animal revolution that deposed jones and worried that this could also happen to him. mr. whymper a man hired by napoleon to act as the liaison between animal farm and human society. at first, he is used to acquire necessities that cannot be produced on the farm, such

as dog biscuits and paraffin wax, but later he procures luxuries like alcohol for the pigs. equines boxer a loyal, kind, dedicated, extremely strong, hard-working, and respectable cart-horse, although quite naive and gullible. boxer does a large share of the physical labour on the farm. he is shown to hold the belief that "napoleon is always right." at one point, he had challenged squealer's statement that snowball was always against the welfare of the farm, earning him an attack from napoleon's dogs. but boxer's immense strength repels the attack, worrying the pigs that their authority can be challenged. boxer has been compared to alexey stakhanov, a diligent and enthusiastic role model of the stakhanovite movement. he has been described as "faithful and strong"; he believes any problem can be solved if he

works harder. when boxer is injured, napoleon sells him to a local knacker to buy himself whisky, and squealer gives a moving account, falsifying boxer's death. mollie a self-centred, self-indulgent, and vain young white mare who quickly leaves for another farm after the revolution, in a manner similar to those who left russia after the fall of the tsar. she is only once mentioned again. clover a gentle, caring mare, who shows concern especially for boxer, who often pushes himself too hard. clover can read all the letters of the alphabet, but cannot "put words together". she seems to catch on to the sly tricks and schemes set up by napoleon and squealer. benjamin a donkey, one of the oldest, wisest animals on the farm, and one of the few who can read properly. he is sceptical, temperamental and cynical:

his most frequent remark is, "life will go on as it has always gone on that is, badly." the academic morris dickstein has suggested there is "a touch of orwell himself in this creature's timeless scepticism" and indeed, friends called orwell "donkey george", "after his grumbling donkey benjamin, in animal farm." other animals muriel a wise old goat who is friends with all of the animals on the farm. similarly to benjamin, muriel is one of the few animals on the farm who is not a pig but can read. the puppies offspring of jessie and bluebell, the puppies were taken away at birth by napoleon and raised by him to serve as his powerful security force. moses the raven, "mr. jones's especial pet, was a spy and a tale-bearer, but he was also a clever talker." initially following mrs. jones into exile, he reappears

several years later and resumes his role of talking but not working. he regales animal farm's denizens with tales of a wondrous place beyond the clouds called "sugarcandy mountain, that happy country where we poor animals shall rest forever from our labours!" orwell portrays established religion as "the black raven of priestcraft promising pie in the sky when you die, and faithfully serving whoever happens to be in power." his preaching to the animals heartens them, and napoleon allows moses to reside at the farm "with an allowance of a gill of beer daily", akin to how stalin brought back the russian orthodox church during the second world war. the sheep they are not given individual names or personalities. they show limited understanding of animalism and the political atmosphere of the farm, yet nonetheless

they are the voice of blind conformity as they bleat their support of napoleon's ideals with jingles during his speeches and meetings with snowball. their constant bleating of "four legs good, two legs bad" was used as a device to drown out any opposition or alternative views from snowball, much as stalin used hysterical crowds to drown out trotsky. towards the end of the book, squealer (the propagandist) trains the sheep to alter their slogan to "four legs good, two legs better", which they dutifully do. the hens also unnamed, the hens are promised at the start of the revolution that they will get to keep their eggs, which are stolen from them under mr. jones. however, their eggs are soon taken from them under the premise of buying goods from outside animal farm. the hens are among the first to rebel, albeit

unsuccessfully, against napoleon. the cows also unnamed, the cows are enticed into the revolution by promises that their milk will not be stolen but can be used to raise their own calves. their milk is then stolen by the pigs, who learn to milk them. the milk is stirred into the pigs' mash every day, while the other animals are denied such luxuries. the cat unnamed and never seen to carry out any work, the cat is absent for long periods and is forgiven because her excuses are so convincing and she "purred so affectionately that it was impossible not to believe in her good intentions." she has no interest in the politics of the farm, and the only time she is recorded as having participated in an election, she is found to have actually "voted on both sides." the ducks also unnamed. the roosters one arranges

to wake boxer early, and a black one acts as a trumpeter for napoleon. the geese also unnamed. one gander commits suicide by eating nightshade berries. genre and style george orwell's animal farm is an example of a political satire that was intended to have a "wider application", according to orwell himself, in terms of its relevance. stylistically, the work shares many similarities with some of orwell's other works, most notably 1984, as both have been considered works of swiftian satire. furthermore, these two prominent works seem to suggest orwell's bleak view of the future for humanity; he seems to stress the potential/current threat of dystopias similar to those in animal farm and 1984. in these kinds of works, orwell distinctly references the disarray and traumatic conditions of europe following the

second world war. orwell's style and writing philosophy as a whole were very concerned with the pursuit of truth in writing. orwell was committed to communicating in a way that was straightforward, given the way that he felt words were commonly used in politics to deceive and confuse. for this reason, he is careful, in animal farm, to make sure the narrator speaks in an unbiased and uncomplicated fashion. the difference is seen in the way that the animals speak and interact, as the generally moral animals seem to speak their minds clearly, while the wicked animals on the farm, such as napoleon, twist language in such a way that it meets their own insidious desires. this style reflects orwell's close proximation to the issues facing europe at the time and his determination to comment critically on stalin's soviet

