acorn barnacle - Acorn Barnacle; acorn barnacle - A popular name for the genus balanus of the Crustacea known as cirripedia. Common objects of the seaside, they are enclosed in a shell. but in the larval state the young at first swim freely about, developing, after several moults, a sucker by which they adhere to any convenient object. These acorn barnacles encrust rocks and shels. One species attaches itself to the skin of the whale, and another burrows into the skin. But they are not true parasites, since, unlike these, they do not derive sustenance from their host.