The acorn worms or Enteropneusta are a hemichordate class of invertebrates consisting of one order of the same name. 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 lived in the sediment deep in the ocean, subsisting as 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). 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.

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