russia. background origin and writing george orwell wrote the manuscript between november 1943 and february 1944 after his experiences during the spanish civil war, which he described in homage to catalonia (1938). in the preface of a 1947 ukrainian edition of animal farm, he explained how escaping the communist purges in spain taught him "how easily totalitarian propaganda can control the opinion of enlightened people in democratic countries." this motivated orwell to expose and strongly condemn what he saw as the stalinist corruption of the original socialist ideals. homage to catalonia sold poorly; after seeing arthur koestler's best-selling, darkness at noon, about the moscow trials, orwell decided that fiction was the best way to describe totalitarianism. immediately prior to writing the book, orwell

had quit the bbc. he was also upset about a booklet for propagandists the ministry of information had put out. the booklet included instructions on how to quell ideological fears of the soviet union, such as directions to claim that the red terror was a figment of nazi imagination. in the preface, orwell described the source of the idea of setting the book on a farm: in 1944, the manuscript was almost lost when a german v-1 flying bomb destroyed his london home. orwell spent hours sifting through the rubble to find the pages intact. publication publishing orwell initially encountered difficulty getting the manuscript published, largely due to fears that the book might upset the alliance between britain, the united states, and the soviet union. four publishers refused to publish animal farm, yet one had initially

accepted the work, but declined it after consulting the ministry of information. eventually, secker and warburg published the first edition in 1945. during the second world war, it became clear to orwell that anti-soviet literature was not something which most major publishing houses would touch including his regular publisher gollancz. he also submitted the manuscript to faber and faber, where the poet t. s. eliot (who was a director of the firm) rejected it; eliot wrote back to orwell praising the book's "good writing" and "fundamental integrity", but declared that they would only accept it for publication if they had some sympathy for the viewpoint "which i take to be generally trotskyite". eliot said he found the view "not convincing", and contended that the pigs were made out to be the best to run the

farm; he posited that someone might argue "what was needed... was not more communism but more public-spirited pigs". orwell let andre deutsch, who was working for nicholson & watson in 1944, read the typescript, and deutsch was convinced that nicholson & watson would want to publish it; however, they did not, and "lectured orwell on what they perceived to be errors in animal farm." in his london letter on 17 april 1944 for partisan review, orwell wrote that it was "now next door to impossible to get anything overtly anti-russian printed. anti-russian books do appear, but mostly from catholic publishing firms and always from a religious or frankly reactionary angle." the publisher jonathan cape, who had initially accepted animal farm, subsequently rejected the book after an official at the british ministry of

information warned him off although the civil servant who it is assumed gave the order was later found to be a soviet spy. writing to leonard moore, a partner in the literary agency of christy & moore, publisher jonathan cape explained that the decision had been taken on the advice of a senior official in the ministry of information. such flagrant anti-soviet bias was unacceptable, and the choice of pigs as the dominant class was thought to be especially offensive. it may reasonably be assumed that the "important official" was a man named peter smollett, who was later unmasked as a soviet agent. orwell was suspicious of smollett/smolka, and he would be one of the names orwell included in his list of crypto-communists and fellow-travellers sent to the information research department in 1949. the publisher wrote

to orwell, saying: frederic warburg also faced pressures against publication, even from people in his own office and from his wife pamela, who felt that it was not the moment for ingratitude towards stalin and the heroic red army, which had played a major part in defeating adolf hitler. a russian translation was printed in the paper posev, and in giving permission for a russian translation of animal farm, orwell refused in advance all royalties. a translation in ukrainian, which was produced in germany, was confiscated in large part by the american wartime authorities and handed over to the soviet repatriation commission. in october 1945, orwell wrote to frederic warburg expressing interest in pursuing the possibility that the political cartoonist david low might illustrate animal farm. low had written a letter

saying that he had had "a good time with animal farm an excellent bit of satire it would illustrate perfectly." nothing came of this, and a trial issue produced by secker & warburg in 1956 illustrated by john driver was abandoned, but the folio society published an edition in 1984 illustrated by quentin blake and an edition illustrated by the cartoonist ralph steadman was published by secker & warburg in 1995 to celebrate the fiftieth anniversary of the first edition of animal farm. preface orwell originally wrote a preface complaining about british self-censorship and how the british people were suppressing criticism of the ussr, their world war ii ally: although the first edition allowed space for the preface, it was not included, and as of june 2009 most editions of the book have not included it. secker

and warburg published the first edition of animal farm in 1945 without an introduction. however, the publisher had provided space for a preface in the author's proof composited from the manuscript. for reasons unknown, no preface was supplied, and the page numbers had to be renumbered at the last minute. in 1972, ian angus found the original typescript titled "the freedom of the press", and bernard crick published it, together with his own introduction, in the times literary supplement on 15 september 1972 as "how the essay came to be written". orwell's essay criticised british self-censorship by the press, specifically the suppression of unflattering descriptions of stalin and the soviet government. the same essay also appeared in the italian 1976 edition of animal farm with another introduction by crick, claiming

to be the first edition with the preface. other publishers were still declining to publish it. reception contemporary reviews of the work were not universally positive. writing in the american new republic magazine, george soule expressed his disappointment in the book, writing that it "puzzled and saddened me. it seemed on the whole dull. the allegory turned out to be a creaking machine for saying in a clumsy way things that have been said better directly." soule believed that the animals were not consistent enough with their real-world inspirations, and said, "it seems to me that the failure of this book (commercially it is already assured of tremendous success) arises from the fact that the satire deals not with something the author has experienced, but rather with stereotyped ideas about a country which

he probably does not know very well". the guardian on 24 august 1945 called animal farm "a delightfully humorous and caustic satire on the rule of the many by the few". tosco fyvel, writing in tribune on the same day, called the book "a gentle satire on a certain state and on the illusions of an age which may already be behind us." julian symons responded, on 7 september, "should we not expect, in tribune at least, acknowledgement of the fact that it is a satire not at all gentle upon a particular state soviet russia? it seems to me that a reviewer should have the courage to identify napoleon with stalin, and snowball with trotsky, and express an opinion favourable or unfavourable to the author, upon a political ground. in a hundred years time perhaps, animal farm may be simply a fairy story; today it is a

