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Lancelet

The lancelets known as amphioxi, consist of some 30–35 species of "fish-like" benthic filter feeding chordates in the order Amphioxiformes. They are the modern representatives of the subphylum Cephalochordata. Lancelets resemble, are believed to be related to, 530-million-year-old Pikaia, fossils of which are known from the Burgess Shale. Zoologists are interested in them because they provide evolutionary insight into the origins of vertebrates. Lancelets contain many organs and organ systems that are related to those of modern fish, but in more primitive form. Therefore, they provide a number of examples of possible evolution exaptation. For example, the gill-slits of lancelets are used for feeding only, not for respiration; the circulatory system carries food throughout their body, but does not have red blood cells or hemoglobin for transporting oxygen. Lancelet genomes hold clues about the early evolution of vertebrates, by comparing genes from lancelets with the same genes in vetebrates, the change in gene function as vetebrates evolved can be discovered.

The genome of a few species in the genus Branchiostoma have been sequenced: B. floridae, B. belcheri, B. lanceolatum. In Asia, lancelets are harvested commercially as food for domesticated animals. In Japan, amphioxus has been listed in the registry of “Endangered Animals of Japanese Marine and Fresh Water Organisms.” Lancelets are distributed in shallow subtidal sand flats in temperate and tropical seas around the world. The only exception is Asymmetron inferum, a species known from the vicinity of whale falls at a depth of about 225 m. Although they are able to swim, adult amphioxi are benthic, they live in sandy bottoms whose granulometry depends on the species and the site, they are found half-buried in sand. When disturbed, they leave their burrow, swim a short distance, rapidly burrow again, posterior end first, into the sand. Adults can tolerate salinities as low as 6‰ and temperatures from 3 to 37 °C, their habitat preference reflects their feeding method: they only expose the front end to the water and filter-feed on plankton by means of a branchial ciliary current that passes water through a mucous sheet.

Branchiostoma floridae is capable of trapping particles from microbial to small phytoplankton size, while B. lanceolatum preferentially traps bigger particles. Lancelets are gonochoric animals, i.e. having two sexes, reproduce via external fertilization. They only reproduce during their spawning season, which varies between species - corresponding to spring and summer months. All lancelets species spawn shortly synchronously or asynchronously. Nicholas and Linda Holland were the first researchers to describe a method of obtaining amphioxus embryos by induction of spawning in captivity and in vitro fertilization. Spawning can be artificially induced in the lab by thermal shock; the first representative organism of the group to be described was Branchiostoma lanceolatum. It was described by Peter Simon Pallas in 1774 as molluscan slugs in the genus Limax, it was not until 1834 that Gabriel Costa brought the phylogenetic position of the group closer to the agnathan vertebrates, including it in the new genus Branchiostoma.

In 1836, Yarrel renamed the genus as Amphioxus, now considered an obsolete synonym of the genus Branchiostoma. Today, the term "amphioxus" is still used as a common name for the Amphioxiformes, along with "lancelet," in the English language. Observations of amphioxus anatomy began in the middle of the 19th century. First, the adult the embryonic anatomy were described. Alexander Kowalevsky first described the key anatomical features of the adult amphioxus. De Quatrefages first described the nervous system of amphioxus. Other important contributions to amphioxus adult anatomy were given by Heinrich Rathke and John Goodsir. Kowalevsky released the first complete description of amphioxus embryos, while Schultze and Leuckart were the first to describe the larvae. Other important contributions to amphioxus embryonic anatomy were given by Hatschek, Conklin and by Tung. Depending on the exact species involved, the maximum length of lancelets is 2.5 to 8 cm. Branchiostoma belcheri and B. lanceolatum are among the largest.

Except for the size, the species are similar in general appearance, differing in the number of myotomes and the pigmentation of their larvae. They without any paired fins or other limbs. A poorly developed tail fin is present, so they are not good swimmers. While they do possess some cartilage-like material stiffening the gill slits and tail, they have no true skeleton. In common with vertebrates, lancelets have a hollow nerve cord running along the back, pharyngeal slits and a tail that runs past the anus. Like vertebrates, the muscles are arranged in blocks called myomeres. Unlike vertebrates, the dorsal nerve cord is not protected by bone but by a simpler notochord made up of a cylinder of cells that are packed to form a toughened rod; the lancelet notochord, unlike the vertebrate spine, extends into the head. This gives the subphylum its name; the nerve cord is only larger in the head region than in

Linear circuit

A linear circuit is an electronic circuit which obeys the superposition principle. This means that the output of the circuit F when a linear combination of signals ax1 + bx2 is applied to it is equal to the linear combination of the outputs due to the signals x1 and x2 applied separately: F = a F + b F It is called a linear circuit because the output of such a circuit is a linear function of its inputs. An equivalent definition is that a linear circuit is a circuit in which, when a sinusoidal input voltage or current of frequency f is applied, any steady-state output of the circuit is sinusoidal with frequency f. Informally, a linear circuit is one in which the electronic components' values do not change with the level of voltage or current in the circuit. Linear circuits are important because they can amplify and process electronic signals without distortion. An example of an electronic device that uses linear circuits is a sound system. A linear circuit is one. Examples of linear circuits are amplifiers and integrators, linear electronic filters, or any circuit composed of ideal resistors, inductors, op-amps, other "linear" circuit elements.

