any of various small burrowing marine animals of the genus Balanoglossus and related genera, having an elongated wormlike body with an acorn-shaped eversible proboscis at the head end: subphylum Hemichordata (hemichordates)
acorn worm in American English
noun
any of several burrowing, often brilliantly colored hemichordates of the class Enteropneusta, usually found in intertidal sand and mud, having an acorn-shaped proboscis and collar
Also: acornworm
Word origin
[1885–90]This word is first recorded in the period 1885–90. Other words that entered Englishat around the same time include: booster, lineup, retread, toner, twofer