Vertebrata (subphylum)

A subphylum including include fishes, reptiles, amphibians, birds, and mammals. They are characterized by a bony or cartilaginous endoskeleton that includes a spinal column surrounding and virtually replacing the notochord as the chief stiffener of the body in locomotion. They also typically have a general body plan consisting of a head, trunk, two pairs of appendages, a postanal tail (which may be highly modified in many vertebrates and sometimes absent), bilateral symmetry, a muscular system attached to the skeleton, a digestive system with large glands, a ventral heart with two to four chambers, blood with red and white corpuscles, and two sexes that each have paired gonads. Vertebrates originated around 500 million years ago during the Cambrian period.