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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 worms. They are divided into two classes: the Enteropneusta and the Pterobranchia.
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