A vestigial structure is an anatomical feature that has lost most or all of its original function through evolution. The human appendix is widely considered a vestigial organ, as it is a remnant of a larger structure (the cecum) used by herbivorous ancestors to digest tough plant materials like cellulose. In modern humans, it has no essential function in digestion, although some research suggests it may play a minor role in the immune system or in maintaining gut bacteria.

The elephant trunk is a highly functional and essential organ, used for breathing, smelling, touching, grasping, and drinking. It is a vital adaptation, not a vestigial structure.

In most birds, wings are essential, functional structures used for flight. While flightless birds like ostriches have vestigial wings in terms of flight function, the general statement "Bird wings" refers to a functional structure, not a vestige.

Gills are vital, functional organs that fish use to extract oxygen from water to breathe. They are essential for survival in their aquatic environment and are not vestigial.