List several characteristics of class Chondrichthyes that separate it from class Agnatha and class Osteichthyes.

The Chondrichthyes are the cartilaginous fishes including the sharks, rays, skates and chimaeras. The Osteichthyes are the bony fish. Both of these groups became established during the Devonian Period about 400 million years ago. The Chondrichthyes differ from the Osteichthyes in many respects, including their skeletons, scales, teeth, buoyancy, swimming, gills and reproduction.