Lampreys are a very ancient and primitive group of jawless vertebrates. Rare fossil remains from over 300 million years ago
strongly suggest that today's lampreys have changed little over this time.
Most species of lamprey are parasites and have long, eel-like bodies that lack scales. They use their jawless mouths to attach to a host fish by suction before sucking out the living tissues. There are nearly 50 species of lamprey, most of whom spend their lives out at sea and return to freshwater only to spawn.
Lampreys live mostly in coastal and fresh waters, although some species, (e.g. Geotria australis, Petromyzon marinus, Entosphenus tridentatus) travel significant distances in the open ocean, as evidenced by their lack of reproductive isolation between populations. They are found in most temperate regions except those in Africa. Their larvae (ammocoetes) have a low tolerance for high water temperatures, which may explain why they are not distributed in the tropics.
Adults physically resemble eels, in that they have no scales, and can range anywhere from 13 to 100 centimetres (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 of all living jawed vertebrates (gnathostomes), and are usually considered the most basal group of the Vertebrata.They will generally not attack humans unless starved.