What is Hemichordata?
{{Taxobox | image = Eichelwurm.jpg | image_caption = Acorn worm, a hemichordate. | image_width = 220px | fossil_range = [[Cambrian]] – [[Holocene]] | regnum = [[Animal]]ia | subregnum = [[Eumetazoa]] | branch = [[Bilateria]] | superphylum = [[Deuterostomia]] | phylum = '''Hemichordata''' | phylum_authority = [[William Bateson|Bateson]], 1885 | subdivision_ranks = Classes | subdivision = *'''Graptolithina''' *'''Enteropneusta''' *'''Pterobranchia''' }} The '''Hemichordata''' is a [[Phylum (biology)|phylum]] of worm-shaped [[Ocean|marine]] [[deuterostome]] [[animal]]s. They are probably the sister group of the [[echinoderm]]s. They date back to the Lower or Middle [[Cambrian]] and include an important class of [[fossil]]s called '''graptolites''', most of which became extinct in the [[Carboniferous]].<ref>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. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC18258/]</ref><ref>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.</ref><ref>Cameron C.B. 2005. A phylogeny of the hemichordates based on morphological characters. ''Canadian Journal of Zoology''. '''83''': 196-2</ref> Living hemichordates are a phylum with two classes: the enteropneusts and the pterobranchs. Since the hemichordates are the closest living relatives to the [[chordate]]s, they are of great interest to those studying the origins of chordate development. ==Structure== The bodies of Hemichordates are divided into three parts, [[proboscis]], collar and trunk. They have open [[circulatory system]]s and a complete [[digestive tract]] but the musculature in their gut is very poorly developed, and food is mostly transported through it by using the [[Cilium|cilia]] that cover its inside s