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The lancelets (from "lancet") — also known as amphioxi (singular, amphioxus) — comprise some 32 species〔(【引用サイトリンク】title="Cephalochordata: EoL species" )〕 of fish-like marine chordates in the order Amphioxiformes, with a global distribution in shallow temperate (as far north as Scotland) and tropical seas, usually found half-buried in sand. They are the modern representatives of the subphylum Cephalochordata, formerly thought to be the sister group of the craniates. In Asia, they are harvested commercially as food for humans and domesticated animals. They are an important object of study in zoology as they provide indications about the evolutionary origins of the vertebrates. Lancelets serve as an intriguing comparison point for tracing how vertebrates have evolved and adapted. Although lancelets split from vertebrates more than 520 million years ago, their genomes hold clues about evolution, particularly how vertebrates have employed old genes for new functions.〔(Worm-like Marine Animal Providing Fresh Clues About Human Evolution ) Newswise, Retrieved on July 8, 2008.〕 They are regarded as similar to the archetypal vertebrate form. 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 (hagfish and lampreys), including it in the new genus ''Branchiostoma'' (from the Greek, branchio = "gills", stoma = "mouth").〔Garcia‐Fernàndez, J., & Benito‐Gutiérrez, È. (2009). It's a long way from amphioxus: descendants of the earliest chordate. ''Bioessays'', 31(6), 665-675, ().〕 In 1836, Yarrel renamed the genus as ''Amphioxus'' (from the Greek: "pointed on both sides"), 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", especially in the English language. A non-technical review of all aspects of lancelet biology is: Stokes, M. D. and Holland, N. D. 1998. American Scientist 86: 552-560. The genome of the Florida lancelet (''Branchiostoma floridae'') has been sequenced. == Physical features == Lancelets are typically long, or at the longest. They have a translucent, somewhat fish-like body, but without any paired fins or other limbs. A relatively poorly developed tail fin is present, so they are not especially good swimmers. While they do possess some cartilage-like material stiffening the gill slits, mouth, 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. Also 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 closely packed to form a toughened rod. The lancelet notochord, unlike the vertebrate spine, extends into the head. This gives the subphylum its name (''cephalo-'' meaning 'relating to the head'). The nerve cord is only slightly larger in the head region than in the rest of the body, so that lancelets do not appear to possess a true brain. However, developmental gene expression and Transmission electron microscopy indicate the presence of a diencephalic forebrain, a possible midbrain, and a hindbrain. Lancelets have two known kinds of light-sensing structures, Joseph cells, and Hesse organs, as well as a "frontal eye" and lamellar body. The frontal eye is widely thought to be light-sensitive, although this has not been confirmed by electrophysiological measurement. The lamellar body, a possible homologue of the pineal body, was once thought to be light-sensitive, although this has been called into question. All of these organs and structure are located in the neural tube, with the frontal eye at the front, followed by the lamellar body, the Joseph cells, and the Hesse organs. Lancelets also have oral ''cirri'', thin tentacle-like strands that hang in front of the mouth and act as sensory devices and as a filter for the water passing into the body. Water passes from the mouth into the large pharynx, which is lined by numerous gill-slits. The ventral surface of the pharynx contains a groove, called the endostyle, which, connected to a structure known as Hatschek's pit, produces a film of mucus. Ciliary action pushes the mucus in a film over the surface of the gill slits, trapping suspended food particles as it does so. The mucus is collected in a second, dorsal, groove, and passed back to the rest of the digestive tract. Having passed through the gill slits, the water enters an atrium surrounding the pharynx, then exits the body via the ''atriopore''.〔 The remainder of the digestive system consists of a simple tube running from the pharynx to the anus. A single blind-ending caecum branches off from the underside of the gut, with a lining able to phagocytize the food particles, a feature never found in vertebrates. It used to be thought that this hepatic caecum might be homologous to the liver of vertebrates, but this is now thought to be less likely.〔 Lancelets have no respiratory system, breathing solely through their skin, which consists of a simple epithelium. Despite the name, little if any respiration occurs in the gill slits, which are solely devoted to feeding. The circulatory system does resemble that of primitive fish in its general layout, but is much simpler, and does not include a heart. There are no blood cells, and no haemoglobin.〔 The excretory system consists of segmented "kidneys" containing protonephridia instead of nephrons, and quite unlike those of vertebrates. Also unlike vertebrates, there are numerous, segmented gonads.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「lancelet」の詳細全文を読む スポンサード リンク
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