Vertebrates are members of the subphylum Vertebrata (within the phylum Chordata), specifically, those chordates with backbones or spinal columns. About 57,739 species of vertebrates have been described. Vertebrates started to evolve about 530 million years ago during the Cambrian explosion, which is part of the Cambrian period. Their name derives from the bones of the spinal column (or vertebral column), the vertebrae. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Fish (including lampreys, but traditionally not hagfish, though this is now disputed), amphibians, reptiles, birds, and mammals (including humans) are vertebrates. Characteristics of the subphylum are a muscular system that mostly consists of paired masses, as well as a central nervous system which is partly located inside the backbone (if one is present). Usually, the defining characteristic of a vertebrate is considered the backbone or spinal cord, a brain case, and an internal skeleton, but the latter do not hold true for lampreys, and the former is arguably present in some other chordates. Rather, all vertebrates are most easily distinguished from all other chordates by having an unequivocal head, that is, sensory organs - especially eyes are concentrated at the fore end of the body and there is pronounced cephalization.