Petromyzones

(lampreys), a subclass of vertebrates of the class Cyclostomata; it comprises a single order. The single nostril is on the upper part of the head and is not joined to the pharynx cavity. The rnouth is at the bottom of the sucking disk; both the disk and the tongue have horny teeth. There are seven gill pouches, each having a separate opening to both a branchial artery and the outer environment. The circulatory system is closed; the heart is two-chambered. The body measures 15–100 cm long.

Lampreys reproduce in rivers: small lampreys deposit 2,000–3,000 eggs, middle-sized lampreys up to 40,000 eggs, and large sea lampreys (Petromyzon marinus) up to 240,000 eggs. After spawning the adults die. Ammocoetes larvae emerge from the eggs. Species that inhabit brooks spend their entire life cycle there. Migratory species travel to the sea, where they live for several years attached to other fishes, upon whose blood and muscle tissue they feed. They return to rivers to spawn.

There are about 30 species of Petromyzones, distributed in temperate waters of both the northern and southern hemispheres and in the basin of the Arctic Ocean. Nine species are found in the USSR. The river lamprey (Lampetra fluviatilis), the arctic lamprey (L. japonica\\ and the Caspian, or Volga, lamprey (Caspiomyzon wagneri) have commercial value.