Biography.

Early life.

On 21 September 1951, Aslan Aliyevich Maskhadov was born in Karaganda Region of the Kazakh Soviet Socialist Republic (SSR) of the Soviet Union, in the small village of Shakai, during the mass deportation of the Chechen people ordered in 1944 by Joseph Stalin. His family was of the Alaroy teip. In 1957, his family returned to Chechnya where they settled in Zebir-Yurt, Nadterechny District.  Maskhadov joined the Soviet Army, trained in the neighbouring Georgian SSR and graduated from the Tbilisi Artillery School in 1972. He then graduated with honours from the Leningrad Kalinin Higher Artillery in 1981. He was posted to Hungary with a self-propelled artillery regiment until 1986 and then from 1986 in the Baltic Military District. He served from 1990 as the chief of staff of Soviet missile and artillery forces in Vilnius, capital of the Lithuanian SSR. In January 1991, Maskhadov participated in the January Events, the seizure of the television tower by Soviet troops (which he regretted later), but didn't participate in the assault itself. During his service in the Soviet Army, he was presented with two Orders For Service to Homeland. Maskhadov retired from the Soviet Army in 1992 with the rank of a colonel and returned to his native land. He was at the head of ChRI civil defence from late 1992 to November 1993.  After the dissolution of the Soviet Union, in the summer of 1993, Maskhadov took part in raids on the armed opposition against the government of Dzhokhar Dudayev in the Urus-Martan, Nadterechny, and Gudermes districts. An unsuccessful anti-Dudayev mutiny in November 1993 resulted in the dismissal of Viskhan Shakhabov as chief of staff of the Chechen armed forces, Maskhadov was appointed as the acting chief of staff and, in March 1994, as the chief of staff.

First Chechen War.

In December 1994, when the First Chechen War broke out, he was the senior military figure on the Chechen side during the war and was widely seen as being instrumental to the Chechen victory over the Russian forces. As the First Deputy Chairman of the ChRI State Defence Council (ChRI President Dudayev was the chairman) and the chief of staff, Maskhadov organised defence of the Chechen capital during the Battle of Grozny. Maskhadov commanded the city from the Presidential Palace in Grozny, where on one occasion a Russian bunker buster bomb landed 20 meters from him but failed to explode. In February 1995, Dudayev promoted Aslan to Divisional General.  Beginning in June 1995, Maskhadov took part in peace talks in Grozny to resolve the crisis in Chechnya. In June 1996, at the negotiations in Nazran, Ingushetia, Maskhadov, on behalf of the ChRI administration, signed the Protocol of the Commission's Meeting on Ceasefire and Measures to Resolve the Armed Conflict in the CRI. In August 1996, after Grozny's seizure by Chechen units he repeatedly held talks with Alexander Lebed and on 31 August 1996, the signing of the Khasav-Yurt Accord took place, a ceasefire agreement, and peace treaty which marked the end of the First Chechen War.

President of the Chechen Republic of Ichkeria.

On 17 October 1996, Maskhadov was appointed Prime Minister of Ichkeria, while he also remained Chief of Staff and Defence Minister. Maskhadov nominated himself for President of Ichkeria on 3 December 1996, for the January 1997 free democratic presidential and parliamentary elections held in Chechnya under the aegis of the OSCE, running primarily against Shamil Basayev and Zelimkhan Yandarbiyev.  The elections were conducted on the basis of the Chechen constitution adopted in March 1992, according to which the Chechen Republic was an independent state. Representatives of more than 20 countries, as well as the United Nations and the OSCE, attended the elections as observers.  Running with Vakha Arsanov, who became his vice president, Mashkadov won a majority of 60% of the votes and was congratulated by the Russian President Boris Yeltsin, who pledged to work towards rebuilding relations with Chechnya. Maskhadov was inaugurated on 12 February 1997, and at the same time he assumed the office of prime minister and abolished the office of Defence Minister he had occupied since late 1996. Maskhadov remained commander-in-chief of the republican armed forces. On 12 May 1997, Maskhadov then attained the apex of his political career when he signed a peace treaty with Yeltsin at the Kremlin.  By the end of 1996, when Maskhadov assumed his office, nearly half a million people (40% of Chechya prewar population) had been internally displaced and lived in refugee camps or overcrowded villages. The economy was destroyed and the warlords had no intention to disband their militias. Under such circumstances, Maskhadov's political fortunes began to wane. His political standing within Chechnya became increasingly insecure as he lost control to Basayev and other warlords. Even his Vice-President Arsanov became his political enemy. Just like in the years before the First Chechen War under Dudayev, the years of Chechen independence were notorious for organized crime, including kidnapping, leading to several public executions of criminals.  Maskhadov attempted with only limited success to curb the growth of Wahhabism and other fundamentalist Muslim groups supported by Basayev, producing a split in the Chechen separatist movement between Islamic fundamentalism and secular Chechen nationalists. In February 1999, as a concession to radical Islamists, Maskhadov introduced Islamic Sharia law. The Sharia courts that were established sentenced people to death, flogging, executing people for crimes such as adultery.  Maskhadov survived assassination attempts on his life three times, on 23 July 1998 and 21 March and 10 April in 1999, in which the attackers used anti-tank missiles and bombs. Russian secret services were officially blamed.

Second Chechen War.

In the summer of 1999, Maskhadov condemned an attempt by Basayev and Ibn Al-Khattab to spread war to the neighboring republic of Dagestan (known as the Invasion of Dagestan). This raid, and the Russian Apartment Bombings, were both blamed on the Chechen Republic of Ichkeria. On 1 October 1999, the then Russian Prime Minister Vladimir Putin declared the authority of President Maskhadov and his parliament illegitimate. Putin sent Russian forces into Chechnya, and his promise of a quick and decisive victory propelled him to the Russian Presidency.  On 11 October 1999, Maskhadov outlined a peace plan offering a crackdown on renegade warlords, the offer was rejected by the Russian side. In response, President Maskhadov declared a "gazavat" (holy war) to confront the approaching Russian army. Soon, martial law was declared in Ichkeria and reservists were called, and the building of his Presidential Palace was one of the supposed targets of the disastrous Grozny ballistic missile attack on 23 October 1999.  Maskhadov was one of the main commanders in the Battle of Grozny (1999–2000) along with Shamil Basayev, Ruslan Gelayev, Ibn Al-Khattab, Aslambek Ismailov and Khunkarpasha Israpilov. Maskhadov along with his men launched daring counter-attacks against the Russian troops while fighting in Grozny and also effectively used the sewer systems to attack Russian troops from behind. After a meeting with top rebel commanders, Maskhadov and others agreed to withdraw from Grozny and continue to attack Russian forces in the cities and towns surrounding the city. Maskhadov was the first to withdraw because of his importance to the rebel cause and because he was the official President of Chechnya. As Maskhadov and his men retreated, they set up a vast amount of booby traps and landmines to hinder Russian forces and make most of Grozny impassable.:  After Chechen forces' withdrawal from Grozny following another battle for the city, Maskhadov returned to a life of a guerrilla leader, living in hiding as Russia's second most wanted man after Basayev, with Russia placing a $10 million bounty on his capture. He was seen as the official political leader of the separatist forces during the war, but it is unclear what kind of a military role he played. Maskhadov offered his readiness for unconditional peace talks with Moscow several times in 2000 alone, continuing in the following years, but his appeals for a political solution were always ignored by the Russian side.  Maskhadov advocated armed resistance to what he saw as a Russian occupation of Chechnya but condemned attacks on civilians. He allegedly supported the assassination of pro-Russian Chechen President Akhmad Kadyrov in Chechnya, whilst condemning the Russian assassination of Chechen separatist ex-President Yandarbiyev in Qatar in 2004. Maskhadov often denied responsibility for the increasingly brutal terrorist acts against Russian civilians by Basayev's followers, continually issuing denunciations of such incidents through spokesmen abroad, such as Akhmed Zakayev in London. However, on 24 October 2002, radio communications were intercepted from Maskhadov's messages wherein he called for intensification of terrorist activities and sabotage in Russian territory. Evidence for Maskhadov's complicity in the 2002 Moscow theater hostage crisis was provided by its two principal perpetrators, Movsar Barayev and Abu Said. Although he initially denied responsibility for the 2004 Nazran raid, in which 98 police officers/troops were killed, in July 2004 Maskhadov publicly accepted responsibility for the attacks. In the same month, Maskahdov promised similar attacks would happen, and vowed that the winner of Chechnya's upcoming presidential election would be illegitimate and would be attacked if necessary. He described the rebels behind the Beslan school siege as "madmen" driven out of their senses by Russian acts of brutality and called the terrorist attack an atrocity.  On 15 January 2005, Maskhadov issued a special order to stop all military operations except those in self-defense, both inside and outside Chechnya, until the end of February (the date marking the anniversary of the Stalin's Vainakh deportations of 1944) as a gesture of good will, and again called for a negotiated end to the Chechen conflict. Umar Khambiev, his designated negotiator, said that the separatists were no longer seeking independence, but only "guarantees for the existence of the Chechen nation". This surprise, unilateral ceasefire was supported by Basayev but flatly rejected by the Russian and pro-Russian leaders who, once again, refused to negotiate. Maskhadov's order to temporarily cease all offensive actions was largely followed by the rebel movement, except in Dagestan.

Death.

