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, which went extinct in the Carboniferous. Modern forms are marine worms, commonly called acorn worms. They are divided into two classes: the Enteropneusta and the Pterobranchia.
