Classification Hemichordate

amplexograptus, graptolite hemichordate, ordovician near caney springs, tennessee.

hemichordata divided 2 classes: enteropneusta, commonly called acorn worms, , pterobranchia, may include graptolites. third class, planctosphaeroidea, proposed based on single species known larvae. phylum contains 120 living species. hemichordata appears sister echinodermata ambulacraria; xenoturbellida may basal grouping. pterobranchia may derived within enteropneusta, making enteropneusta paraphyletic. possible extinct organism etacystis member of hemichordata, either within or close affinity pterobranchia.

there 130 described species of hemichordata , many new species being discovered, in deep sea.