On 8 March 2005, less than a month after Maskhadov announced the cease-fire, the head of the Federal Security Service of the Russian Federation (FSB) Nikolay Patrushev announced that special forces attached to the FSB had ""today carried out an operation in the settlement of Tolstoy-Yurt, as a result of which the international jihadist and leader of armed groups Maskhadov was killed, and his closest comrades-in-arms detained"". He said the special operations unit had wanted to take Maskhadov alive for interrogation, but claim that they killed him accidentally with a grenade thrown into a bunker where Maskhadov was hiding. Akhmed Zakayev, one of his closest allies who acted as his spokesman and foreign minister, told a Russian radio station that it was probable that Maskhadov had indeed been killed; he indicated later that a new Chechen leader could be chosen within days. Vladimir Putin awarded those responsible for the killings with medals. Shortly after Maskhadov's death, the Chechen rebel council announced that Abdul-Halim Sadulayev had assumed the leadership.  Four Chechens, Vakhit Murdashev, Viskhan Hadzhimuradov, Skanarbek Yusupov and Ilias Iriskhanov, were captured by the special operation. According to the ballistic evidence at their trial in the Supreme Court of the Chechen Republic, Maskhadov was killed by a shot from the pistol of Viskhan Hadzhimuradov, his nephew and bodyguard. Hadzhimuradov testified that he does not remember whether he shot Maskhadov or not since he was stunned by an explosion but after the capture Hadzhimuradov reportedly said: "My uncle always told me to shoot him if he is wounded and his capture is imminent. He said that if he is taken prisoner, he would be mistreated like Saddam Hussein had been".

Unmarked grave.

On 24 April 2006, the General Procurator's Office of Russia officially refused to turn the remains of Aslan Maskhadov over to his relatives for burial. The refusal was described as legal: Maskhadov A. A., in connection with terrorism, was criminally responsible for many separate serious crimes on the territory of the Russian Federation. Taking this into account, it was decided to suppress Maskhadov's activities and Maskhadov was being pursued for our protection. The burial of such persons is carried out in accordance with the rules concerning the burial of those whose death was a result of the suppression of their terrorist actions, affirmed by the government of the Russian Federation on 20 March 2003, in Order No. 164. In this case, the body is not handed over for burial, and the location of the burial is not communicated.  Maskhadov's family has been campaigning for the release of his remains or a disclosure of what happened to his body.

Family life.

He married at the age of 17. His wife Kusama holds a graduate degree in teaching. They had two children: a son, Anzor, who took part in military action during the First Russian-Chechen War, and a daughter, Fatima.

Aslan (Khalid) Aliyevich Maskhadov (; ; 21 September 1951 – 8 March 2005) was a politician who served as the third President of the unrecognized Chechen Republic of Ichkeria.

Aslan Alijevič Maschadov, rusky Аслан Алиевич Масхадов (21. září 1951 – 8. března 2005) Tolstoj-Jurt, byl prezidentem Čečny a představitel čečenské opozice vůči Rusku. Byl zabit ruskou tajnou službou FSB při plánované operaci. Ruská tajná služba jej dopadla za pomoci informací z místních zdrojů, od místních obyvatel a také díky informaci od zajatého povstalce.


History.

Prague–Ruzyně Airport began operations on 5 April 1937, but Czechoslovak civil aviation history started at the military airport in Prague–Kbely in 1919. The Prague Aviation Museum is now found at Kbely Airport.  Due to insufficient capacity of Kbely Airport by the mid-1930s, the government decided to develop a new state civil airport in Ruzyně. One of the major awards Prague Ruzyně Airport received include Diploma and Gold Medal granted in 1937 at the occasion of the International Art and Technical Exhibition in Paris ("Exposition Internationale des Arts et Techniques dans la Vie Moderne" also known as "Paris 1937 World's Fair") for the technical conception of the central airport, primarily the architecture of the check-in building (nowadays known as Terminal 4) designed by architect Adolf Benš.  In one of the most dramatic moments in its history, the airport was seized by Soviet paratroopers on the night of 20–21 August 1968, who then facilitated the landing of Soviet troops and transports for the invasion of Czechoslovakia.  Moreover, the Ruzyně fields provide opportunities for further expansion of the airport according to the increasing capacity demand. The airport serves as a hub of the trans-European airport network.  The political and economic changes affected the seventy years of existence of Prague–Ruzyně Airport. Some new air transportation companies and institutions were founded and some ceased operation since then. Ten entities have been responsible for airport administration over time, including the new construction and development. Until the 1990s, there were two or three-decade gaps before the major modernisation of Prague–Ruzyně Airport began to match the current capacity requirements.  The airport stood in for Miami International Airport in the 2006 James Bond film Casino Royale.  An online petition organised by one of the best-known Slovak film directors, Fero Fenič, calling on the government and the Parliament to rename Prague Ruzyně Airport to Václav Havel International Airport attracted – in just one week after 20 December 2011 – the support of over 65,000 signatories both within and outside the Czech Republic. A rendition of the airport with the proposed Václav Havel name in the form of his signature followed by his typical heart symbol suffix was included in the blog's article in support of renaming of the airport. This name change took place on 5 October 2012 on what would have been Havel's 76th birthday. However, the PRG name of the airport for IATA and ICAO will remain the same.

Further development.

As the capacity of the airport has been reaching its limit for the last couple of years (as of 2005), further development of the airport is being considered. Besides regular repairs of the existing runways, Prague Airport () began the preparations for building a new runway, parallel to the 06/24 runway. The construction with estimated costs of CZK 5–7 billion was scheduled to begin in 2007, and the new runway marked 06R/24L (also called the BIS runway) was to be put into service in 2010. However, because of many legal problems and the protests of people who live close to the airport premises, the construction has not yet begun. Despite these problems, the project has support from the government, and is expected to be completed by the end of 2014.  It will be over long. Located about southeast of the present main runway, the 24L runway will be equipped with a category III ILS, allowing landing and taking off under bad weather conditions.  Prague Airport states that besides increasing the airport capacity, the new runway system will greatly reduce the noise level in some densely inhabited areas of Prague. This should be achieved by reorganising the air traffic space around the airport, and shifting the traffic corridors after putting the two parallel runways into service. The vision of heavy traffic raised many protests from the suburban communities directly surrounding the airport. On 6 November 2004, local referenda were held in two Prague suburbs – Nebušice and Přední Kopanina – giving official support to the local authorities for active opposition against the construction of the parallel runway.  The construction of a railway connection between the airport and Prague city centre is also in the planning stage. The track will be served by express trains with special fares, connecting non-stop the airport with the city centre, and local trains fully integrated into Prague integrated transit system.

General runway reconstruction.

The main runway 06/24 was reconstructed from 2012 - 2013 due to poor technical conditions. During reconstruction, runway 12/30 was the only usable runway as runway 04/22 is closed permanently. The runway reconstruction was originally planned for three stages. The first stage in 2012, the second stage in 2013 and the last stage in 2014. However, runway 12/30 (which would be used during the reconstruction of the main runway) is not equipped for low visibility landings as it offers only ILS CAT I landings. In addition, the approach path of runway 12/30 goes above high-density population areas (such as Prague 6 and Kladno). Therefore, the second and the third stage of the runway reconstruction had to be merged so the works could be finished in 2013.

Infrastructure.

Terminals.

Prague Airport has two main passenger terminals, two general aviation terminals, as well as a cargo facility. Most flights depart Prague Airport from the North Terminals (Terminal 1 and 2). The South Terminals (Terminal 3 and 4) handle a few irregular flights, as well as VIP flights, special flights and small aircraft.  There are also two freight terminals, "Cargo Terminal 1" is operated by Menzies Aviation Czech while "Cargo Terminal 2" is operated by Skyport.

Runways.

The airport contains two runways in service: 06/24 (till April 1993 07/25) and 12/30 (till May 2012 13/31). Former runway 04/22 is permanently closed for take-offs and landings and is used for taxiing and parking only. The most used runway is 24 due to the prevailing western winds. Runway 30 is also used often. Runway 06 is used rarely, while runway 12 is used only exceptionally.

Operations.

The company operating the airport is Prague Airport ("Letiště Praha, a. s."), a joint-stock company that has one shareholder, the Ministry of Finance. The company was founded in February 2008, as part of a privatisation process involving the Airport Prague ("Správa Letiště Praha, s.p.") state enterprise. This action was in accordance with the Czech Republic Government Memorandum Nr. 888, which had been passed on 9 July 2008. On 1 December 2008, Prague Airport took all rights and duties formerly held by Správa Letiště Praha, s.p., and Prague Airports took all business authorisations, certificates, employees, and licenses from the former company. The head office of Prague Airport is in Prague 6. The former state-owned enterprise had its head office on the airport property.

Statistics.

Annual passenger numbers.

It was the 35th busiest airport in Europe in 2019 and the second busiest (after Warsaw Chopin Airport) in the newer EU member states.

Busiest routes.

The top 15 destinations by passengers handled in 2019 were:

Other facilities.

Czech Airlines has its head office, the APC Building, on the grounds of Prague Airport. On 30 December 2009 CSA announced that it will sell its head office to the airport for CZK 607 million.  Smartwings have their head office on the airport property.  In addition the Civil Aviation Authority also has its head office on the airport property.  There are a number of drinking water fountains across the airport departure halls for travellers to refill their reusable water bottles.

Ground transportation.

Buses of DPP, the Prague Public Transit Co., stop at both terminals 1 and 2 frequently.  A Czech Railways public bus service, AE – AirportExpress, connects Terminal 1 with Praha hlavní nádraží.  From bus station in front of Terminal 1 there are also regular buses to Kladno, intercity buses of Regiojet run every 30–60 minutes to Karlovy Vary and Cheb.  There are plans to build a rail connection to the airport. Preliminary work commenced in 2018, with procurement proceedings launched the following year. Main construction is likely to start around 2023.

Václav Havel Airport Prague (), formerly "Prague Ruzyně International Airport" (, ), is the international airport of Prague, the capital of the Czech Republic. The airport was founded in 1937, when it replaced the Kbely Airport (founded in 1918). It was reconstructed and extended in 1956, 1968, 1997, and 2006. In 2012, it was renamed after the last president of Czechoslovakia and the first President of Czech Republic Václav Havel. It is located in the edge of Prague-Ruzyně area, next to Kněževes village, west of the centre of Prague and southeast of the city Kladno.

