The Osteichthyes are the bony fish, a class of the Chordata (see chordate) that includes the Acanthodii, Crossopterygii, Dipnoi (see lungfish), and Actinopterygii. Members of the class Osteichthyes are found in almost every environment. Their characteristics include a bony skeleton, a single flap (operculum) covering the gill openings, and erythrocytes (red blood cells) with nuclei. Most members of this class have scales. Osteichthyes first appeared during the Devonian period, when they were heavily armored and lived in freshwater.