lamprey name for several primitive marine and freshwater fishes of the order Cyclostomata, or jawless fishes (see cyclostome ). As in the other member of the order, the hagfish , the adult lamprey retains the notochord , the supporting structure that in higher vertebrates is found only in the embryo. An ancient fish that still resembles fossils that are 360 million years old, the lamprey lacks a sympathetic nervous system, a spleen, and scales. Most adult lampreys are parasitic, sucking the blood of other fishes. The horny teeth, set in the circular, jawless mouth, attach to the prey and the lamprey feeds as it is carried along. Lampreys have an anticoagulant in the saliva that keeps the blood of the victim fluid. Some freshwater lampreys eat flesh as well as blood. Lampreys resemble eels in external appearance and, although not related to the true eels, are sometimes called lamprey eels. When not attached to prey, they swim with undulating movements. The marine lampreys normally migrate into freshwater to spawn, and some populations have become landlocked in freshwater. The sexes are separate in lampreys and fertilization is external. The parents die shortly after the eggs are deposited in a nest. The larvae, called ammocoetes, are about 1/4 in. (6 mm) long. They are transparent, eyeless filter-feeders and live in muddy river bottoms, eating particles of organic matter. Ammocoetes are used in zoology courses to demonstrate a theoretically primitive vertebrate...