All chordates possess the following characteristics at one stage of their life or another:

• A notochord, or, a backbone that provides sites for muscle attachment.
• A hollow dorsal nerve cord. Usually lying along the notochord, one end of this structure usually develops into a brain.
• Pharyngeal gills. Located in the space behind the mouth, these gill slits can either be functional, or merely cosmetic.
• A tail. Pretty straightforward.

Many thanks to wharfinger.

Ver"te*brate (?), n. Zool.

One of the Vertebrata.

© Webster 1913.

Ver"te*brate (?), Ver"te*bra`ted (?), a. [L. vertebratus.]

1. Anat.

Having a backbone, or vertebral column, containing the spinal marrow, as man, quadrupeds, birds, amphibia, and fishes.

2. Bot.

Contracted at intervals, so as to resemble the spine in animals.

Henslow.

3. Zool.

Having movable joints resembling vertebrae; -- said of the arms ophiurans.

4. Zool.

Of or pertaining to the Vertebrata; -- used only in the form vertebrate.

© Webster 1913.