Popularly included among fishes, the lampreys and hags of the class Cyclostomata (roundmouths) differ from true fishes by the possession of a suctorial mouth devoid of functional jaws, by the single olfactory organ, and by the absence of lateral appendages, or paired fins. They have an eellike form and method of travel, and some species are a yard in length. They are bisexual, discharging both eggs and milt into the water to become fertilized by accidental contact. Lampreys ascend the rivers to spawn, however, and there make little heaps of pebbles, carried and piled with the mouth, in which the eggs find some protection from the many egg-eaters in all streams. Most, if not all, of the migratory parents die after spawning. From the eggs hatch larvæ that undergo a metamorphosis. Lampreys live on small crustaceans, worms, and so forth, eat carrion, and also attack living fishes. The tongue, like the interior of the mouth, is armed with teeth. They are in the habit of attaching themselves to stones in order to hold themselves against a river current, breathing meanwhile by taking water directly into the pouchlike gill chambers and expelling it, instead of sucking it through the mouth and passing it out of the gill slits. In ancient Rome the big sea lampreys of the Mediterranean were eaten as a delicacy, and even cultivated in landlocked ponds, and they are still highly prized in some parts of Europe.

The hags are an even more primitive group of cyclostomes that live in the mud of shallow seas and are too abundant on both our coasts, where they are a pest of the fisheries. Their general habits are similar to those of lampreys, but wherever possible they attach themselves to fish on which they feed. The hag is particularly destructive to fishes caught on "set lines" of hooks, or in nets, and the loss thus resulting on the coasts of California, in Japan, and in some European fisheries is very serious. As these cyclostomes have no scales or other hard parts to be preserved except a few teeth, no fossil remains are certainly known, but it is the opinion of paleontologists that otherwise the class might be traced to the earliest Paleozoic time.