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The coccyx, commonly referred to as the tailbone, is the final segment of the human vertebral column. It is the region of the spine just below the sacrum.

Vestigial organs are homologous characters of organisms which have lost all or most of their original function in a species through evolution. They are remnants of organs that had important functions in ancestors. Eg. - Appendix in humans.

Morphology is the branch of biology that refers to the outward appearance (shape, structure, colour, pattern) of an organism or taxon and its component parts. It deals with the structure of animals and plants. This is in contrast to physiology, which deals primarily with function.