Chordates are defined as organisms that possess a structure called a notochord, at least during some part of their development. The notochord is a rod that extends most of the length of the body when it is fully developed. Lying dorsal to the gut but ventral to the central nervous system, it stiffens the body and acts as support during locomotion. Other characteristics shared by chordates include the following (from Hickman and Roberts, 1994):
- bilateral symmetry
- segmented body, including segmented muscles
- three germ layers and a well-developed coelom.
- single, dorsal, hollow nerve cord, usually with an enlarged anterior end (brain)
- tail projecting beyond (posterior to) the anus at some stage of development
- pharyngeal pouches present at some stage of development
- ventral heart, with dorsal and ventral blood vessels and a closed blood system
- complete digestive system
- bony or cartilaginous endoskeleton usually present.