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Lampreys are anadromous or fresh water, eel-shaped jawless fishes. They can be readily recognized by the large, rounded sucker which surrounds their mouth and by their single "nostril" on the top of their head. The skin of lampreys is entirely naked ans slimy, and their seven gill openings extend behind the eyes. Whether marine or fresh water, lampreys always spaw and lay eggs in brooks and rivers. During most of their life (about seven years), they are larval; then they undergo a metamorphose and become an adult. Anadromous lampreys, when adult, return to the sea, where they become mature, and live there for one or two years. Then they return to rivers, reproduce and generally die. Many lampreys are parasites. They attach on other fishes by means of their sucker, scrape their skin with their rasping tongue, and suck their blood. All lampreys, however can also feed on small invertebrates. The sucker is also for them a means to travel upstreams in rivers. They use it to attach on stones to rest (Petromyzon, the name of the European lamprey, means "stone sucker") or on more powerful fishes which trail them. Although lampreys are sometimes regarded as a delicacy and fished in Europe, the main cause of their disappearance is water pollution, to which they (in particular larvae) are particularly sensitive.
Recent lampreys, or Petromyzontiformes, include ten genera: Ichthyomyzon, Petromyzon, Caspiomyzon, Geotria, Mordacia, Eudontomyzon, Tetrapleurodon, Entosphenus, Lethenteron, and Lampetra. Lampreys have an amphitropical distribution and are restricted to relatively cold waters. Geotria and Mordacia are the only lampreys of the southern hemisphere, all other genera live in the northern hemisphere.