Mezinárodní Letiště Václava Havla Praha neboli Praha/Ruzyně (IATA: PRG, ICAO: LKPR) je veřejné mezinárodní letiště umístěné na severozápadním okraji Prahy, v městské části Praha 6, katastrálním území Ruzyně, u Kněževsi. Letiště je určeno pro mezinárodní i vnitrostátní, pravidelný i nepravidelný letecký provoz. Leteckou základnu zde mají České aerolinie, Smartwings a Ryanair. Provozovatelem letiště je akciová společnost Letiště Praha, a. s., v jejímž čele stojí čtyřčlenné představenstvo, jehož předsedou je Václav Řehoř.


History.

Early development.

Soon after the dawn of modern computers in the late 1940s – early 1950s, researchers started realizing the immense potential these machines had for modern society. One of the first challenges was to make such machine capable of “thinking” like humans. In particular, making these machines capable of making important decisions the way humans do. The medical / healthcare field presented the tantalizing challenge to enable these machines to make medical diagnostic decisions.  Thus, in the late 1950s, right after the information age had fully arrived, researchers started experimenting with the prospect of using computer technology to emulate human decision-making. For example, biomedical researchers started creating computer-aided systems for diagnostic applications in medicine and biology. These early diagnostic systems used patients’ symptoms and laboratory test results as inputs to generate a diagnostic outcome. These systems were often described as the early forms of expert systems. However, researchers had realized that there were significant limitations when using traditional methods such as flow-charts  statistical pattern-matching, or probability theory.

Formal introduction & later developments.

This previous situation gradually led to the development of expert systems, which used knowledge-based approaches. These expert systems in medicine were the MYCIN expert system, the INTERNIST-I expert system and later, in the middle of the 1980s, the CADUCEUS.  Expert systems were formally introduced around 1965 by the Stanford Heuristic Programming Project led by Edward Feigenbaum, who is sometimes termed the "father of expert systems"; other key early contributors were Bruce Buchanan and Randall Davis. The Stanford researchers tried to identify domains where expertise was highly valued and complex, such as diagnosing infectious diseases (Mycin) and identifying unknown organic molecules (Dendral). The idea that "intelligent systems derive their power from the knowledge they possess rather than from the specific formalisms and inference schemes they use" – as Feigenbaum said – was at the time a significant step forward, since the past research had been focused on heuristic computational methods, culminating in attempts to develop very general-purpose problem solvers (foremostly the conjunct work of Allen Newell and Herbert Simon). Expert systems became some of the first truly successful forms of artificial intelligence (AI) software.  Research on expert systems was also active in France. While in the US the focus tended to be on rules-based systems, first on systems hard coded on top of LISP programming environments and then on expert system shells developed by vendors such as Intellicorp, in France research focused more on systems developed in Prolog. The advantage of expert system shells was that they were somewhat easier for nonprogrammers to use. The advantage of Prolog environments was that they were not focused only on "if-then" rules; Prolog environments provided a much better realization of a complete first order logic environment.  In the 1980s, expert systems proliferated. Universities offered expert system courses and two thirds of the Fortune 500 companies applied the technology in daily business activities. Interest was international with the Fifth Generation Computer Systems project in Japan and increased research funding in Europe.  In 1981, the first IBM PC, with the PC DOS operating system, was introduced. The imbalance between the high affordability of the relatively powerful chips in the PC, compared to the much more expensive cost of processing power in the mainframes that dominated the corporate IT world at the time, created a new type of architecture for corporate computing, termed the client-server model. Calculations and reasoning could be performed at a fraction of the price of a mainframe using a PC. This model also enabled business units to bypass corporate IT departments and directly build their own applications. As a result, client server had a tremendous impact on the expert systems market. Expert systems were already outliers in much of the business world, requiring new skills that many IT departments did not have and were not eager to develop. They were a natural fit for new PC-based shells that promised to put application development into the hands of end users and experts. Until then, the main development environment for expert systems had been high end Lisp machines from Xerox, Symbolics, and Texas Instruments. With the rise of the PC and client server computing, vendors such as Intellicorp and Inference Corporation shifted their priorities to developing PC based tools. Also, new vendors, often financed by venture capital (such as Aion Corporation, Neuron Data, Exsys, and many others), started appearing regularly.  The first expert system to be used in a design capacity for a large-scale product was the SID (Synthesis of Integral Design) software program, developed in 1982. Written in LISP, SID generated 93% of the VAX 9000 CPU logic gates. Input to the software was a set of rules created by several expert logic designers. SID expanded the rules and generated software logic synthesis routines many times the size of the rules themselves. Surprisingly, the combination of these rules resulted in an overall design that exceeded the capabilities of the experts themselves, and in many cases out-performed the human counterparts. While some rules contradicted others, top-level control parameters for speed and area provided the tie-breaker. The program was highly controversial, but used nevertheless due to project budget constraints. It was terminated by logic designers after the VAX 9000 project completion.  During the years before the middle of the 1970s, the expectations of what expert systems can accomplish in many fields tended to be extremely optimistic. At the beginning of these early studies, researchers were hoping to develop entirely automatic (i.e., completely computerized) expert systems. The expectations of people of what computers can do were frequently too idealistic. This situation radically changed after Richard M. Karp published his breakthrough paper: “Reducibility among Combinatorial Problems” in the early 1970s. Thanks to Karp's work it became clear that there are certain limitations and possibilities when one designs computer algorithms. His findings describe what computers can do and what they cannot do. Many of the computational problems related to this type of expert systems have certain pragmatic limitations. These findings laid down the groundwork that led to the next developments in the field.  In the 1990s and beyond, the term "expert system" and the idea of a standalone AI system mostly dropped from the IT lexicon. There are two interpretations of this. One is that "expert systems failed": the IT world moved on because expert systems did not deliver on their over hyped promise. The other is the mirror opposite, that expert systems were simply victims of their success: as IT professionals grasped concepts such as rule engines, such tools migrated from being standalone tools for developing special purpose "expert" systems, to being one of many standard tools. Many of the leading major business application suite vendors (such as SAP, Siebel, and Oracle) integrated expert system abilities into their suite of products as a way of specifying business logic – rule engines are no longer simply for defining the rules an expert would use but for any type of complex, volatile, and critical business logic; they often go hand in hand with business process automation and integration environments.

Current approaches to expert systems.

The limitations of the previous type of expert systems have urged researchers to develop new types of approaches. They have developed more efficient, flexible and powerful approaches in order to emulate the human decision-making process. Some of the approaches that researchers have developed are based on new methods of artificial intelligence (AI), and in particular in machine learning and data mining approaches with a feedback mechanism. Related is the discussion on the disadvantages section.  Modern systems can incorporate new knowledge more easily and thus update themselves easily. Such systems can generalize from existing knowledge better and deal with vast amounts of complex data. Related is the subject of big data here. Sometimes these type of expert systems are called “intelligent systems.”

Software architecture.

An expert system is an example of a knowledge-based system. Expert systems were the first commercial systems to use a knowledge-based architecture. A knowledge-based system is essentially composed of two sub-systems: the knowledge base and the inference engine.  The knowledge base represents facts about the world. In early expert systems such as Mycin and Dendral, these facts were represented mainly as flat assertions about variables. In later expert systems developed with commercial shells, the knowledge base took on more structure and used concepts from object-oriented programming. The world was represented as classes, subclasses, and instances and assertions were replaced by values of object instances. The rules worked by querying and asserting values of the objects.  The inference engine is an automated reasoning system that evaluates the current state of the knowledge-base, applies relevant rules, and then asserts new knowledge into the knowledge base. The inference engine may also include abilities for explanation, so that it can explain to a user the chain of reasoning used to arrive at a particular conclusion by tracing back over the firing of rules that resulted in the assertion.  There are mainly two modes for an inference engine: forward chaining and backward chaining. The different approaches are dictated by whether the inference engine is being driven by the antecedent (left hand side) or the consequent (right hand side) of the rule. In forward chaining an antecedent fires and asserts the consequent. For example, consider the following rule:  formula_1  A simple example of forward chaining would be to assert Man(Socrates) to the system and then trigger the inference engine. It would match R1 and assert Mortal(Socrates) into the knowledge base.  Backward chaining is a bit less straight forward. In backward chaining the system looks at possible conclusions and works backward to see if they might be true. So if the system was trying to determine if Mortal(Socrates) is true it would find R1 and query the knowledge base to see if Man(Socrates) is true. One of the early innovations of expert systems shells was to integrate inference engines with a user interface. This could be especially powerful with backward chaining. If the system needs to know a particular fact but does not, then it can simply generate an input screen and ask the user if the information is known. So in this example, it could use R1 to ask the user if Socrates was a Man and then use that new information accordingly.  The use of rules to explicitly represent knowledge also enabled explanation abilities. In the simple example above if the system had used R1 to assert that Socrates was Mortal and a user wished to understand why Socrates was mortal they could query the system and the system would look back at the rules which fired to cause the assertion and present those rules to the user as an explanation. In English if the user asked "Why is Socrates Mortal?" the system would reply "Because all men are mortal and Socrates is a man". A significant area for research was the generation of explanations from the knowledge base in natural English rather than simply by showing the more formal but less intuitive rules.  As expert systems evolved, many new techniques were incorporated into various types of inference engines. Some of the most important of these were:

Advantages.

The goal of knowledge-based systems is to make the critical information required for the system to work explicit rather than implicit. In a traditional computer program the logic is embedded in code that can typically only be reviewed by an IT specialist. With an expert system the goal was to specify the rules in a format that was intuitive and easily understood, reviewed, and even edited by domain experts rather than IT experts. The benefits of this explicit knowledge representation were rapid development and ease of maintenance.  Ease of maintenance is the most obvious benefit. This was achieved in two ways. First, by removing the need to write conventional code, many of the normal problems that can be caused by even small changes to a system could be avoided with expert systems. Essentially, the logical flow of the program (at least at the highest level) was simply a given for the system, simply invoke the inference engine. This also was a reason for the second benefit: rapid prototyping. With an expert system shell it was possible to enter a few rules and have a prototype developed in days rather than the months or year typically associated with complex IT projects.  A claim for expert system shells that was often made was that they removed the need for trained programmers and that experts could develop systems themselves. In reality, this was seldom if ever true. While the rules for an expert system were more comprehensible than typical computer code, they still had a formal syntax where a misplaced comma or other character could cause havoc as with any other computer language. Also, as expert systems moved from prototypes in the lab to deployment in the business world, issues of integration and maintenance became far more critical. Inevitably demands to integrate with, and take advantage of, large legacy databases and systems arose. To accomplish this, integration required the same skills as any other type of system.

