Movable, non- animal posterior that extents well beyond the alimentary canal.
The movability of the post- comes from post-anal muscles in combination with a flexibility. That is, while there are non-chordate animals that certainly possess post-anal extensions, those extensions tend to not be muscular.
Alternatively, a number of chordates have tails that are reduced to the point of no longer being external to the animal, e.g., such as especially the great apes.
The post-anal muscular tail is one of four defining chordate characteristics. The other three are a dorsal hollow nerve cord, notochord, and pharyngeal slits (or