And it was originally the last cranial nerve in vertebrates, before the first spinal nerves have been incorporated in the cranium as the nerves XI and XII.

It and several of the nerves preceding it, which originally innervated the branchial arches, remain different in nature from the following nerves (spinal nerves + cranial nerves XI and XII), whose original main purpose was to innervate the somatic muscles. This difference is more important than whether the nerves happen to exit the central nervous systems between adjacent vertebrae or by holes through the cranium (which are the result of the first vertebrae becoming fused to the occipital bone).

The nervus vagus and its associated ganglia of the vertebrates has a correspondent in the nervous system of most animals, including the arthropods, as a vegetative nervous system of the gastro-intestinal tract, which is connected to the central nervous system from the head, e.g. with the tritocerebrum of the arthropods.