Chondrichthyes

Chondrichthyes, one of the three classes of living fishes (see Fish), characterized by a cartilaginous skeleton. Sharp, toothlike scales known as placoid scales are common to all species of Chondrichthyes and are sometimes modified into spines, as in stingrays. The teeth, which are also modified scales, are usually not fused to the jaw, and are progressively shed and replaced. The swim bladder, which gives bony fishes their buoyancy, is absent in members of Chondrichthyes; the light weight of their cartilaginous skeletons helps keep Chondrichthyes afloat. Some cartilaginous fishes have additional buoyancy modifications, including forms that provide hydraulic lift while swimming; other species are bottom-dwelling. Chimaeras, sharks, skates, and rays all belong to the class Chondrichthyes.