political satire with a good deal of point." animal farm has been subject to much comment in the decades since these early remarks. the cia, from 1952 to 1957 in operation aedinosaur, sent millions of balloons carrying copies of the novel into poland, hungary and czechoslovakia, whose air forces tried to shoot the balloons down. time magazine chose animal farm as one of the 100 best english-language novels (1923 to 2005); it also featured at number 31 on the modern library list of best 20th-century novels. it won a retrospective hugo award in 1996 and is included in the great books of the western world selection. popular reading in schools, animal farm was ranked the uk's favourite book from school in a 2016 poll. animal farm has also faced an array of challenges in school settings around the us. the following

are examples of this controversy that has existed around orwell's work: the john birch society in wisconsin challenged the reading of animal farm in 1965 because of its reference to masses revolting. new york state english council's committee on defense against censorship found that in 1968, animal farm had been widely deemed a "problem book". a censorship survey conducted in dekalb county, georgia, relating to the years 19791982, revealed that many schools had attempted to limit access to animal farm due to its "political theories". a superintendent in bay county, florida, banned animal farm at the middle school and high school levels in 1987. the board quickly brought back the book, however, after receiving complaints of the ban as "unconstitutional". animal farm was removed from the stonington, connecticut

school district curriculum in 2017. animal farm has also faced similar forms of resistance in other countries. the ala also mentions the way that the book was prevented from being featured at the international book fair in moscow, russia, in 1977 and banned from schools in the united arab emirates for references to practices or actions that defy arab or islamic beliefs, such as pigs or alcohol. in the same manner, animal farm has also faced relatively recent issues in china. in 2018, the government made the decision to censor all online posts about or referring to animal farm. however the book itself, as of 2019, remains sold in stores. amy hawkins and jeffrey wasserstrom of the atlantic stated in 2019 that the book is widely available in mainland china for several reasons: censors believe the general public

is unlikely to read a highbrow book , because the elites who do read books feel connected to the ruling party anyway, and because the communist party sees being too aggressive in blocking cultural products as a liability. the authors stated "it wasand remainsas easy to buy 1984 and animal farm in shenzhen or shanghai as it is in london or los angeles." an enhanced version of the book, launched in india in 2017, was widely praised for capturing the author's intent, by republishing the proposed preface of the first edition and the preface he wrote for the ukrainian edition. analysis animalism the pigs snowball, napoleon, and squealer adapt old major's ideas into "a complete system of thought", which they formally name animalism, an allegoric reference to communism, not to be confused with the philosophy animalism.

soon after, napoleon and squealer partake in activities associated with the humans (drinking alcohol, sleeping in beds, trading), which were explicitly prohibited by the seven commandments. squealer is employed to alter the seven commandments to account for this humanisation, an allusion to the soviet government's revising of history in order to exercise control of the people's beliefs about themselves and their society. the original commandments are: whatever goes upon two legs is an enemy. whatever goes upon four legs, or has wings, is a friend. no animal shall wear clothes. no animal shall sleep in a bed. no animal shall drink alcohol. no animal shall kill any other animal. all animals are equal. these commandments are also distilled into the maxim "four legs good, two legs bad!" which is primarily

used by the sheep on the farm, often to disrupt discussions and disagreements between animals on the nature of animalism. later, napoleon and his pigs secretly revise some commandments to clear themselves of accusations of law-breaking. the changed commandments are as follows, with the changes bolded: eventually, these are replaced with the maxims, "all animals are equal, but some animals are more equal than others", and "four legs good, two legs better" as the pigs become more human. this is an ironic twist to the original purpose of the seven commandments, which were supposed to keep order within animal farm by uniting the animals together against the humans and preventing animals from following the humans' evil habits. through the revision of the commandments, orwell demonstrates how simply political dogma

can be turned into malleable propaganda. significance and allegory orwell biographer jeffrey meyers has written, "virtually every detail has political significance in this allegory." orwell himself wrote in 1946, "of course i intended it primarily as a satire on the russian revolution... [and] that kind of revolution (violent conspiratorial revolution, led by unconsciously power-hungry people) can only lead to a change of masters [-] revolutions only effect a radical improvement when the masses are alert." in a preface for a 1947 ukrainian edition, he stated, "for the past ten years i have been convinced that the destruction of the soviet myth was essential if we wanted a revival of the socialist movement. on my return from spain [in 1937] i thought of exposing the soviet myth in a story that could be easily

understood by almost anyone and which could be easily translated into other languages." the revolt of the animals against farmer jones is orwell's analogy with the october 1917 bolshevik revolution. the battle of the cowshed has been said to represent the allied invasion of soviet russia in 1918, and the defeat of the white russians in the russian civil war. the pigs' rise to preeminence mirrors the rise of a stalinist bureaucracy in the ussr, just as napoleon's emergence as the farm's sole leader reflects stalin's emergence. the pigs' appropriation of milk and apples for their own use, "the turning point of the story" as orwell termed it in a letter to dwight macdonald, stands as an analogy for the crushing of the left-wing 1921 kronstadt revolt against the bolsheviks, and the difficult efforts of the animals

