What is the embryonic characteristic that evolutionarily makes echinoderms proximal to chordates?
Echinoderms and chordates are deuterostomes, i.e., in their embryonic development the blastopore turns into the anus. All other animals with complete digestive system are protostomes, i.e., their blastopore originates the mouth. The blastopore is the first opening of the digestive tube that appears in the embryonic development.
