Hemichordates

Hemichordates

Hemichordata is a phylum of marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta (acorn worms), and Pterobranchia. A third class, Planctosphaeroidea, is known only from the larva of a single species. The extinct class Graptolithina is closely related to the pterobranchs.

Acorn worms are solitary worm-shaped organisms. They generally live in burrows and are deposit feeders, but some species are pharyngeal filter feeders. Pterobranchs are filter-feeders, mostly colonial, living in a collagenous tubular structure called a coenecium.