Cartilaginous fish
Sharks, rays, skates and chimaeras, whose skeletons are made of cartilage rather than bone.
Also: Chondrichthyes
Cartilaginous fish have skeletons of cartilage, the flexible material of the human nose and ear, often hardened with calcium. There are roughly 1,200 living species: the sharks, the rays and skates, and the strange deep-sea chimaeras or ratfish. They have no swim bladder, and many float with the help of a large, oily liver. Their teeth are replaced throughout life, and their skin is covered in placoid scales. Most grow slowly and have few young, so they recover slowly from fishing, which is why many are now threatened.