Petromyzontiformes, commonly known as lampreys, are a group of jawless fish belonging to the class Agnatha. They are characterized by their elongated bodies, round mouths filled with sharp teeth, and a lack of paired fins. Lampreys are primarily found in freshwater and marine environments, where they can be either parasitic or non-parasitic. These creatures have a unique life cycle that includes a larval stage known as ammocoetes, which burrow into sediment. As adults, some species attach to other fish using their suction-cup-like mouths to feed on blood and tissues, while others do not feed at all during their short adult lives.