Zoology 10th Edition by Stephen Miller,John Harley

Exercise 10

The clade Ambulacraria and the phylum Chordata comprise the Deuterostome lineage. The ancestral deuterostome probably used pharyngeal slits in filter feeding. Pharyngeal slits were lost early in the echinoderm lineage. Ancient echinoderm fossils are most similar to members of the class Crinoidea. The form and function of the water-vascular system in crinoids probably reflect its ancestral form and function. The mouthdown orientation of most modern echinoderms is probably an adaptation to predatory and scavenging lifestyles. Echinoids and holothuroids are closely related; however, other evolutionary relationships within the phylum are debated.
Why are the crinoids considered the group of extant echinoderms most closely related to the echinoderm ancestors?