to build the windmill suggest the various five year plans. the puppies controlled by napoleon parallel the nurture of the secret police in the stalinist structure, and the pigs' treatment of the other animals on the farm recalls the internal terror faced by the populace in the 1930s. in chapter seven, when the animals confess their non-existent crimes and are killed, orwell directly alludes to the purges, confessions and show trials of the late 1930s. these contributed to orwell's conviction that the bolshevik revolution had been corrupted and the soviet system become rotten. peter edgerly firchow and peter davison contend that the battle of the windmill, specifically referencing the battle of stalingrad and the battle of moscow, represents world war ii. during the battle, orwell first wrote, "all the animals,

including napoleon" took cover. orwell had the publisher alter this to "all the animals except napoleon" in recognition of stalin's decision to remain in moscow during the german advance. orwell requested the change after he met jozef czapski in paris in march 1945. czapski, a survivor of the katyn massacre and an opponent of the soviet regime, told orwell, as orwell wrote to arthur koestler, that it had been "the character [and] greatness of stalin" that saved russia from the german invasion. other connections that writers have suggested illustrate orwell's telescoping of russian history from 1917 to 1943 include the wave of rebelliousness that ran through the countryside after the rebellion, which stands for the abortive revolutions in hungary and in germany (ch iv); the conflict between napoleon and snowball

(ch v), parallelling "the two rival and quasi-messianic beliefs that seemed pitted against one another: trotskyism, with its faith in the revolutionary vocation of the proletariat of the west; and stalinism with its glorification of russia's socialist destiny"; napoleon's dealings with whymper and the willingdon markets (ch vi), paralleling the treaty of rapallo; and frederick's forged bank notes, parallelling the hitler-stalin pact of august 1939, after which frederick attacks animal farm without warning and destroys the windmill. the book's close, with the pigs and men in a kind of rapprochement, reflected orwell's view of the 1943 tehran conference that seemed to display the establishment of "the best possible relations between the ussr and the west" but in reality were destined, as orwell presciently predicted,

to continue to unravel. the disagreement between the allies and the start of the cold war is suggested when napoleon and pilkington, both suspicious, each "played an ace of spades simultaneously". similarly, the music in the novel, starting with "beasts of england" and the later anthems, parallels "the internationale" and its adoption and repudiation by the soviet authorities as the anthem of the ussr in the 1920s and 1930s. adaptations stage productions in 2021, the national youth theatre toured a stage version of animal farm. a solo version, adapted and performed by guy masterson, premiered at the traverse theatre edinburgh in january 1995 and has toured worldwide since. a theatrical version, with music by richard peaslee and lyrics by adrian mitchell, was staged at the national theatre london on 25 april

1984, directed by peter hall. it toured nine cities in 1985. a new adaptation written and directed by robert icke, designed by bunny christie with puppetry designed and directed by toby olie opened at the birmingham repertory theatre in january 2022 before touring the uk. films animal farm has been adapted to film twice. both differ from the novel and have been accused of taking significant liberties, including sanitising some aspects. animal farm (1954) is an animated film, in which napoleon is eventually overthrown in a second revolution. in 1974, e. howard hunt revealed that he had been sent by the cia's psychological warfare department to obtain the film rights from orwell's widow, and the resulting 1954 animation was funded by the agency. animal farm (1999) is a live-action tv version that shows napoleon's

regime collapsing in on itself, with the farm having new human owners, reflecting the collapse of soviet communism. andy serkis is directing a film adaptation for netflix, with matt reeves producing. serkis began work on the film after finishing directing duties for venom: let there be carnage. radio dramatisations a bbc radio version, produced by rayner heppenstall, was broadcast in january 1947. orwell listened to the production at his home in canonbury square, london, with hugh gordon porteous, amongst others. orwell later wrote to heppenstall that porteous, "who had not read the book, grasped what was happening after a few minutes." a further radio production, again using orwell's own dramatisation of the book, was broadcast in january 2013 on bbc radio 4. tamsin greig narrated, and the cast included nicky

henson as napoleon, toby jones as the propagandist squealer, and ralph ineson as boxer. comic strip in 1950, norman pett and his writing partner don freeman were secretly hired by the information research department (ird), a secret wing of the british foreign office, to adapt animal farm into a comic strip. this comic was not published in the u.k. but ran in brazilian and burmese newspapers. see also information research department authoritarian personality history of soviet russia and the soviet union (19171927) history of the soviet union (19271953) ideocracy new class anthems in animal farm animals, an album based on animal farm books gulliver's travels was a favourite book of orwell's. swift reverses the role of horses and human beings in the fourth book. orwell brought to animal farm "a dose

of swiftian misanthropy, looking ahead to a time 'when the human race had finally been overthrown.'" bunt (revolt), published in 1924, is a book by polish nobel laureate wadysaw reymont with a theme similar to animal farms. white acre vs. black acre, published in 1856 and written by william m. burwell, is a satirical novel that features allegories for slavery in the united states similar to animal farms portrayal of soviet history. george orwell's own nineteen eighty-four, a classic dystopian novel about totalitarianism. references explanatory notes citations general sources further reading o'neill, terry, readings on animal farm (1998), greenhaven press. . external links animal farm book notes from literapedia excerpts from orwell's letters to his agent concerning animal farm literary journal

review orwell's original preface to the book animal farm revisited by john molyneux, international socialism, 44 (1989) animal farm at the british library animal farm (1954) 1945 british novels allegory british novellas british novels adapted into films british novels adapted into plays british novels adapted into television shows british political novels british satirical novels cats in literature cattle in literature censored books dogs in literature dystopian novels english novels hugo award for best novella winning works novels about animals novels about propaganda novels about revolutionaries novels about totalitarianism novels adapted into comics novels adapted into radio programs novels by george orwell pigs in literature political literature roman a clef novels satirical novels secker & warburg

