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the larva of any of various lampreys (Petro-myzones). In some species the ammocoetes is the same length as the adult form. The body is eel-like, and the mouth is in the form of a triangular cleft. There are no teeth, and the eyes are underdeveloped. The gills are interior, and the fins are unpaired and poorly developed. Ammocoetes live in rivers, streams, and brooks. They spend much of their lives buried in the soil. The larvae feed on small invertebrates and detritus. At the age of three to six years they undergo metamorphosis and become adults.