Class Chondrichthyes consists of the cartilaginous fish, fish with a flexible skeleton made of cartilage instead of bone. Only their teeth and sometimes the vertebrae have calcium, like boned animals have. These fish are very good predators, although they feed on anything from mollusks to large fish. Sharks, one type of cartilaginous fish, have a good sense of smell but poor eyesight. To make up for this, they have a lateral line system, a line of sensory organs running down the side of the body that can detect very small changes in water pressure, suggesting a swimming fish nearby. Skates and rays are also examples of cartilaginous fish, a group that consists of about 1,000 living species.