Phylum

There are 35 recognized animal phyla — from Porifera (sponges) to Chordata (vertebrates and relatives). Phyla represent the deepest animal body-plan divisions, most of which appeared during the Cambrian Explosion. Phylum membership has conservation implications: a single representative of an otherwise extinct phylum carries irreplaceable evolutionary history. Horseshoe crabs, sole survivors of their class, exemplify such phylogenetically unique taxa.