An internal, bony skeleton is also common and provides sites for muscle attachment for efficient movement. Vertrbrates have a vertebral column that replaces the notochord in adults and surrounds the dorsal nerve cord.
Vertebrates also have a distinct head.
There are seven classes of living vertebrates, three of them fishes and four of them terrestrial tetrapods. Advanced features of bony fish include a bony endoskeleton, modified gill arches, and internal air bladders for balance and buoyancy. Although sharks are built for speed, the maneuverability of bony fishes is much greater. Gills are protected by a movable fill cover called an operculum. Running along each side and branching over the head of most fishes is a lateral-line system consisting of sensory pits in the skin.