Petromyzontida

The order comprising the lampreys, sometimes called Petromyzontiformes, which are eellike, jawless vertebrates (class Agnatha). Lampreys differ from Myxiniformes as follows: the single nasal opening is on the dorsal side of the head and ends internally as a blind sac; the mouth is surrounded by a circular oral disk and provided with a rasping tongue (both disk and tongue are set with horny teeth); seven pairs of gill pouches open separately to the exterior; two pairs of semicircular canals are present; adults have well-developed eyes; dorsal and caudal fins are separate, and both are supported by fin rays; there are separate sexes and a distinct larval stage; and they either live in fresh water or enter fresh water to breed if they live in the sea. See also Myxiniformes.

After they have metamorphosed to adults, most lampreys aggressively attack other fish to which they attach themselves by suction, using their oral disks; then they rasp through skin and scales with their tongues and suck the blood and flesh of the host. This parasitic habit has resulted in serious damage to commercial fisheries. Lampreys are rarely used as food.

The Petromyzontida have a worldwide distribution and are divided into eight Recent genera, of which Petromyzon and Lampetra are well known. Two genera of fossil lampreys are currently known. Details of the structure of the four genera indicate a relationship to the Osteostraci and Anaspida, extinct ostracoderms of the Silurian and Devonian periods, from which they may be descended. See also Anaspida; Osteostraci.