Class Osteichthyes

Osteichthyes (Greek osteon, bone; ichthys, fish) includes both marine and freshwater fishes with a bony endoskeleton and a typically spindle-shaped (fusiform) body — the largest and most diverse class of fish. The skin is covered by ganoid, cycloid or ctenoid scales depending on the species.

Respiration is carried out by four pairs of filamentous gills, each protected on either side by a bony flap called the operculum — a key structural difference from the cartilaginous fishes. Many bony fishes possess an air bladder, which may or may not connect to the gut; it aids gas exchange in lungfishes and helps most ray-finned fishes control their buoyancy in the water column. The heart is ventrally placed and two-chambered. Excretory organs are mesonephric kidneys, and bony fishes are ammonotelic (excreting nitrogenous waste mainly as ammonia, suited to their aquatic environment). A well-developed lateral line sense organ, used to detect water movement and pressure changes, runs along the body. …