Deuterostomes form the second division of triblastics, the first and the most important in terms of phyla is the division of protostomes.
What distinguishes deuterostomes is the order in which the mouth and anus appear during embryonic development. As opposed to protostomes, the orifice (blastopore) formed during one of the first embryological stages (gastrulation) forms the opening of the anus, while a secondary orifice forms the mouth.
The mouth of these organisms therefore appears second, hence their name deutero-, second, -stome, mouth.
Deuterostomes have fewer phyla than protostomes, but are very diversified. Echinoderms and vertebrates are part of this group.
This crinoid is an echinoderm and therefore a deuterostome. © danielguip CC by-nc-sa 2.0