Which characteristic is unique to the phylum Chordata?

The characteristic that is unique to the phylum Chordata is the presence of a notochord at some stage of development. The notochord is a flexible, rod-like structure that provides support and defines the primitive axis of the body. In most vertebrates, the notochord is replaced by the backbone during development, but its presence during embryonic stages is a defining feature of all chordates. This characteristic distinguishes chordates from other animal phyla, which may have different types of support structures or none at all.

In contrast, while jointed appendages and segmented bodies are features found in various groups within the animal kingdom, they are not exclusive to chordates. For example, jointed appendages are common in arthropods, and segmented bodies can be observed in annelids. Similarly, a hard exoskeleton is primarily a characteristic of arthropods, again not unique to chordates. Thus, the presence of a notochord is a key trait that sets chordates apart from other animal phyla.