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any of the soft-bodied invertebrates of the class Enteropneusta, phylum Hemichordata. The front end of these animals is shaped like an acorn, hence their common name. The "acorn" consists of a proboscis and a collar that may be used to burrow into soft sand or mud. The animals vary in length from about 5 cm (about 2 inches) in certain Saccoglossus species to more than 180 cm (about 6 feet) in Balanoglossus gigas.