What is the defining feature of organisms in the phylum Echinodermata?

The defining feature of organisms in the phylum Echinodermata is radial symmetry, particularly in their adult forms. Echinoderms, such as starfish, sea urchins, and sea cucumbers, typically exhibit a body plan that can be divided into similar halves around a central axis. This characteristic is especially evident in the arrangement of their arms or body sections. While many organisms indeed have bilateral symmetry during some stages of their lives, echinoderms' adult body structure is characteristically radially symmetrical, which is significant for their ecological roles and movement in marine environments.

The options of bilateral symmetry, aquatic habitat, and exoskeleton do not uniquely define echinoderms. Although echinoderms do inhabit aquatic environments, many other organisms also share this trait. While they possess a calcareous endoskeleton made up of ossicles, this feature is not the primary distinguishing characteristic of the phylum compared to their radial symmetry.