amphibians are ectothermic, tetrapod vertebrates of the class amphibia. all living amphibians belong to the group lissamphibia. they inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arboreal or freshwater aquatic ecosystems. thus amphibians typically start out as larvae living in water, but some species have developed behavioural adaptations to bypass this. the young generally undergo metamorphosis from larva with gills to an adult air-breathing form with lungs. amphibians use their skin as a secondary respiratory surface and some small terrestrial salamanders and frogs lack lungs and rely entirely on their skin. they are superficially similar to lizards but, along with mammals and birds, reptiles are amniotes and do not require water bodies in which to breed. with their

complex reproductive needs and permeable skins, amphibians are often ecological indicators; in recent decades there has been a dramatic decline in amphibian populations for many species around the globe. the earliest amphibians ("crown") evolved in the carboniferous period from sarcopterygian fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land. they diversified and became dominant during the carboniferous and permian periods, but were later displaced by reptiles and other vertebrates. over time, amphibians shrank in size and decreased in diversity, leaving only the modern subclass lissamphibia. the three modern orders of amphibians are anura (the frogs), urodela (the salamanders), and apoda (the caecilians). the number of known amphibian species is approximately 8,000, of

which nearly 90% are frogs. the smallest amphibian (and vertebrate) in the world is a frog from new guinea (paedophryne amauensis) with a length of just . the largest living amphibian is the south china giant salamander (andrias sligoi), but this is dwarfed by the extinct prionosuchus from the middle permian of brazil. the study of amphibians is called batrachology, while the study of both reptiles and amphibians is called herpetology. classification the word amphibian is derived from the ancient greek term (), which means 'both kinds of life', meaning 'of both kinds' and meaning 'life'. the term was initially used as a general adjective for animals that could live on land or in water, including seals and otters. traditionally, the class amphibia includes all tetrapod vertebrates that are not amniotes. amphibia

in its widest sense () was divided into three subclasses, two of which are extinct: subclass lepospondyli (small paleozoic group, which are more closely related to amniotes than lissamphibia) subclass temnospondyli (diverse paleozoic and early mesozoic grade) subclass lissamphibia (all modern amphibians, including frogs, toads, salamanders, newts and caecilians) salientia (frogs, toads and relatives): jurassic to present7,360 current species in 53 families caudata (salamanders, newts and relatives): jurassic to present764 current species in 9 families gymnophiona (caecilians and relatives): jurassic to present215 current species in 10 families allocaudata (albanerpetontidae) middle jurassic - early pleistocene the actual number of species in each group depends on the taxonomic classification followed. the

two most common systems are the classification adopted by the website amphibiaweb, university of california, berkeley and the classification by herpetologist darrel frost and the american museum of natural history, available as the online reference database "amphibian species of the world". the numbers of species cited above follows frost and the total number of known amphibian species as of march 31, 2019 is exactly 8,000, of which nearly 90% are frogs. with the phylogenetic classification, the taxon labyrinthodontia has been discarded as it is a polyparaphyletic group without unique defining features apart from shared primitive characteristics. classification varies according to the preferred phylogeny of the author and whether they use a stem-based or a node-based classification. traditionally, amphibians

as a class are defined as all tetrapods with a larval stage, while the group that includes the common ancestors of all living amphibians (frogs, salamanders and caecilians) and all their descendants is called lissamphibia. the phylogeny of paleozoic amphibians is uncertain, and lissamphibia may possibly fall within extinct groups, like the temnospondyli (traditionally placed in the subclass labyrinthodontia) or the lepospondyli, and in some analyses even in the amniotes. this means that advocates of phylogenetic nomenclature have removed a large number of basal devonian and carboniferous amphibian-type tetrapod groups that were formerly placed in amphibia in linnaean taxonomy, and included them elsewhere under cladistic taxonomy. if the common ancestor of amphibians and amniotes is included in amphibia, it becomes

a paraphyletic group. all modern amphibians are included in the subclass lissamphibia, which is usually considered a clade, a group of species that have evolved from a common ancestor. the three modern orders are anura (the frogs), caudata (or urodela, the salamanders), and gymnophiona (or apoda, the caecilians). it has been suggested that salamanders arose separately from a temnospondyl-like ancestor, and even that caecilians are the sister group of the advanced reptiliomorph amphibians, and thus of amniotes. although the fossils of several older proto-frogs with primitive characteristics are known, the oldest "true frog" is prosalirus bitis, from the early jurassic kayenta formation of arizona. it is anatomically very similar to modern frogs. the oldest known caecilian is another early jurassic species, eocaecilia

micropodia, also from arizona. the earliest salamander is beiyanerpeton jianpingensis from the late jurassic of northeastern china. authorities disagree as to whether salientia is a superorder that includes the order anura, or whether anura is a sub-order of the order salientia. the lissamphibia are traditionally divided into three orders, but an extinct salamander-like family, the albanerpetontidae, is now considered part of lissamphibia alongside the superorder salientia. furthermore, salientia includes all three recent orders plus the triassic proto-frog, triadobatrachus. evolutionary history the first major groups of amphibians ("stem") developed in the devonian period, around 370 million years ago, from lobe-finned fish which were similar to the modern coelacanth and lungfish. these ancient lobe-finned

fish had evolved multi-jointed leg-like fins with digits that enabled them to crawl along the sea bottom. some fish had developed primitive lungs that help them breathe air when the stagnant pools of the devonian swamps were low in oxygen. they could also use their strong fins to hoist themselves out of the water and onto dry land if circumstances so required. eventually, their bony fins would evolve into limbs and they would become the ancestors to all tetrapods, including modern amphibians, reptiles, birds, and mammals. despite being able to crawl on land, many of these prehistoric tetrapodomorph fish still spent most of their time in the water. they had started to develop lungs, but still breathed predominantly with gills. many examples of species showing transitional features have been discovered. ichthyostega