Disadvantages.

The most common disadvantage cited for expert systems in the academic literature is the knowledge acquisition problem. Obtaining the time of domain experts for any software application is always difficult, but for expert systems it was especially difficult because the experts were by definition highly valued and in constant demand by the organization. As a result of this problem, a great deal of research in the later years of expert systems was focused on tools for knowledge acquisition, to help automate the process of designing, debugging, and maintaining rules defined by experts. However, when looking at the life-cycle of expert systems in actual use, other problems – essentially the same problems as those of any other large system – seem at least as critical as knowledge acquisition: integration, access to large databases, and performance.  Performance could be especially problematic because early expert systems were built using tools (such as earlier Lisp versions) that interpreted code expressions without first compiling them. This provided a powerful development environment, but with the drawback that it was virtually impossible to match the efficiency of the fastest compiled languages (such as C). System and database integration were difficult for early expert systems because the tools were mostly in languages and platforms that were neither familiar to nor welcome in most corporate IT environments – programming languages such as Lisp and Prolog, and hardware platforms such as Lisp machines and personal computers. As a result, much effort in the later stages of expert system tool development was focused on integrating with legacy environments such as COBOL and large database systems, and on porting to more standard platforms. These issues were resolved mainly by the client-server paradigm shift, as PCs were gradually accepted in the IT environment as a legitimate platform for serious business system development and as affordable minicomputer servers provided the processing power needed for AI applications.  Another major challenge of expert systems emerges when the size of the knowledge base increases. This causes the processing complexity to increase. For instance, when an expert system with 100 million rules was envisioned as the ultimate expert system, it became obvious that such system would be too complex and it would face too many computational problems. An inference engine would have to be able to process huge numbers of rules to reach a decision.  How to verify that decision rules are consistent with each other is also a challenge when there are too many rules. Usually such problem leads to a satisfiability (SAT) formulation. This is a well-known NP-complete problem Boolean satisfiability problem. If we assume only binary variables, say n of them, and then the corresponding search space is of size 2formula_2. Thus, the search space can grow exponentially.  There are also questions on how to prioritize the use of the rules in order to operate more efficiently, or how to resolve ambiguities (for instance, if there are too many else-if sub-structures within a single rule) and so on.  Other problems are related to the overfitting and overgeneralization effects when using known facts and trying to generalize to other cases not described explicitly in the knowledge base. Such problems exist with methods that employ machine learning approaches too.  Another problem related to the knowledge base is how to make updates of its knowledge quickly and effectively. Also how to add a new piece of knowledge (i.e., where to add it among many rules) is challenging. Modern approaches that rely on machine learning methods are easier in this regard.  Because of the above challenges, it became clear that new approaches to AI were required instead of rule-based technologies. These new approaches are based on the use of machine learning techniques, along with the use of feedback mechanisms.  The key challenges that expert systems in medicine (if one considers computer-aided diagnostic systems as modern expert systems), and perhaps in other application domains, include issues related to aspects such as: big data, existing regulations, healthcare practice, various algorithmic issues, and system assessment.

Applications.

Hayes-Roth divides expert systems applications into 10 categories illustrated in the following table. The example applications were not in the original Hayes-Roth table, and some of them arose well afterward. Any application that is not footnoted is described in the Hayes-Roth book. Also, while these categories provide an intuitive framework to describe the space of expert systems applications, they are not rigid categories, and in some cases an application may show traits of more than one category.  Hearsay was an early attempt at solving voice recognition through an expert systems approach. For the most part this category of expert systems was not all that successful. Hearsay and all interpretation systems are essentially pattern recognition systems—looking for patterns in noisy data. In the case of Hearsay recognizing phonemes in an audio stream. Other early examples were analyzing sonar data to detect Russian submarines. These kinds of systems proved much more amenable to a neural network AI solution than a rule-based approach.  CADUCEUS and MYCIN were medical diagnosis systems. The user describes their symptoms to the computer as they would to a doctor and the computer returns a medical diagnosis.  Dendral was a tool to study hypothesis formation in the identification of organic molecules. The general problem it solved—designing a solution given a set of constraints—was one of the most successful areas for early expert systems applied to business domains such as salespeople configuring Digital Equipment Corporation (DEC) VAX computers and mortgage loan application development.  SMH.PAL is an expert system for the assessment of students with multiple disabilities.  Mistral is an expert system to monitor dam safety, developed in the 1990s by Ismes (Italy). It gets data from an automatic monitoring system and performs a diagnosis of the state of the dam. Its first copy, installed in 1992 on the Ridracoli Dam (Italy), is still operational 24/7/365. It has been installed on several dams in Italy and abroad (e.g., Itaipu Dam in Brazil), and on landslide sites under the name of Eydenet, and on monuments under the name of Kaleidos. Mistral is a registered trade mark of CESI.

In artificial intelligence, an expert system is a computer system that emulates the decision-making ability of a human expert. Expert systems are designed to solve complex problems by reasoning through bodies of knowledge, represented mainly as if–then rules rather than through conventional procedural code. The first expert systems were created in the 1970s and then proliferated in the 1980s. Expert systems were among the first truly successful forms of artificial intelligence (AI) software. An expert system is divided into two subsystems: the inference engine and the knowledge base. The knowledge base represents facts and rules. The inference engine applies the rules to the known facts to deduce new facts. Inference engines can also include explanation and debugging abilities.

Expertní systém je počítačový program, který má za úkol poskytovat expertní rady, rozhodnutí nebo doporučit řešení v konkrétní situaci. Expertní systémy jsou navrženy tak, aby mohly zpracovávat nenumerické a neurčité informace a řešit tak úlohy, které nejsou řešitelné tradičními algoritmickými postupy.


Terminology.

Eastern traditions.

In Eastern Orthodoxy this day, known as Holy and Great Saturday, is also called The Great Sabbath since it is on this day that Christ "rested" physically in the tomb. But it is also believed that it was on this day he performed in spirit the Harrowing of Hell and raised up to Paradise, having liberated those who had been held captive. In the Coptic, Ethiopian and Eritrean Orthodox Churches, this day is known as Joyous Saturday, otherwise known as the night of light and joy.

Western traditions.

In Western traditions, the day is usually called Holy Saturday, although in the Moravian Church, the day is referred to as the Great Sabbath and in the Anglican Communion, the Book of Common Prayer refers to the day as Easter Even. Although the term Easter Saturday is usually applied to the Saturday in Easter week, in English-speaking countries it is sometimes applied to Holy Saturday, including in legislation in the Australian states of New South Wales and Queensland, and by Australian government agencies. In the Catholic tradition, the Blessed Virgin Mary as Our Lady of Sorrows is on this day assigned the title "Our Lady of Solitude", referring to her solace and grief at the death of her son Jesus. However, nowhere in Catholic liturgical documents is this day referred to as the feast of Our Lady of Solitude.

Religious and cultural practices.

Eastern traditions.

Matins of Holy and Great Saturday (in parishes usually held on Friday evening) takes the form of a funeral service for Christ. The entire service takes place around the "Epitaphios", an icon in the form of a cloth embroidered with the image of Christ prepared for burial. The first part of the service consists of chanting Psalm 118, as usual at both Saturday matins and at funerals, but interspersed with hymns ("enkomia" or "lamentations") between the verses. The predominant theme of the service is not so much one of mourning, but of watchful expectation:  Today Thou dost keep holy the seventh day,
 Which Thou has blessed of old by resting from Thy works.
 Thou bringest all things into being and Thou makest all things new,
 Observing the Sabbath rest, my Saviour, and restoring strength.  Near the end of matins, at the end of the Great Doxology, the Epitaphios is taken up and carried in procession around the outside of the church, while the Trisagion is sung, as is done when carrying the body to the cemetery in an Orthodox burial. On Saturday, a vesperal Divine Liturgy of Saint Basil the Great is celebrated, called the First Resurrection Service (Greek: ), named so because chronologically it was composed earlier than the Paschal Canon by St John Damascene, rather than because it occurs earlier liturgically. This is the longest Divine Liturgy of the entire year and the latest. After the Little Entrance there are 15 Old Testament readings which recall the history of salvation. In the Russian tradition, just before the Gospel reading () the hangings, altar cloths and vestments are changed from dark to bright and the deacon performs a censing of the church. In the Greek tradition, the clergy strew laurel leaves and flower petals all over the church to symbolize the shattered gates and broken chains of hell and Jesus' victory over death. While the liturgical atmosphere changes from sorrow to joy at this service, the faithful continue to fast and the Paschal greeting, "Christ is risen!", is not exchanged until after midnight during the Paschal Vigil since this service represents the proclamation of Jesus' victory over death to those in Hades, but the Resurrection has not yet been announced to those on earth which takes place during the Paschal Vigil. Great Lent was originally the period of catechesis for new converts in order to prepare them for baptism and chrismation and when there are converts received, that occurs during the Old testament readings during the vesperal divine liturgy. Before the midnight service, the faithful gather in church for the reading of the Acts of the Apostles in its entirety. Preceding midnight the Paschal Vigil begins with the Midnight Office, during which the Canon of Holy Saturday is repeated, toward the end of which the epitaphios is removed from the center of the church and placed on the altar table where it remains until the Ascension. Then, all of the candles and lights in the church are extinguished, and all wait in darkness and silence for the proclamation of the Resurrection of Christ.  Prior to the composition of the current Paschal Vigil of St. John of Damascus, this day's vesperal liturgy was the main Easter celebration.

