acorn worm

Any worm of the class Enteropneusta, phylum Hemichordata. They have a proboscis surrounded by a collar at their front end, which they use to dig burrows in sand and mud. They live, usually in U-shaped burrows, on the seashore and on the ocean bottom and filter their food out of the sea water. There are about 70 species, varying in length from 5 cm/2 in to 2 m/6.5 ft.