What is the key distinguishing feature of deuterostomes?

The key distinguishing feature of deuterostomes is that the anus forms from the blastopore during embryonic development. This developmental pattern is characteristic of deuterostome animals, which includes groups such as chordates (including vertebrates) and echinoderms (like starfish).

In contrast to protostomes, where the blastopore typically develops into the mouth, deuterostomes exhibit this unique embryological process where the initial opening becomes the anus, and the mouth forms later from a separate opening. This distinction not only sets deuterostomes apart from protostomes but also plays a fundamental role in the overall body plan and development of these organisms.

The other options do not accurately reflect this critical characteristic. Radial symmetry is not exclusive to deuterostomes, as some protostomes also exhibit radial symmetry, and claiming that they are exclusively marine is inaccurate since many deuterostomes, such as mammals and birds, inhabit terrestrial environments.