Western traditions.

In the Roman Catholic Church, the chancel remains stripped completely bare (following the Mass on Maundy Thursday). All Masses are severely limited; no Mass at all appears in the normal liturgy for this day, although Mass can be said on Good Friday and on Holy Saturday for an extremely grave or solemn situation with a dispensation from the Vatican or the local bishop. The celebration of the Sacraments is extremely limited: Holy Communion is given only as Viaticum to the dying; while Penance, and Anointing of the Sick may be administered because they, like Viaticum, are helpful to ensuring salvation for the dying. The day is the second day of the Paschal fast as outlined in Sacrosanctum Concilium, Article 110, although fasting may not be as stringent as on Good Friday.  Many of the churches of the Anglican Communion as well as Lutheran, Methodist, and some other Churches observe most of the same customs of the Catholic Church; however, their altars may be covered in black instead of being stripped. In some Anglican churches, including the Protestant Episcopal Church in the United States of America, provision is made for a simple Liturgy of the Word on this day, with readings commemorating the burial of Christ. Daily Offices are still observed. In the Moravian Church, the day is known as Great Sabbath. The Anglican Book of Common Prayer uses Easter Even to designate the day. In the Roman Catholic, Lutheran, Anglican and Methodist traditions, Holy Saturday lasts until nightfall, after which the Easter Vigil is celebrated, marking the official start of the Easter season. The rubrics state that the Easter Vigil must take place in the night; it must begin after nightfall and end before dawn. The service may start with a fire and the lighting of the new Paschal candle. In Roman Catholic and some Anglican observance, the Mass is the first Mass since that of Maundy Thursday, and during it, the "Gloria" — which has been absent during Lent — is used as the statues and icons, covered with purple veils during Passiontide, are dramatically unveiled. Some Anglican churches prefer to celebrate Easter and the lighting of the new Paschal candle at dawn on Easter Day. Baptisms may take place in this service and Baptismal vows are often renewed.  In the predominantly Roman Catholic Philippines, the day is legally and colloquially known in English as "Black Saturday", given the colour's role in mourning. Traditional taboos from the previous day are carried over and are sometimes broken; swimming is allowed in the afternoon. Most commercial establishments resume operations, with smaller enterprises remaining closed until Easter. Television and radio stations broadcast on shorter hours with special programming or remain off-air. After the liturgical changes of the Second Vatican Council, the term "Sábado de Gloria" (Spanish for "Gloria" Saturday) became widely used, referring to the return of chanting the "Gloria in Excelsis Deo" during the Easter Vigil. In predominantly Roman Catholic Poland, Święconka (), meaning "the blessing of the Easter baskets", on Holy Saturday, is one of the most enduring and beloved traditions.

Holy Saturday (), the Saturday of Holy Week, also known as Holy and Great Saturday, the Great Sabbath, Black Saturday, Joyous Saturday, Hallelujah Saturday (in Portugal and Brazil), Glorious Saturday (in the Philippines) or Easter Eve, and called "Joyous Saturday" or "the Saturday of Light" among Coptic Christians, is the day after Good Friday. It is the day before Easter and the last day of Holy Week in which Christians prepare for Easter. It commemorates the day that Jesus' body lay in the tomb and the Harrowing of Hell.

Bílá sobota (latinsky "sabbatum sanctum", "magnum" či "luminum" – tj. svatá, velká nebo světlá, světelná) je v liturgickém kalendáři den před Velikonoční nedělí, součást Svatého týdne - druhý den velikonočního tridua. Připomíná den, kdy Ježíš ležel v hrobě.


Recording.

The first attempt by Led Zeppelin to record songs for "Physical Graffiti" took place in November 1973 at Headley Grange in Hampshire, England, where they had previously recorded their untitled fourth album. The recording equipment consisted of Ronnie Lane's Mobile Studio. Guitarist and producer Jimmy Page and drummer John Bonham recorded an instrumental which was later reworked as "Kashmir" during this visit. However, these sessions came to a halt quickly and the studio time was turned over to Bad Company, who used it to record songs for their eponymous debut album. The press reported that bassist/keyboardist John Paul Jones was ill and unable to record. However, he became disillusioned with the group and fed up with touring, and told manager Peter Grant he was considering quitting. Grant asked him to reconsider and take the rest of the year off to recuperate.  The group reconvened at Headley Grange in January and February 1974, where they recorded eight tracks engineered by Ron Nevison. Lead singer Robert Plant later referred to these eight tracks as "the belters", including "off-the-wall stuff that turned out really nice". As with previous sessions at Headley Grange, the informal atmosphere allowed the group to improvise and develop material while recording. Sometimes the group would rehearse or record a track several times, discuss what went wrong or what could be improved and then realised they had worked out an alternative arrangement for it which was better. Bonham was a driving force at the sessions, regularly suggesting ideas or the best ways in which a complicated arrangement could be played successfully. This led to him getting a lead songwriting credit on several tracks.  The eight songs extended beyond the length of a conventional album, almost spanning three sides of an LP, so the group decided to create a double album, adding material they had recorded for previous albums but never issued. This included various jam sessions such as "Boogie With Stu", which Page thought would be unsuitable as a track on a single album. Additional overdubs were laid down, and the final mixing of the album was performed in July 1974 by Keith Harwood at Olympic Studios, London. The title "Physical Graffiti" was coined by Page to illustrate the whole physical and written energy that had gone into producing the set.

Songs.

The album spanned several years of recording and covered a range of musical styles, including hard rock ("Custard Pie", "The Rover", "The Wanton Song", "Sick Again", "Houses of the Holy"), eastern-influenced orchestral rock ("Kashmir"), progressive rock ("In the Light"), driving funk ("Trampled Under Foot"), acoustic rock and roll ("Boogie with Stu", "Black Country Woman"), love ballad ("Ten Years Gone"), blues rock ("In My Time of Dying"), soft rock ("Down by the Seaside"), country rock romp ("Night Flight"), and acoustic guitar instrumental ("Bron-Yr-Aur").  Several tracks from the album became live staples at Led Zeppelin concerts. In particular, "In My Time of Dying", "Trampled Under Foot", "Kashmir", "Ten Years Gone", "Black Country Woman", and "Sick Again" became regular components of the band's live concert set lists following the release of the album.

Side one.

"Custard Pie" was recorded at Headley Grange in early 1974. The first take was played at a faster tempo than the finished version, with various improvised vocals. After a basic run-through, the group then discussed possibilities for rearranging it. Page played the guitar solo through an ARP synthesiser, while Jones overdubbed a Hohner Clavinet part and Plant played harmonica.  "The Rover" was written in 1970 at Bron-Yr-Aur, a cottage near Machynlleth, Wales. It was first recorded at Headley Grange in May 1970 as an acoustic number for "Led Zeppelin III". It was reworked as an electric number in 1972 for "Houses of the Holy", which formed the basis for the backing track. Page subsequently added guitar overdubs in 1974 with Keith Harwood engineering.  "In My Time of Dying" is based on a traditional song that Bob Dylan recorded on his debut album in 1962. The track was recorded live, with Page later adding further slide guitar overdubs. The arranging and structuring was led by Bonham, who worked out where the various stop / start sections in the track should be, and how the group would know where to come back in. The very end of the song features his off-mic cough, causing the rest of the group to break down at that point. Bonham subsequently shouted "That's got to be the one, hasn't it?", feeling it was the best take. It was left on the album to show fans that Led Zeppelin were a working band that took care in their recordings.

Side two.

"Houses of the Holy" was recorded as the title track for the album of the same name in June 1972 at Electric Lady Studios with Eddie Kramer engineering. It was left off that album because of its similarity to other tracks such as "Dancing Days", which were felt to be better. Unlike some of the other older material on "Physical Graffiti", it required no further overdubbing or remixing.  "Trampled Under Foot" developed from a jam session driven by Jones at the Clavinet. The song went through several arrangement changes before arriving at the version heard on the album, with the group rehearsing various different ideas and arguing about the overall style. Bonham decided the track was too "souly" and rearranged it into a funk style, suggesting that Page should play a guitar riff throughout in place of chords. The lyrics are a series of double entendres around driving and cars. The song quickly became a popular live piece that was played at every live show from 1975 onwards, and was later revived by Plant for his solo tours. It was released as a single in the US on 2 April (with "Black Country Woman" as the B-side) and was a top 40 hit.  "Kashmir" was an idea from Page and Bonham, and was first attempted as an instrumental demo in late 1973. Plant wrote the lyrics while on holiday in Morocco. Jones played Mellotron on the track, and arranged strings and brass parts that were played by session musicians. The song was one of the most critically acclaimed on the album, and was played at every gig from 1975 onwards. Page and Plant played it on their 1994 tour, and it was reworked in 1998 by Sean "Puffy" Combs for his single "Come With Me" which featured Page on guitar.

Side three.

"In the Light" was recorded at Headley Grange in early 1974. It was initially called "In the Morning" and went through several rehearsals and takes to work out a basic structure. A drone / chant introduction was later added to the piece.  "Bron-Yr-Aur" was a solo acoustic piece by Page, named after the cottage where he had composed and arranged much of "Led Zeppelin III" with Plant. It was recorded at Island Studios in mid-1970. The track was later used as background music in the group's film "The Song Remains the Same".  "Down by the Seaside" was originally written as an acoustic track at Bron-Yr-Aur in 1970, and was influenced by Neil Young. It was reworked as an electric track during sessions for the fourth album the following year. Page and Bonham led the arranging, changing tempo from the slow to fast section and then back again.  "Ten Years Gone" was mostly composed by Plant about an old love affair, and was combined with an instrumental piece from Page, featuring overdubbed electric and acoustic guitar parts. When the track was performed live, Jones played a triple-neck guitar featuring mandolin, six- and twelve-string guitars, in order to try to reproduce the various guitar overdubs on the studio recording.

