Question

why are echinoderms considered closer to chordates than any other phylum?

Answer 1

Echinoderms are closest to chorddates because
1. They are coelomates
2. They have digestive tract
3. Their endoderm is mostly made up of Calcium and thus it is similar to the cartiage or bone skeleton.

Answer 2

Echinoderms are closer to Chordates than any other phylum:

Chordates and echinoderms are both classified as deuterostomes unlike most phyla, which are categorized as protostomes.

During embryonic development, the first aperture that is formed in deuterostomes is a primitive anus. Alternatively, in protostomes the first aperture, also known as a blastopore, develops to form the mouth. Both echinoderms and chordates show this characteristic trait.

Thus, it is assumed that the two might have a common ancestor and are considered closer than any other phylum.