The larva of a lamprey (see cyclostomata). Ammocoetes have a slender elongate body, typically reaching 150–70 mm long, with nonfunctional eyes, a series of gill slits behind the head on either side, a horseshoe-shaped mouth with a prominent upper lip called the oral hood, and one or two shallow dorsal fins that merge into the caudal fin at the tail. They live in soft silty beds of rivers and streams for several years, filter feeding on microscopic particles by means of a network of tentacle-like cirri in the mouth entrance. Metamorphosis into the adult lamprey may be delayed, sometimes for many years.