Which feature is distinct to chordates?

Chordates are characterized by the presence of a notochord during some stage of their development. The notochord is a flexible, rod-like structure that provides support and serves as a precursor to the vertebral column in vertebrates. This defining feature is critical in the development of the nervous system and helps establish the basic body plan of chordates.

In terms of evolutionary biology, the notochord is an ancestral trait that sets chordates apart from other animal groups. While notochords are prominent in embryonic stages, they may be replaced by a backbone in adult forms, demonstrating the evolutionary significance of this structure.

Features such as a hard exoskeleton (common in arthropods) or segmentation (not exclusive to chordates, as seen in annelids) do not serve to distinctly identify chordates. Similarly, radial body symmetry is characteristic of other groups, such as cnidarians, rather than chordates, which typically exhibit bilateral symmetry. Thus, it is the presence of the notochord that distinctly defines chordates among animal phyla.