What key characteristic defines the Phylum Chordata?

The defining characteristic of the Phylum Chordata is the presence of both a notochord and a backbone (vertebral column). The notochord is a flexible, rod-like structure that provides support and serves as a precursor to the vertebral column in vertebrates. This trait is essential in the early development of chordates, occurring during embryonic development.

Most species within the Chordata phylum are vertebrates, which possess a backbone that protects the spinal cord and provides structural support. Although some chordates, such as tunicates and lancelets, do not develop a backbone as adults, they do exhibit a notochord at some stage of their life cycle, making them members of the Chordata phylum.

The other choices highlight different biological features. For example, exoskeletons and segmentation are characteristics of the Phylum Arthropoda, radial symmetry is common in Phylum Cnidaria and Echinodermata, and jointed appendages are specific to arthropods as well. Thus, the presence of a backbone and notochord distinctly identifies the members of Chordata.