Hemichordata

Classification

Hemichordata are divided into two classes: the Enteropneusta, commonly called acorn worms, and the Pterobranchia, which includes the graptolites. A third class, Planctosphaeroidea, is proposed based on a single species known only from larvae. The phylum contains about 120 living species. Hemichordata appears to be sister to the Echinodermata as Ambulacraria; Xenoturbellida may be basal to that grouping. Pterobranchia may be derived from within Enteropneusta, making Enteropneusta paraphyletic. It is possible that the extinct organism Etacystis is a member of the Hemichordata, either within or with close affinity to the Pterobranchia.