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The Acorn worms or Enteropneusta are a hemichordate class of invertebrates consisting of one order of the same name.〔Van der Land, J., (Enteropneusta ), World Register of Marine Species, accessed on March 1, 2011 and October 6, 2012〕 They are closely related to the echinoderms. There are about 90 species of acorn worm in the world, the main species for research being ''Saccoglossus kowalevskii''. Until recently, it was thought that all species were infaunal benthos that either may be deposit feeders or suspension feeders. However, the last decade has seen the description of a new family, the Torquaratoridae, evidently limited to the deep sea, in which most of the species crawl on the surface of the ocean bottom and alternatively rise into the water column, evidently to drift to new foraging sites (Smith et al. 2005; Holland et al. 2005; Holland et al. 2009; Osborn et al. 2012, Priede et al. 2012).〔Smith KL, Holland ND, and Ruhl HA (2005) Enteropneust production of spiral fecal trails on the deep-sea floor observed with time-lapse photography.〕 〔Holland ND, Clague DA, Gordon DP, Gebruk A, Pawson DL, and Vecchione M (2005) "Lophenteropneust" hypothesis refuted by collection and photos of new deep-sea hemichordates. Nature 434: 374-376.〕 〔Holland ND, Jones WJ, Elena J, Ruhl HA, Smith KL (2009) A new deep-sea species of epibenthic acorn worm (Hemichordata, Enteropneusta). Zoosystema 31: 333—346.〕 〔Osborn KL, Kuhnz LA, Priede IG, Urata M, Gebruk AV, and Holland ND (2012) Diversication of acorn worms (Hemichordata, Enteropneusta) revealed in the deep sea. Proc. Roy. Soc. Lond. B 279: 1646—1654.〕 〔Priede IG, Osborn KJ, Gebruk AV, Jones D, Shale D, Rogacheva A, Holland ND (2012) Observations on torquaratorid acorn worms (Hemichordata, Enteropneusta) from the North Atlantic with descriptions of a new genus and three new species. Invert. Biol. 131: 244-257.〕 Some of these worms may grow to be very long; one particular species may reach a length of 2.5 metres (8.2 ft), although most acorn worms are much smaller. ==Anatomy== Most acorn worms range from in length, with the largest species, ''Balanoglossus gigas'', reaching or more. The body is made up of three main parts: an acorn-shaped proboscis, a short fleshy collar that lies behind it, and a long, worm-like trunk. The creature's mouth is located at the collar behind the proboscis. The skin is covered with cilia as well as glands that secrete mucus. Some produce a bromide compound that gives them a medicinal smell and might protect them from bacteria and predators. Acorn worms move only sluggishly, using ciliary action and peristalsis of the proboscis.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Acorn worm」の詳細全文を読む スポンサード リンク
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