Petromyzontomorphi (pet-ro-mi-ZON-to-MORF-i) is a combination of three Greek roots that mean rock [petra (πέτρα)] sucking [myzo (μυζώ)] forms [morphi (μορφή)].  The name refers to the way that lampreys attach to stones, particularly in streams.

Like hagfish, lampreys are jawless, armorless fish without paired appendages.  However, their mouth is rigid and conical with a rasping tongue at the apex.  Many lampreys are "parasitic" (really predatory), and use their mouths to attach to their prey, usually fish, after which they use their tooth-covered tongue to dig into the bodies of their prey.  They draw water into a common respiratory tube which exits through numerous gill openings.  This mechanism allows them to continue to draw water across their gills while they are feeding (at which time their mouth is completely occluded).  

Lampreys breed in freshwater, where they may spend all or some of their lives.  The ammocetes larva is a small, amphioxus-like stage that is free-living in streams.  Usually, they return to the sea where they become sexually mature.

Lampreys, though superficially similar to the hagfish (Myxinomorphi) differ from them in several important ways, chief among them is the occurrence of vertebrae in lampreys.  They also differ in the ways they draw water into the pharynx and across the gills.  Furthermore, lampreys have two semicircular canals in the middle ear.

Lampreys were once thought to be living descendants of armored jawless fishes (see the Anaspidomorphi for a discussion of this); however, their developmental history and fossil history (Mississippian to the present) suggest that they are primitive rather than being secondarily reduced.  Benton (2005) suggests that the lampreys are sisters to the Conodontomorphi (see the cladogram of the craniate classes based on Benton 2005), and the most basal of all the vertebrate groups. Either way, they are the most basal group of living vertebrates.