The title of this article may appear to only invite a tautology (a worm is a worm) or a mere example (an earthworm is a worm). But tautologies and examples make a good definition not, just as bizarrely worded sentences make not eloquent constructions. So, what is a worm? The matter is not exactly simple, and the answer to this deceptively easy question will prove very surprising to some, as a worm is not anything at all, yet it is. Allow us to explain.