Final answer:
Echinoderms form their anus first, which is a characteristic of deuterostomes.
Step-by-step explanation:
Echinoderms are deuterostome marine organisms. They form their anus first, which means that the blastopore becomes the anus. This is in contrast to protostomes, where the blastopore becomes the mouth. The development of the anus first is a characteristic of deuterostomes, which include echinoderms and chordates.