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Hemichordata is a phylum of deuterostome animals, generally considered the sister group of our own, the chordates. They date back to the Lower or Middle Cambrian and include an important class of fossils called graptolites, most of which became extinct in the Carboniferous. One surviving species is known, Cephalodiscus graptolitoides .

All other modern forms are marine worms, commonly called acorn worm s. They are divided into two classes: the Enteropneusta and the Pterobranchia .