Side four.

"Night Flight" was recorded at Headley Grange in 1971 for the fourth album. Jones plays Hammond organ on the track, and Page plays guitar through a Leslie speaker. Plant wrote the lyrics after reading a news headline entitled "Nuclear Damage Test Threat" and wondered why there seemed to be little peace and love in the world.  "The Wanton Song" was built around a Page guitar riff. Unlike some of the other tracks recorded at the 1974 Headley Grange sessions, it was straightforward to arrange, with the group building the song around the riffs.  "Boogie with Stu" was a jam session with Rolling Stones pianist Ian Stewart based around the Ritchie Valens song "Ooh My Head". It was recorded in 1971 at Headley Grange during the same session that produced "Rock and Roll" for the fourth album. It did not credit Valens or Bob Keane, instead crediting Valens' mother. Eventually, a lawsuit was filed by Keane, and half of the award went to Valens' mother, although she was not part of the suit.  "Black Country Woman" was recorded in the garden at Stargroves in 1972 for "Houses of the Holy", as part of the group's desire to work in "off the wall" locations outside a traditional studio environment. The track was nearly abandoned when an aeroplane cruised overhead, but it was left on the final recording for effect.  "Sick Again" was written by Page and Plant about the 1973 tour and their experience with meeting groupies. The track was driven by Bonham's drumming and Page's guitar riffs. The arrangement had been worked out before recording, and was straightforward to put down on tape.

Unreleased material.

As "Physical Graffiti" collected various out-takes from earlier albums, little was left over from the recording sessions that was not eventually released. An early arrangement of "Custard Pie", different from the final version, was reworked as "Hots on For Nowhere" on the following album, "Presence". A number of other outtakes from earlier album sessions that had not been put on "Physical Graffiti" were later included on the 1982 album "Coda".

Artwork and packaging.

The album was originally released with a die-cut sleeve design depicting a New York City tenement block, through whose windows various cultural icons could be interchangeably viewed. The album designer, Peter Corriston, was looking for a building that was symmetrical with interesting details, that was not obstructed by other objects and would fit the square album cover. He subsequently came up with the rest of the cover based on people moving in and out of the tenement, with various sleeves that could be placed under the main cover and filling the windows with various pieces of information.  The two five-storey buildings photographed for the album cover are located at 96 and 98 St. Mark's Place in New York City. The original photograph underwent a number of tweaks to arrive at the final image. The fifth floor of the building had to be cropped out to fit the square album cover format.  Eschewing the usual gatefold design in favour of a special die-cut cover, the original album jacket included four covers made up of two inners (for each disc), a middle insert cover and an outer cover. The middle insert cover is white and details all the album track listings and recording information. The outer cover has die-cut windows on the building, so when the middle cover is wrapped around the inner covers and slid into the outer cover, the title of the album is shown on the front cover, spelling out the name "Physical Graffiti". Images in the windows touched upon a set of American icons and a range of Hollywood ephemera. Pictures of W. C. Fields and Buzz Aldrin alternated with the snapshots of Led Zeppelin. Photographs of Lee Harvey Oswald, Marcel Duchamp and Pope Leo XIII are also featured. Per the liner notes, package concept and design was by AGI/Mike Doud (London) and Peter Corriston (New York). Photography was by Elliot Erwitt, B. P. Fallon, and Roy Harper. "Tinting Extraordinaire:" Maurice Tate, and window illustration by Dave Heffernan. In 1976 the album was nominated for a Grammy Award in the category of best album package.

Release and critical reception.

"Physical Graffiti" was Led Zeppelin's first release on their own Swan Song Records label, which had been launched in May 1974. Until this point, all of Led Zeppelin's albums had been released on Atlantic Records, who would distribute Swan Song. The album was first announced to the press on 6 November with a planned release date of 29 November and an accompanying US tour (the band's tenth) starting in January. Delays in the production of the album's sleeve design prevented its release prior to the commencement of the tour. It was finally released on 24 February 1975. The album was a commercial and critical success, having built up a huge advance order following the delayed release date, and when eventually issued it reached No. 1 in the UK charts. In the US, it debuted at No. 3 on "Billboard's" Pop Albums chart, rising to No. 1 the following week and staying there for six weeks. "Physical Graffiti" has since proven to be one of the most popular releases by the group, shipping 8 million copies in the United States. It was the first album to go platinum on advance orders alone. Shortly after its release, all previous Led Zeppelin albums simultaneously re-entered the top-200 album chart.  The group debuted several songs from "Physical Graffiti" live for the first time at a warm-up gig in Rotterdam, Netherlands on 11 January, a week before the US tour, which lasted until 27 March. The tour was also successful, and was followed up by a series of shows at Earl's Court, London. Tickets for the shows sold out within four hours, described by promoter Mel Bush as "unprecedented demand in the history of rock music", so a further two dates were added. The shows attracted rave reviews, and critics noted the band enjoyed playing the new material on "Physical Graffiti" more than the older songs.  "NME"s Nick Kent reviewed the album three months before it was released. He speculated it could be the group's best work to date, saying "the album's tonal density is absolutely the toughest, most downright brutal I've heard all year". In March 1975, "Billboard" magazine's reviewer wrote: "["Physical Graffiti"] is a tour de force through a number of musical styles, from straight rock to blues to folky acoustic to orchestral sounds." Similarly, Jim Miller stated in "Rolling Stone" that the double album was "the band's "Tommy", "Beggar's Banquet" and "Sgt. Pepper" rolled into one: "Physical Graffiti" is Led Zeppelin's bid for artistic respectability". "Village Voice" critic Robert Christgau was less impressed, writing that except for side two, the material often wanders into "wide tracks, misconceived opi, and so forth", and "after a while Robert Plant begins to grate".  Plant later felt that "Physical Graffiti" represented the band at its creative peak, and has since said that it is his favourite Led Zeppelin album. Page has also said the album was a "high-water mark" for the group, and the creative energy from jamming and gradually working out song structures together led to some strong material. Reviewing the album for BBC Music in 2007, Chris Jones described it as "a towering monument to the glory of Zeppelin in their high-flying heyday".

Reissues.

"Physical Graffiti" was first issued on CD as a double-disc set in 1987. However, it was done without input from the band, used second and third-generation master tapes, and accidentally edited off the studio banter at the end of "In My Time of Dying". Page was unhappy with the lack of care over the CDs and decided he would produce new versions himself. He booked a week in May 1990 with engineer George Marino to remaster the entire back catalogue. Eight tracks from "Physical Graffiti" appeared on the four-disc "Boxed Set" and three on "Remasters", both in 1990; the remainder appeared on "Boxed Set 2" in 1993, while the album proper was reissued in 1994.  An extended remastered version of "Physical Graffiti" was reissued on 23 February 2015, almost exactly forty years after the original album was released. The reissue comes in six formats: a standard two-CD edition, a deluxe three-CD edition, a standard two-LP version, a deluxe three-LP version, a super deluxe three-CD plus three-LP version with a hardback book, and as high resolution 96k/24-bit digital downloads. The deluxe and super deluxe editions feature bonus material containing alternative takes and arrangements of songs. The reissue was released with an altered colour version of the original album's artwork as its bonus disc's cover.

Personnel.

Led Zeppelin  Additional musicians  Production Artwork

References.

Notes Citations Sources

In mathematics, the domain or set of departure of a function is the set into which all of the input of the function is constrained to fall. It is the set in the notation. Since a function is defined on its entire domain, its domain coincides with its domain of definition, the subset of the domain for which the function associates an image. However this coincidence is no longer true for a partial function since the domain of definition of a partial function can be a proper subset of the domain.

Definiční obor funkce formula_1 je množina všech hodnot, pro které je funkce formula_1 definována. Definiční obor můžeme definovat pro jakékoli množinové zobrazení. Nechť formula_3 je zobrazení z množiny formula_4 do množiny formula_5. Pak definiční obor zobrazení formula_6 tvoří právě ty prvky množiny formula_4, pro něž je definován obraz při zobrazení formula_6. Obecně nemusí být zobrazení formula_6 definováno na celé množině formula_4. V tom případě tvoří jeho definiční obor vlastní podmnožinu množiny formula_4. Občas se kromě názvu "definiční obor" používá také označení doména. Definiční obor zobrazení formula_6 se značí většinou formula_13, formula_14 či formula_15. Posledně uvedený symbol pak vychází z anglického názvu pro definiční obor ("domain") a je běžně používán v cizojazyčné literatuře. V matematické notaci lze definiční obor pro zobrazení formula_3 zapsat jako


Taxonomy.

About 2,150 species of tunicate exist in the world's oceans, living mostly in shallow water. The most numerous group is the ascidians; fewer than 100 species of these are found at depths greater than. Some are solitary animals leading a sessile existence attached to the seabed, but others are colonial and a few are pelagic. Some are supported by a stalk, but most are attached directly to a substrate, which may be a rock, shell, coral, seaweed, mangrove root, dock, piling, or ship's hull. They are found in a range of solid or translucent colours and may resemble seeds, grapes, peaches, barrels, or bottles. One of the largest is a stalked sea tulip, "Pyura pachydermatina", which can grow to be over tall.  The Tunicata were established by Jean-Baptiste Lamarck in 1816. In 1881, Francis Maitland Balfour introduced another name for the same group, "Urochorda", to emphasize the affinity of the group to other chordates. No doubt largely because of his influence, various authors supported the term, either as such, or as the slightly older "Urochordata", but this usage is invalid because "Tunicata" has precedence, and grounds for superseding the name never existed. Accordingly, the current (formally correct) trend is to abandon the name Urochorda or Urochordata in favour of the original Tunicata, and the name Tunicata is almost invariably used in modern scientific works. It is accepted as valid by the World Register of Marine Species but not by the Integrated Taxonomic Information System.  Various common names are used for different species. Sea tulips are tunicates with colourful bodies supported on slender stalks. Sea squirts are so named because of their habit of contracting their bodies sharply and squirting out water when disturbed. Sea liver and sea pork get their names from the resemblance of their dead colonies to pieces of meat.

