one of the grand divisions of the animal kingdom, comprising all animals that have a backbone composed of bony or cartilaginous vertebrae, together with amphioxus in which the backbone is represented by a simple undivided notochord. the vertebrata always have a dorsal, or neural, cavity above the notochord or backbone, and a ventral, or visceral, cavity below it. the subdivisions or classes of vertebrata are mammalia, aves, reptilia, amphibia, pisces, marsipobranchia, and leptocardia.