Vertebrate (thing)

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.