Classification.

Tunicates are more closely related to craniates, (including hagfish, lampreys, and jawed vertebrates) than to lancelets, echinoderms, hemichordates, "Xenoturbella" or other invertebrates.  The clade consisting of tunicates and vertebrates is called Olfactores.  The Tunicata contain roughly 3,051 described species, traditionally divided into these classes:  Members of the Sorberacea were included in Ascidiacea in 2011 as a result of rDNA sequencing studies. Although the traditional classification is provisionally accepted, newer evidence suggests the Ascidiacea are an artificial group of paraphyletic status.  The following cladogram is based on the phylogenomic study of Delsuc and colleagues.

Fossil record.

Undisputed fossils of tunicates are rare. The best known and earliest unequivocally identified species is "Shankouclava shankouense" from the Lower Cambrian Maotianshan Shale at Shankou village, Anning, near Kunming (South China). There is also a common bioimmuration, ("Catellocaula vallata"), of a possible tunicate found in Upper Ordovician bryozoan skeletons of the upper midwestern United States.  Three enigmatic species were also found from the Ediacaran period – "Ausia fenestrata" from the Nama Group of Namibia, the sac-like "Yarnemia acidiformis", and one from a second new "Ausia"-like genus from the Onega Peninsula of northern Russia, "Burykhia hunti". Results of a new study have shown possible affinity of these Ediacaran organisms to the ascidians. "Ausia" and "Burykhia" lived in shallow coastal waters slightly more than 555 to 548 million years ago, and are believed to be the oldest evidence of the chordate lineage of metazoans. The Russian Precambrian fossil "Yarnemia" is identified as a tunicate only tentatively, because its fossils are nowhere near as well-preserved as those of "Ausia" and "Burykhia", so this identification has been questioned.  Fossils of tunicates are rare because their bodies decay soon after death, but in some tunicate families, microscopic spicules are present, which may be preserved as microfossils. These spicules have occasionally been found in Jurassic and later rocks, but, as few palaeontologists are familiar with them, they may have been mistaken for sponge spicules.

Hybridization studies.

A multi-taxon molecular study in 2010 proposed that sea squirts are descended from a hybrid between a chordate and a protostome ancestor. This study was based on a quartet partitioning approach designed to reveal horizontal gene transfer events among metazoan phyla.

Anatomy.

Body form.

Colonies of tunicates occur in a range of forms, and vary in the degree to which individual organisms, known as zooids, integrate with one another. In the simplest systems, the individual animals are widely separated, but linked together by horizontal connections called stolons, which grow along the seabed. Other species have the zooids growing closer together in a tuft or clustered together and sharing a common base. The most advanced colonies involve the integration of the zooids into a common structure surrounded by the tunic. These may have separate buccal siphons and a single central atrial siphon and may be organized into larger systems, with hundreds of star-shaped units. Often, the zooids in a colony are tiny but very numerous, and the colonies can form large encrusting or mat-like patches.

Body structure.

By far the largest class of tunicates is the Ascidiacea. The body of an ascidiacean is surrounded by a test or tunic, from which the subphylum derives its name. This varies in thickness between species but may be tough, resembling cartilage, thin and delicate, or transparent and gelatinous. The tunic is composed of proteins and complex carbohydrates, and includes tunicin, a variety of cellulose. The tunic is unique among invertebrate exoskeletons in that it can grow as the animal enlarges and does not need to be periodically shed. Inside the tunic is the body wall or mantle composed of connective tissue, muscle fibres, blood vessels, and nerves. Two openings are found in the body wall: the buccal siphon at the top through which water flows into the interior, and the atrial siphon on the ventral side through which it is expelled. A large pharynx occupies most of the interior of the body. It is a muscular tube linking the buccal opening with the rest of the gut. It has a ciliated groove known as an endostyle on its ventral surface, and this secretes a mucous net which collects food particles and is wound up on the dorsal side of the pharynx. The gullet, at the lower end of the pharynx, links it to a loop of gut which terminates near the atrial siphon. The walls of the pharynx are perforated by several bands of slits, known as stigmata, through which water escapes into the surrounding water-filled cavity, the atrium. This is criss-crossed by various rope-like mesenteries which extend from the mantle and provide support for the pharynx, preventing it from collapsing, and also hold up the other organs.  The Thaliacea, the other main class of tunicates, is characterised by free-swimming, pelagic individuals. They are all filter feeders using a pharyngeal mucous net to catch their prey. The pyrosomes are bioluminous colonial tunicates with a hollow cylindrical structure. The buccal siphons are on the outside and the atrial siphons inside. About 10 species are known, and all are found in the tropics. The 23 species of doliolids are small, mostly under long. They are solitary, have the two siphons at opposite ends of their barrel-shaped bodies, and swim by jet propulsion. The 40 species of salps are also small, under long, and found in the surface waters of both warm and cold seas. They also move by jet propulsion, and often form long chains by budding off new individuals.  A third class, the Larvacea (or Appendicularia), is the only group of tunicates to retain their chordate characteristics in the adult state, a product of extensive neoteny. The 70 species of larvaceans superficially resemble the tadpole larvae of amphibians, although the tail is at right angles to the body. The notochord is retained, and the animals, mostly under 1 cm long, are propelled by undulations of the tail. They secrete an external mucous net known as a house, which may completely surround them and is very efficient at trapping planktonic particles.

Physiology and internal anatomy.

Like other chordates, tunicates have a notochord during their early development, but which is lost by the time they have completed their metamorphosis. As members of the Chordata, they are true Coelomata with endoderm, ectoderm, and mesoderm, but they do not develop very clear coelomic body cavities, if any at all. Whether they do or not, by the end of their larval development, all that remain are the pericardial, renal, and gonadal cavities of the adults. Except for the heart, gonads, and pharynx (or branchial sac), the organs are enclosed in a membrane called an epicardium, which is surrounded by the jelly-like mesenchyme. Tunicates begin life in a mobile larval stage that resembles a tadpole. A minority of species, those in the Larvacea, retain the general larval form throughout life, but most Tunicata very rapidly settle down and attach themselves to a suitable surface, later developing into a barrel-like and usually sedentary adult form. The Thaliacea, however, are pelagic throughout their lives and may have complex lifecycles.  Tunicates have a well-developed heart and circulatory system. The heart is a double U-shaped tube situated just below the gut. The blood vessels are simple connective tissue tubes, and their blood has several types of corpuscle. The blood may appear pale green, but this is not due to any respiratory pigments, and oxygen is transported dissolved in the plasma. Exact details of the circulatory system are unclear, but the gut, pharynx, gills, gonads, and nervous system seem to be arranged in series rather than in parallel, as happens in most other animals. Every few minutes, the heart stops beating and then restarts, pumping fluid in the reverse direction.  Tunicate blood has some unusual features. In some species of Ascidiidae and Perophoridae, it contains high concentrations of the transitional metal vanadium and vanadium-associated proteins in vacuoles in blood cells known as vanadocytes. Some tunicates can concentrate vanadium up to a level ten million times that of the surrounding seawater. It is stored in a +3 oxidation form that requires a pH of less than 2 for stability, and this is achieved by the vacuoles also containing sulfuric acid. The vanadocytes are later deposited just below the outer surface of the tunic, where their presence is thought to deter predation, although it is unclear whether this is due to the presence of the metal or low pH. Other species of tunicates concentrate lithium, iron, niobium, and tantalum, which may serve a similar function. Other tunicate species produce distasteful organic compounds as chemical defenses against predators.  Tunicates lack the kidney-like metanephridial organs typical of deuterostomes. Most have no excretory structures, but rely on the diffusion of ammonia across their tissues to rid themselves of nitrogenous waste, though some have a simple excretory system. The typical renal organ is a mass of large clear-walled vesicles that occupy the rectal loop, and the structure has no duct. Each vesicle is a remnant of a part of the primitive coelom, and its cells extract nitrogenous waste matter from circulating blood. They accumulate the wastes inside the vesicles as urate crystals, and do not have any obvious means of disposing of the material during their lifetimes.  Adult tunicates have a hollow cerebral ganglion, equivalent to a brain, and a hollow structure known as a neural gland. Both originate from the embryonic neural tube and are located between the two siphons. Nerves arise from the two ends of the ganglion; those from the anterior end innervate the buccal siphon and those from the posterior end supply the rest of the body, the atrial siphon, organs, gut and the musculature of the body wall. There are no sense organs but there are sensory cells on the siphons, the buccal tentacles and in the atrium.  Tunicates are unusual among animals in that they produce a large fraction of their tunic and some other structures in the form of cellulose. The production in animals of cellulose is so unusual that at first some researchers denied its presence outside of plants, but the tunicates were later found to possess a functional cellulose synthesizing enzyme, encoded by a gene horizontally transferred from a bacterium. When, in 1845, Carl Schmidt first announced the presence in the test of some ascidians of a substance very similar to cellulose, he called it "tunicine", but it is now recognized as cellulose rather than any alternative substance.

Feeding.