was one of the first primitive amphibians, with nostrils and more efficient lungs. it had four sturdy limbs, a neck, a tail with fins and a skull very similar to that of the lobe-finned fish, eusthenopteron. amphibians evolved adaptations that allowed them to stay out of the water for longer periods. their lungs improved and their skeletons became heavier and stronger, better able to support the weight of their bodies on land. they developed "hands" and "feet" with five or more digits; the skin became more capable of retaining body fluids and resisting desiccation. the fish's hyomandibula bone in the hyoid region behind the gills diminished in size and became the stapes of the amphibian ear, an adaptation necessary for hearing on dry land. an affinity between the amphibians and the teleost fish is the multi-folded

structure of the teeth and the paired supra-occipital bones at the back of the head, neither of these features being found elsewhere in the animal kingdom. at the end of the devonian period (360 million years ago), the seas, rivers and lakes were teeming with life while the land was the realm of early plants and devoid of vertebrates, though some, such as ichthyostega, may have sometimes hauled themselves out of the water. it is thought they may have propelled themselves with their forelimbs, dragging their hindquarters in a similar manner to that used by the elephant seal. in the early carboniferous (360 to 345 million years ago), the climate became wet and warm. extensive swamps developed with mosses, ferns, horsetails and calamites. air-breathing arthropods evolved and invaded the land where they provided

food for the carnivorous amphibians that began to adapt to the terrestrial environment. there were no other tetrapods on the land and the amphibians were at the top of the food chain, occupying the ecological position currently held by the crocodile. though equipped with limbs and the ability to breathe air, most still had a long tapering body and strong tail. they were the top land predators, sometimes reaching several metres in length, preying on the large insects of the period and the many types of fish in the water. they still needed to return to water to lay their shell-less eggs, and even most modern amphibians have a fully aquatic larval stage with gills like their fish ancestors. it was the development of the amniotic egg, which prevents the developing embryo from drying out, that enabled the reptiles

to reproduce on land and which led to their dominance in the period that followed. after the carboniferous rainforest collapse amphibian dominance gave way to reptiles, and amphibians were further devastated by the permiantriassic extinction event. during the triassic period (250 to 200 million years ago), the reptiles continued to out-compete the amphibians, leading to a reduction in both the amphibians' size and their importance in the biosphere. according to the fossil record, lissamphibia, which includes all modern amphibians and is the only surviving lineage, may have branched off from the extinct groups temnospondyli and lepospondyli at some period between the late carboniferous and the early triassic. the relative scarcity of fossil evidence precludes precise dating, but the most recent molecular study,

based on multilocus sequence typing, suggests a late carboniferous/early permian origin for extant amphibians. the origins and evolutionary relationships between the three main groups of amphibians is a matter of debate. a 2005 molecular phylogeny, based on rdna analysis, suggests that salamanders and caecilians are more closely related to each other than they are to frogs. it also appears that the divergence of the three groups took place in the paleozoic or early mesozoic (around 250 million years ago), before the breakup of the supercontinent pangaea and soon after their divergence from the lobe-finned fish. the briefness of this period, and the swiftness with which radiation took place, would help account for the relative scarcity of primitive amphibian fossils. there are large gaps in the fossil record,

but the discovery of a gerobatrachus hottoni from the early permian in texas in 2008 provided a missing link with many of the characteristics of modern frogs. molecular analysis suggests that the frogsalamander divergence took place considerably earlier than the palaeontological evidence indicates. newer research indicates that the common ancestor of all lissamphibians lived about 315 million years ago, and that stereospondyls are the closest relatives to the caecilians. as they evolved from lunged fish, amphibians had to make certain adaptations for living on land, including the need to develop new means of locomotion. in the water, the sideways thrusts of their tails had propelled them forward, but on land, quite different mechanisms were required. their vertebral columns, limbs, limb girdles and musculature

needed to be strong enough to raise them off the ground for locomotion and feeding. terrestrial adults discarded their lateral line systems and adapted their sensory systems to receive stimuli via the medium of the air. they needed to develop new methods to regulate their body heat to cope with fluctuations in ambient temperature. they developed behaviours suitable for reproduction in a terrestrial environment. their skins were exposed to harmful ultraviolet rays that had previously been absorbed by the water. the skin changed to become more protective and prevent excessive water loss. characteristics the superclass tetrapoda is divided into four classes of vertebrate animals with four limbs. reptiles, birds and mammals are amniotes, the eggs of which are either laid or carried by the female and are surrounded

by several membranes, some of which are impervious. lacking these membranes, amphibians require water bodies for reproduction, although some species have developed various strategies for protecting or bypassing the vulnerable aquatic larval stage. they are not found in the sea with the exception of one or two frogs that live in brackish water in mangrove swamps; the anderson's salamander meanwhile occurs in brackish or salt water lakes. on land, amphibians are restricted to moist habitats because of the need to keep their skin damp. modern amphibians have a simplified anatomy compared to their ancestors due to paedomorphosis, caused by two evolutionary trends: miniaturization and an unusually large genome, which result in a slower growth and development rate compared to other vertebrates. another reason for

their size is associated with their rapid metamorphosis, which seems to have evolved only in the ancestors of lissamphibia; in all other known lines the development was much more gradual. because a remodeling of the feeding apparatus means they don't eat during the metamorphosis, the metamorphosis has to go faster the smaller the individual is, so it happens at an early stage when the larvae are still small. (the largest species of salamanders don't go through a metamorphosis.) amphibians that lay eggs on land often go through the whole metamorphosis inside the egg. an anamniotic terrestrial egg is less than 1cm in diameter due to diffusion problems, a size which puts a limit on the amount of posthatching growth. the smallest amphibian (and vertebrate) in the world is a microhylid frog from new guinea (paedophryne

