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Parasitic lampreys are divided into three groups; blood feeders, flesh feeders and intermediaries or a combination of the two. Flesh feeders have fewer teeth than blood feeders which can have over 100. Lampreys are perfectly adapted to their needs and so efficient little has changed in 360 million years. There are at least 40 species of Lampreys, ranging in size from  the 12 cm or 4.45 inch long  Lampetra planeri, to the 90 centimeter or 35.5 inch Petromyzon marinus. Lampreys typically live 7 to 9 years, but they can live as many as 11  to as few as 6 years. They are scaleless and only have posterior tail and caudal fins. Some blood feeders produce larger amounts of lamphredin to prevent coagulation of the blood and breakdown of tissue. Newly born lampreys called ammocoets have no eyes and spend a large of their early life in the river they were born in. Upon reaching mature form, after several years as ammocoets they leave the river for the ocean. Upon reaching sexual maturity, after years in the sea they return to rivers to prepare nests.  The lamprey's cloaca (reproductive opening) stay open after reproduction allowing killing bacteria to enter, so they die after reproduction.

Nonparasitic lampreys such as Ichthyomyzon fosser are less known. Like their parasitic cousins they begin there lives as filter feeding ammocoets, but they do not feed as adults.  As adults most nonparasitic forms are small,  usually less than 23 cm or 8 inches long, they develop eyes and a fleshy oral disk like other lampreys but they lack a functional digestive tract. There are teeth in the oral disk of nonparasitic lampreys, but they are less specialized and simpler in design. There are exceptions to the rule, Caspiomyzon wagneri is significantly larger and it feeds on fish carrion and plant matter.

The large fleshy oral disk or mouth works like a suction cup, and a strongly muscled tongue like plunger creates suction.  The epidermal teeth are embedded  into the host locking the lampreys mouth in position, and three rake like teeth on the plunger rasp away flesh. Lampreys also use there mouths as a hold-fast in flowing water, they lock down on rocks and other things to stay in position. Lampreys are jawless, and they have cartilage structures supporting the oral disk and much of the head. The individual gill pouches are arranged along the respiratory chamber, each with it's own external opening. When attached or feeding, a flap separates the respiratory camber and water flows directly through the gill pouches as it enters and exits the gill slits. Lampreys have a simple digestive system, no more than a esophagus and intestine.

Fossil lampreys are not common but they have been recorded from as far back as the Famennian period of the Devonian. Priscomyzon riniensis is a small lamprey from the Devonian, and three, Mayomyzon, Pipiscius and Hardistiella are known the Carboniferous. Cretaceous rocks from Mongolia yielded Mesomyzon mengae, an exceptional lamprey fossil that has the same characteristics as it's modern cousins. Dental arrangements in fossils are only known from the 4.2 cm long (1.58 inch) Devonian specimen, a ring of 14 simple conical teeth surrounding the mouth on a large oral disk. Priscomyzon riniensis is the only fossil preserved in a completely ventral plane possibly because of it's large disk, other lamprey fossils are found on lateral planes without clear dental structures. Recent research, has shown lampreys decay  quickly in stagnant water, specific structures  are lost quickly leaving indistinguishable remains.

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