Petromyzontida is a class of jawless fish commonly known as lampreys. They are characterized by their elongated bodies, a round mouth 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, feeding on the blood and tissues of other fish or on organic matter. These creatures have a unique life cycle that includes a larval stage called ammocoetes, which can last several years in freshwater habitats. Adult lampreys migrate to spawn in rivers, where they lay eggs. After hatching, the young lampreys remain in the sediment until they mature, continuing the cycle of life.