Lampreys mate in a nestlike depression in gravel beds of streams, where they deposit numerous eggs, about 1 mm (0.04 inch) in diameter, lodge in the gravel around the nest. Eggs hatch into ammocoete larvae—a blind, wormlike animal that burrows in silt. After about three years, when the ammocoete undergoes a radical metamorphosis, in which the eyes complete their development, the upper lip becomes transformed into a suctorial oral disk, the endostyle (a mucus-secreting organ in the pharynx) changes into a thyroid gland, and the fins along the back increase in height.
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