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[?].