Hemichordata

Hemichordata Temporal range: Cambrian – Holocene
Acorn worm, a hemichordate.
Scientific classification
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Superphylum:	Deuterostomia
Phylum:	Hemichordata Bateson, 1885
Classes
Graptolithina (extinct) Enteropneusta Pterobranchia

The Hemichordata is a phylum of worm-shaped marine deuterostome animals. They are probably the sister group of the echinoderms.

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.^[1][2][3]

Living hemichordates are a phylum with two classes: the enteropneusts and the pterobranchs. Since the hemichordates are the closest living relatives to the chordates, they are of great interest to those studying the origins of chordate development.

1. ↑ Cameron C.B; Swalla B.J. and Garey J.R. 2000. Evolution of the chordate body plan: new insights from phylogenetic analysis of deuterostome phyla. Proceedings of the National Academy of Sciences (USA) 97: 4469-4474. [1]
2. ↑ Winchell C.J. et al 2002. Evaluating hypotheses of deuterostome phylogeny and chordate evolution with new LSU and SSU ribosomal DNA data. Molecular Biology and Evolution. 19: 762-776.
3. ↑ Cameron C.B. 2005. A phylogeny of the hemichordates based on morphological characters. Canadian Journal of Zoology. 83: 196-2