The acorn worms or Enteropneusta are a hemichordate class of invertebrates. The closest non-hemichordate relatives of the Enteropneusta are the echinoderms. There are 111 known species of acorn worm in the world, the main species for research being Saccoglossus kowalevskii. Two families—Harrimaniidae and Ptychoderidae—separated at least 370 million years ago.

Until recently, it was thought that all species lived in the sediment on the seabed, subsisting as deposit feeders or suspension feeders. However, the early 21st century 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. It is assumed that the ancestors of acorn worms used to live in tubes like their relatives Pterobranchia, but that they eventually started to live a safer and more sheltered existence in sediment burrows instead. The body length normally range from 2 centimetres (0.79 in) to 2.5 metres (8 ft 2 in) (Balanoglossus gigas), but one species, Meioglossus psammophilus, only reach 0.6 millimetres (0.024 in). Due to secretions containing elements like iodine, the animals have an iodoform-like smell.

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