Hemichordata

Hemichordata is a phylum of bilaterally symmetrical marine deuterostome animals generally considered the sister group of the echinoderms, first appearing in the fossil record in the Lower or Middle Cambrian. It includes two main living classes, the worm-shaped acorn worms of Enteropneusta and the small, mostly colonial, tube-dwelling Pterobranchia, the latter of which now also encompasses the graptolites, once thought entirely extinct but represented today by the living genus Rhabdopleura.