amauensis) first discovered in 2012. it has an average length of and is part of a genus that contains four of the world's ten smallest frog species. the largest living amphibian is the chinese giant salamander (andrias davidianus) but this is a great deal smaller than the largest amphibian that ever existedthe extinct prionosuchus, a crocodile-like temnospondyl dating to 270 million years ago from the middle permian of brazil. the largest frog is the african goliath frog (conraua goliath), which can reach and weigh . amphibians are ectothermic (cold-blooded) vertebrates that do not maintain their body temperature through internal physiological processes. their metabolic rate is low and as a result, their food and energy requirements are limited. in the adult state, they have tear ducts and movable eyelids, and

most species have ears that can detect airborne or ground vibrations. they have muscular tongues, which in many species can be protruded. modern amphibians have fully ossified vertebrae with articular processes. their ribs are usually short and may be fused to the vertebrae. their skulls are mostly broad and short, and are often incompletely ossified. their skin contains little keratin and lacks scales, apart from a few fish-like scales in certain caecilians. the skin contains many mucous glands and in some species, poison glands (a type of granular gland). the hearts of amphibians have three chambers, two atria and one ventricle. they have a urinary bladder and nitrogenous waste products are excreted primarily as urea. most amphibians lay their eggs in water and have aquatic larvae that undergo metamorphosis

to become terrestrial adults. amphibians breathe by means of a pump action in which air is first drawn into the buccopharyngeal region through the nostrils. these are then closed and the air is forced into the lungs by contraction of the throat. they supplement this with gas exchange through the skin. anura the order anura (from the ancient greek a(n)- meaning "without" and oura meaning "tail") comprises the frogs and toads. they usually have long hind limbs that fold underneath them, shorter forelimbs, webbed toes with no claws, no tails, large eyes and glandular moist skin. members of this order with smooth skins are commonly referred to as frogs, while those with warty skins are known as toads. the difference is not a formal one taxonomically and there are numerous exceptions to this rule. members of the

family bufonidae are known as the "true toads". frogs range in size from the goliath frog (conraua goliath) of west africa to the paedophryne amauensis, first described in papua new guinea in 2012, which is also the smallest known vertebrate. although most species are associated with water and damp habitats, some are specialised to live in trees or in deserts. they are found worldwide except for polar areas. anura is divided into three suborders that are broadly accepted by the scientific community, but the relationships between some families remain unclear. future molecular studies should provide further insights into their evolutionary relationships. the suborder archaeobatrachia contains four families of primitive frogs. these are ascaphidae, bombinatoridae, discoglossidae and leiopelmatidae which have few

derived features and are probably paraphyletic with regard to other frog lineages. the six families in the more evolutionarily advanced suborder mesobatrachia are the fossorial megophryidae, pelobatidae, pelodytidae, scaphiopodidae and rhinophrynidae and the obligatorily aquatic pipidae. these have certain characteristics that are intermediate between the two other suborders. neobatrachia is by far the largest suborder and includes the remaining families of modern frogs, including most common species. ninety-six percent of the over 5,000 extant species of frog are neobatrachians. caudata the order caudata (from the latin cauda meaning "tail") consists of the salamanderselongated, low-slung animals that mostly resemble lizards in form. this is a symplesiomorphic trait and they are no more closely related to

lizards than they are to mammals. salamanders lack claws, have scale-free skins, either smooth or covered with tubercles, and tails that are usually flattened from side to side and often finned. they range in size from the chinese giant salamander (andrias davidianus), which has been reported to grow to a length of , to the diminutive thorius pennatulus from mexico which seldom exceeds in length. salamanders have a mostly laurasian distribution, being present in much of the holarctic region of the northern hemisphere. the family plethodontidae is also found in central america and south america north of the amazon basin; south america was apparently invaded from central america by about the start of the miocene, 23 million years ago. urodela is a name sometimes used for all the extant species of salamanders. members

of several salamander families have become paedomorphic and either fail to complete their metamorphosis or retain some larval characteristics as adults. most salamanders are under long. they may be terrestrial or aquatic and many spend part of the year in each habitat. when on land, they mostly spend the day hidden under stones or logs or in dense vegetation, emerging in the evening and night to forage for worms, insects and other invertebrates. the suborder cryptobranchoidea contains the primitive salamanders. a number of fossil cryptobranchids have been found, but there are only three living species, the chinese giant salamander (andrias davidianus), the japanese giant salamander (andrias japonicus) and the hellbender (cryptobranchus alleganiensis) from north america. these large amphibians retain several

larval characteristics in their adult state; gills slits are present and the eyes are unlidded. a unique feature is their ability to feed by suction, depressing either the left side of their lower jaw or the right. the males excavate nests, persuade females to lay their egg strings inside them, and guard them. as well as breathing with lungs, they respire through the many folds in their thin skin, which has capillaries close to the surface. the suborder salamandroidea contains the advanced salamanders. they differ from the cryptobranchids by having fused prearticular bones in the lower jaw, and by using internal fertilisation. in salamandrids, the male deposits a bundle of sperm, the spermatophore, and the female picks it up and inserts it into her cloaca where the sperm is stored until the eggs are laid. the

