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Lampreys (sometimes also called lamprey eels) are any jawless fish of the order Petromyzontiformes. The adult lamprey may be characterized by a toothed, funnel-like sucking mouth. The common name "lamprey" is probably derived from Latin ''lampetra'', which may mean "stone licker" (''lambere'' "to lick" + ''petra'' "stone"), though the etymology is uncertain.〔(【引用サイトリンク】publisher=Oxford University Press )〕 Currently there are about 38 known extant species of lampreys. Although they are well known for boring into the flesh of other fish to suck their blood, in fact only a minority do so; only 18 species of lampreys are actually parasitic. The lampreys are a very ancient lineage of vertebrates, though their exact relationship to hagfishes and jawed vertebrates is still a matter of dispute. ==Characteristics== File:Lamprey anatomy.png|Basic external anatomy of a lamprey Adults physically resemble eels, in that they have no scales, and can range from 13 to 100 cm (5 to 40 inches) long. Lacking paired fins, adult lampreys have large eyes, one nostril on the top of the head, and seven gill pores on each side of the head. The unique morphological characteristics of lampreys, such as their cartilaginous skeleton, suggest they are the sister taxon (see cladistics) of all living jawed vertebrates (gnathostomes), and are usually considered the most basal group of the Vertebrata. Instead of true vertebrae, they have a series of cartilaginous structures called arcualia arranged above the notochord. Parasitic lampreys feed on prey as adults by attaching their mouthparts to the target animal's body, then using their teeth to cut through surface tissues until they reach blood and body fluid. Although attacks on humans do occur, they will generally not attack humans unless starved. Non-parasitic lampreys, which are usually freshwater species, do not feed as adults; they live off reserves acquired as ammocoetes (larvae), which they obtain through filter feeding. Hagfish, which resemble lampreys, have traditionally been considered the sister taxon of the true vertebrates (lampreys and gnathostomes) 〔 but DNA evidence suggests that they are in fact the sister taxon of lampreys. The pharynx is subdivided; the ventral part forming a respiratory tube that is isolated from the mouth by a valve called the velum. This is an adaptation to how the adults feed, by preventing the prey's body fluids from escaping through the gills or interfering with gas exchange, which takes place by pumping water in and out of the gill pouches instead of taking it in through the mouth. Near the gills are the eyes, which are poorly developed and buried under skin in the larvae. The eyes complete their development during metamorphosis, and in adults are covered by a thin and transparent layer of skin that becomes opaque in preservatives.〔(The Dissection of Vertebrates: A Laboratory Manual )〕 Lampreys provide valuable insight into adaptive immune systems, as they possess a convergently evolved adaptive immunity with cells that function like the T cells and B cells seen in higher vertebrates. Lamprey leukocytes express surface variable lymphocyte receptors (VLRs) generated from somatic recombination of leucine-rich repeat gene segments in a recombination activating gene-independent manner. Northern lampreys (Petromyzontidae) have the highest number of chromosomes (164-174) among vertebrates. Pouched lamprey (''Geotria australis'') larvae also have a very high tolerance for free iron in their bodies, and have well-developed biochemical systems for detoxification of the large quantities of these metal ions. 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「lamprey」の詳細全文を読む スポンサード リンク
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