Perch are bony fish covered in thick scales that help them reduce friction and protect their soft tissues while swimming swiftly. They have two eyes without eyelids, four pairs of gills protected by an operculum that forces water over the gills, and five types of fins. Perch also possess a lateral line to detect pressure changes, internal organs accessed through a side window, and complex digestive, circulatory, and reproductive systems aided by a swim bladder for buoyancy.