Sharks, which comprise only about 1 percent of all living fishes, are highly evolved representatives of a large and unique group of fishes - the cartilaginous fishes, or class Chondrichthyes.
Sharks are jawed fishes. Like all fishes they are aquatic, water-breathing vertebrates with a brain and spinal cord; fins; plate-like pairs of internal gills; and paired sense organs. Unlike most bony fishes, sharks do not have swim bladders or lungs. Sharks' jaws are simple but effective structures, armed with transverse rows of hard teeth that are replaced slowly but continuously. These teeth are not in sockets, but attached to the jaws by soft tissue.
All sharks have paired fins - pectoral and pelvic - on the underside of the body. The pectoral, or breast, fins are located just behind the gill region of the head. The pelvic fins are on the rear of the abdomen just in front of the tail. Most sharks have a cylindrical or slightly depressed head and body; a strong tail with caudal, or tail, fin; one or two dorsal fins on their back; and an anal fin on the underside of the tail behind the vent.
Also see Great White Sharks.