Some examples of nonlinear electronic components are: diodes and iron core inductors and transformers when the core is saturated. Some examples of circuits that operate in a nonlinear way are mixers, rectifiers, radio receiver detectors and digital logic circuits. Linear circuits are important because they can process analog signals without introducing intermodulation distortion; this means that separate frequencies in the signal stay separate and do not mix, creating new frequencies. They are easier to understand and analyze; because they obey the superposition principle, linear circuits are governed by linear differential equations, can be analyzed with powerful mathematical frequency domain techniques, including Fourier analysis and the Laplace transform. These give an intuitive understanding of the qualitative behavior of the circuit, characterizing it using terms such as gain, phase shift, resonant frequency, bandwidth, Q factor and zeros; the analysis of a linear circuit can be done by hand using a scientific calculator.

In contrast, nonlinear circuits do not have closed form solutions. They must be analyzed using approximate numerical methods by electronic circuit simulation computer programs such as SPICE, if accurate results are desired; the behavior of such linear circuit elements as resistors and inductors can be specified by a single number. In contrast, a nonlinear element's behavior is specified by its detailed transfer function, which may be given by a curved line on a graph. So specifying the characteristics of a nonlinear circuit requires more information than is needed for a linear circuit. "Linear" circuits and systems form a separate category within electronic manufacturing. Manufacturers of transistors and integrated circuits divide their product lines into'linear' and'digital' lines. "Linear" here means "analog". Nonlinear elements such as transistors tend to behave linearly when small AC signals are applied to them. So in analyzing many circuits where the signal levels are small, for example those in TV and radio receivers, nonlinear elements can be replaced with a linear small-signal model, allowing linear analysis techniques to be used.

Conversely, all circuit elements "linear" elements, show nonlinearity as the signal level is increased. If nothing else, the power supply voltage to the circuit puts a limit on the magnitude of voltage output from a circuit. Above that limit, the output ceases to scale in magnitude with the input, failing the definition of linearity. Network analysis

Dorothy Thompson (historian)

Dorothy Katharine Gane Thompson was a social historian, a leading expert on the Chartist movement. Born in Greenwich, south-east London, Thompson entered Girton College, Cambridge, in 1942. During the war, her work as an industrial draughtswoman for Royal Dutch Shell interrupted her formal education. Nonetheless, she was politically active, she joined the Young Communists, married the historian Edward Thompson in 1948, moved to Halifax, where Edward worked in adult education and they were both active in the peace movement. They had three children. Kate Thompson, the award-winning children's writer, is their youngest child. With husband E. P. Thompson, she was part of the dissenting group in the Communist Party of Great Britain which in 1956-7 set up the socialist humanist journal the New Reasoner, where her competence meant her principal role was "business manager". While she found the break with the Communist Party painful, she was inspired working with writers, artists and trade unionists in the formation of new left clubs in many towns.

In 1970 Thompson was appointed a lecturer in the School of History at the University of Birmingham, where she remained until 1988. She was a visiting scholar on a number of occasions at universities in the United States, as well as in Canada and Japan; the Early Chartists was a groundbreaking collection of documents. The Chartists set out all the ways in which Thompson sought to revise how Chartism was seen - from the Irish leaders to the vital contribution of women. In January 1995 Thompson was presented with The Duty of Discontent. Edited by Owen Ashton, Stephen Roberts and Robert Fyson, the volume consists of 12 essays spanning the whole range of nineteenth- and twentieth-century British social history; the title was taken from a lecture by Chartist poet Thomas Cooper. The importance of Thompson's writings on Chartism and Irish and women's history is recognised by scholars internationally, her work, like that of her husband, was always been informed by a passionate radicalism and a deep sympathy for the underdog.

Thompson's position as the most influential historian of Chartism has been reinforced by two volumes of essays: Outsiders and The Dignity of Chartism. She was a leading member of the Communist Party Historians Group; the Early Chartists Bibliography of the Chartist Movement, 1837-1976 The Chartist Experience: Studies in Working-class Radicalism and Culture, 1830-60 Over Our Dead Bodies: Women against the Bomb The Chartists: Popular Politics in the Industrial Revolution. Selected Poems by Frank Thompson, edited by Dorothy Thompson and Kate Thompson The Dignity of Chartism: Essays by Dorothy Thompson; the Duty of Discontent, a festschrift edited by Owen Ashton and Stephen Roberts, New York: Mansell, ISBN 978-0-7201-2201-5 Obituary by Stephen Roberts, Labour History Review, vol. 76, n. 2, August 2011. "Dorothy Thompson", London Socialist Historians Group, 14 February 2011. Chartism & The Chartists, information & illustrations about the Chartists from Stephen Roberts