largest family in this group is plethodontidae, the lungless salamanders, which includes 60% of all salamander species. the family salamandridae includes the true salamanders and the name "newt" is given to members of its subfamily pleurodelinae. the third suborder, sirenoidea, contains the four species of sirens, which are in a single family, sirenidae. members of this order are eel-like aquatic salamanders with much reduced forelimbs and no hind limbs. some of their features are primitive while others are derived. fertilisation is likely to be external as sirenids lack the cloacal glands used by male salamandrids to produce spermatophores and the females lack spermathecae for sperm storage. despite this, the eggs are laid singly, a behaviour not conducive for external fertilisation. gymnophiona the order

gymnophiona (from the greek gymnos meaning "naked" and ophis meaning "serpent") or apoda comprises the caecilians. these are long, cylindrical, limbless animals with a snake- or worm-like form. the adults vary in length from 8 to 75 centimetres (3 to 30 inches) with the exception of thomson's caecilian (caecilia thompsoni), which can reach . a caecilian's skin has a large number of transverse folds and in some species contains tiny embedded dermal scales. it has rudimentary eyes covered in skin, which are probably limited to discerning differences in light intensity. it also has a pair of short tentacles near the eye that can be extended and which have tactile and olfactory functions. most caecilians live underground in burrows in damp soil, in rotten wood and under plant debris, but some are aquatic. most species

lay their eggs underground and when the larvae hatch, they make their way to adjacent bodies of water. others brood their eggs and the larvae undergo metamorphosis before the eggs hatch. a few species give birth to live young, nourishing them with glandular secretions while they are in the oviduct. caecilians have a mostly gondwanan distribution, being found in tropical regions of africa, asia and central and south america. anatomy and physiology skin the integumentary structure contains some typical characteristics common to terrestrial vertebrates, such as the presence of highly cornified outer layers, renewed periodically through a moulting process controlled by the pituitary and thyroid glands. local thickenings (often called warts) are common, such as those found on toads. the outside of the skin is

shed periodically mostly in one piece, in contrast to mammals and birds where it is shed in flakes. amphibians often eat the sloughed skin. caecilians are unique among amphibians in having mineralized dermal scales embedded in the dermis between the furrows in the skin. the similarity of these to the scales of bony fish is largely superficial. lizards and some frogs have somewhat similar osteoderms forming bony deposits in the dermis, but this is an example of convergent evolution with similar structures having arisen independently in diverse vertebrate lineages. amphibian skin is permeable to water. gas exchange can take place through the skin (cutaneous respiration) and this allows adult amphibians to respire without rising to the surface of water and to hibernate at the bottom of ponds. to compensate for

their thin and delicate skin, amphibians have evolved mucous glands, principally on their heads, backs and tails. the secretions produced by these help keep the skin moist. in addition, most species of amphibian have granular glands that secrete distasteful or poisonous substances. some amphibian toxins can be lethal to humans while others have little effect. the main poison-producing glands, the parotoids, produce the neurotoxin bufotoxin and are located behind the ears of toads, along the backs of frogs, behind the eyes of salamanders and on the upper surface of caecilians. the skin colour of amphibians is produced by three layers of pigment cells called chromatophores. these three cell layers consist of the melanophores (occupying the deepest layer), the guanophores (forming an intermediate layer and containing

many granules, producing a blue-green colour) and the lipophores (yellow, the most superficial layer). the colour change displayed by many species is initiated by hormones secreted by the pituitary gland. unlike bony fish, there is no direct control of the pigment cells by the nervous system, and this results in the colour change taking place more slowly than happens in fish. a vividly coloured skin usually indicates that the species is toxic and is a warning sign to predators. skeletal system and locomotion amphibians have a skeletal system that is structurally homologous to other tetrapods, though with a number of variations. they all have four limbs except for the legless caecilians and a few species of salamander with reduced or no limbs. the bones are hollow and lightweight. the musculoskeletal system

is strong to enable it to support the head and body. the bones are fully ossified and the vertebrae interlock with each other by means of overlapping processes. the pectoral girdle is supported by muscle, and the well-developed pelvic girdle is attached to the backbone by a pair of sacral ribs. the ilium slopes forward and the body is held closer to the ground than is the case in mammals. in most amphibians, there are four digits on the fore foot and five on the hind foot, but no claws on either. some salamanders have fewer digits and the amphiumas are eel-like in appearance with tiny, stubby legs. the sirens are aquatic salamanders with stumpy forelimbs and no hind limbs. the caecilians are limbless. they burrow in the manner of earthworms with zones of muscle contractions moving along the body. on the surface

of the ground or in water they move by undulating their body from side to side. in frogs, the hind legs are larger than the fore legs, especially so in those species that principally move by jumping or swimming. in the walkers and runners the hind limbs are not so large, and the burrowers mostly have short limbs and broad bodies. the feet have adaptations for the way of life, with webbing between the toes for swimming, broad adhesive toe pads for climbing, and keratinised tubercles on the hind feet for digging (frogs usually dig backwards into the soil). in most salamanders, the limbs are short and more or less the same length and project at right angles from the body. locomotion on land is by walking and the tail often swings from side to side or is used as a prop, particularly when climbing. in their normal

gait, only one leg is advanced at a time in the manner adopted by their ancestors, the lobe-finned fish. some salamanders in the genus aneides and certain plethodontids climb trees and have long limbs, large toepads and prehensile tails. in aquatic salamanders and in frog tadpoles, the tail has dorsal and ventral fins and is moved from side to side as a means of propulsion. adult frogs do not have tails and caecilians have only very short ones. salamanders use their tails in defence and some are prepared to jettison them to save their lives in a process known as autotomy. certain species in the plethodontidae have a weak zone at the base of the tail and use this strategy readily. the tail often continues to twitch after separation which may distract the attacker and allow the salamander to escape. both tails