Nearly all tunicates are suspension feeders, capturing planktonic particles by filtering sea water through their bodies. Ascidians are typical in their digestive processes, but other tunicates have similar systems. Water is drawn into the body through the buccal siphon by the action of cilia lining the gill slits. To obtain enough food, an average ascidian needs to process one body-volume of water per second. This is drawn through a net lining the pharynx which is being continuously secreted by the endostyle. The net is made of sticky mucus threads with holes about 0.5 μm in diameter which can trap planktonic particles including bacteria. The net is rolled up on the dorsal side of the pharynx, and it and the trapped particles are drawn into the oesophagus. The gut is U-shaped and also ciliated to move the contents along. The stomach is an enlarged region at the lowest part of the U-bend. Here, digestive enzymes are secreted and a pyloric gland adds further secretions. After digestion, the food is moved on through the intestine, where absorption takes place, and the rectum, where undigested remains are formed into faecal pellets or strings. The anus opens into the dorsal or cloacal part of the peribranchial cavity near the atrial siphon. Here, the faeces are caught up by the constant stream of water which carries the waste to the exterior. The animal orientates itself to the current in such a way that the buccal siphon is always upstream and does not draw in contaminated water.  Some ascidians that live on soft sediments are detritivores. A few deepwater species, such as "Megalodicopia hians", are sit-and-wait predators, trapping tiny crustacea, nematodes, and other small invertebrates with the muscular lobes which surround their buccal siphons. Certain tropical species in the family Didemnidae have symbiotic green algae or cyanobacteria in their tunics, and one of these symbionts, "Prochloron", is unique to tunicates. Excess photosynthetic products are assumed to be available to the host.

Life cycle.

Ascidians are almost all hermaphrodites and each has a single ovary and testis, either near the gut or on the body wall. In some solitary species, sperm and eggs are shed into the sea and the larvae are planktonic. In others, especially colonial species, sperm is released into the water and drawn into the atria of other individuals with the incoming water current. Fertilization takes place here and the eggs are brooded through their early developmental stages. Some larval forms appear very much like primitive chordates with a notochord (stiffening rod) and superficially resemble small tadpoles. These swim by undulations of the tail and may have a simple eye, an ocellus, and a balancing organ, a statocyst.  When sufficiently developed, the larva of the sessile species finds a suitable rock and cements itself in place. The larval form is not capable of feeding, though it may have a rudimentary digestive system, and is only a dispersal mechanism. Many physical changes occur to the tunicate's body during metamorphosis, one of the most significant being the reduction of the cerebral ganglion, which controls movement and is the equivalent of the vertebrate brain. From this comes the common saying that the sea squirt "eats its own brain". However, the adult does possess a cerebral ganglion which may even be larger than in the embryonic stage, so the scientific validity of this joke is questionable. In some classes, the adults remain pelagic (swimming or drifting in the open sea), although their larvae undergo similar metamorphoses to a higher or lower degree. Colonial forms also increase the size of the colony by budding off new individuals to share the same tunic.  Pyrosome colonies grow by budding off new zooids near the posterior end of the colony. Sexual reproduction starts within a zooid with an internally fertilized egg. This develops directly into an oozooid without any intervening larval form. This buds precociously to form four blastozooids which become detached in a single unit when the oozoid disintegrates. The atrial siphon of the oozoid becomes the exhalent siphon for the new, four-zooid colony.  Doliolids have a very complex life cycle that includes various zooids with different functions. The sexually reproducing members of the colony are known as gonozooids. Each one is a hermaphrodite with the eggs being fertilised by sperm from another individual. The gonozooid is viviparous, and at first, the developing embryo feeds on its yolk sac before being released into the sea as a free-swimming, tadpole-like larva. This undergoes metamorphosis in the water column into an oozooid. This is known as a "nurse" as it develops a tail of zooids produced by budding asexually. Some of these are known as trophozooids, have a nutritional function, and are arranged in lateral rows. Others are phorozooids, have a transport function, and are arranged in a single central row. Other zooids link to the phorozooids, which then detach themselves from the nurse. These zooids develop into gonozooids, and when these are mature, they separate from the phorozooids to live independently and start the cycle over again. Meanwhile, the phorozooids have served their purpose and disintegrate. The asexual phase in the lifecycle allows the doliolid to multiply very rapidly when conditions are favourable.  Salps also have a complex lifecycle with an alternation of generations. In the solitary life history phase, an oozoid reproduces asexually, producing a chain of tens or hundreds of individual zooids by budding along the length of a stolon. The chain of salps is the 'aggregate' portion of the lifecycle. The aggregate individuals, known as blastozooids, remain attached together while swimming and feeding and growing larger. The blastozooids are sequential hermaphrodites. An egg in each is fertilized internally by a sperm from another colony. The egg develops in a brood sac inside the blastozooid and has a placental connection to the circulating blood of its "nurse". When it fills the blastozooid's body, it is released to start the independent life of an oozooid.  Larvaceans only reproduce sexually. They are protandrous hermaphrodites, except for "Oikopleura dioica" which is gonochoric, and a larva resembles the tadpole larva of ascidians. Once the trunk is fully developed, the larva undergoes "tail shift", in which the tail moves from a rearward position to a ventral orientation and twists through 90° relative to the trunk. The larva consists of a small, fixed number of cells, and grows by enlargement of these rather than cell division. Development is very rapid and only takes seven hours for a zygote to develop into a house-building juvenile starting to feed.  During embryonic development, tunicates exhibit determinate cleavage, where the fate of the cells is set early on with reduced cell numbers and genomes that are rapidly evolving. In contrast, the amphioxus and vertebrates show cell determination relatively late in development and cell cleavage is indeterminate. The genome evolution of amphioxus and vertebrates is also relatively slow.

Promotion of out-crossing.

"Ciona intestinalis" (class Ascidiacea) is a hermaphrodite that releases sperm and eggs into the surrounding seawater almost simultaneously. It is self-sterile, and thus has been used for studies on the mechanism of self-incompatibility. Self/non-self-recognition molecules play a key role in the process of interaction between sperm and the vitelline coat of the egg. It appears that self/non-self recognition in ascidians such as "C. intestinalis" is mechanistically similar to self-incompatibility systems in flowering plants. Self-incompatibility promotes out-crossing, and thus provides the adaptive advantage at each generation of the masking of deleterious recessive mutations (i.e. genetic complementation) and the avoidance of inbreeding depression.  "Botryllus schlosseri" (class Ascidiacea) is a colonial tunicate, a member of the only group of chordates that are able to reproduce both sexually and asexually. "B. schlosseri" is a sequential (protogynous) hermaphrodite, and in a colony, eggs are ovulated about two days before the peak of sperm emission. Thus self-fertilization is avoided, and cross-fertilization is favored. Although avoided, self-fertilization is still possible in "B. schlosseri". Self-fertilized eggs develop with a substantially higher frequency of anomalies during cleavage than cross-fertilized eggs (23% vs. 1.6%). Also a significantly lower percentage of larvae derived from self-fertilized eggs metamorphose, and the growth of the colonies derived from their metamorphosis is significantly lower. These findings suggest that self-fertilization gives rise to inbreeding depression associated with developmental deficits that are likely caused by expression of deleterious recessive mutations.

A model tunicate.

"Oikopleura dioica" (class Appendicularia) is a semelparous organism, reproducing only once in its lifetime. It employs an original reproductive strategy in which the entire female germ-line is contained within an ovary that is a single giant multinucleate cell termed the "coenocyst". "O. dioica" can be maintained in laboratory culture, and is of growing interest as a model organism because of its phylogenetic position within the closest sister group to vertebrates.

Invasive species.

Over the past few decades, tunicates (notably of the genera "Didemnum" and "Styela") have been invading coastal waters in many countries. The carpet tunicate ("Didemnum vexillum") has taken over a area of the seabed on the Georges Bank off the northeast coast of North America, covering stones, molluscs, and other stationary objects in a dense mat. "D. vexillum", "Styela clava" and "Ciona savignyi" have appeared and are thriving in Puget Sound and Hood Canal in the Pacific Northwest.  Invasive tunicates usually arrive as fouling organisms on the hulls of ships, but may also be introduced as larvae in ballast water. Another possible means of introduction is on the shells of molluscs brought in for marine cultivation. Current research indicates many tunicates previously thought to be indigenous to Europe and the Americas are, in fact, invaders. Some of these invasions may have occurred centuries or even millennia ago. In some areas, tunicates are proving to be a major threat to aquaculture operations.

Use by humans.

Medical uses.

Tunicates contain a host of potentially useful chemical compounds, including:  Tunicates are able to correct their own cellular abnormalities over a series of generations, and a similar regenerative process may be possible for humans. The mechanisms underlying the phenomenon may lead to insights about the potential of cells and tissues to be reprogrammed and to regenerate compromised human organs.

As food.

Various Ascidiacea species are consumed as food around the world. In Japan and Korea, the sea pineapple ("Halocynthia roretzi") is the main species eaten. It is cultivated on dangling cords made of palm fronds. In 1994, over 42,000 tons were produced, but since then, mass mortality events have occurred among the farmed sea squirts (the tunics becoming soft), and only 4,500 tons were produced in 2004.

Other uses.

The use of tunicates as a source of biofuel is being researched. The cellulose body wall can be broken down and converted into ethanol, and other parts of the animal are protein-rich and can be converted into fish feed. Culturing tunicates on a large scale may be possible and the economics of doing so are attractive. As tunicates have few predators, their removal from the sea may not have profound ecological impacts. Being sea-based, their production does not compete with food production as does the cultivation of land-based crops for biofuel projects.  Some tunicates are used as model organisms. "Ciona intestinalis" and "Ciona savignyi" have been used for developmental studies. Both species' mitochondrial and nuclear genomes have been sequenced. The nuclear genome of the appendicularian "Oikopleura dioica" appears to be one of the smallest among metazoans and this species has been used to study gene regulation and the evolution and development of chordates.

A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata. It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords. The subphylum was at one time called Urochordata, and the term urochordates is still sometimes used for these animals. They are the only chordates that have lost their myomeric segmentation, with the possible exception of the seriation of the gill slits.

Pláštěnci (Tunicata, Urochordata) je podkmen mořských strunatců. Hlavní vývojové znaky strunatců (pevná struna hřbetní a nervová trubice) jsou vyvinuty pouze u larev (s výjimkou vršenek). Tělo většiny dospělých obklopuje tuhý rosolovitý plášť ("tunica"), tvořený polysacharidem tunicinem. Pláštěnci žijí především přisedlým životem v koloniích.