Hemichordata
. Acorn Worm, common name for simple, wormlike marine animals in the hemichordate phylum. They are of special interest because of their close relationship to chordates . This connection is evident in the adult anatomy. Some representative acorn worms have gill slits, traces of a supporting structure resembling a notochord, and a tubular nerve cord, which are features characteristic of vertebrates . The larval stages of acorn worms, however, are very much like those of echinoderms such as starfish, indicating a remote common ancestry of echinoderms and vertebrates. The hemichordates are divided into two classes comprising about 50 species. The first class, the acorn worms, consists of animals that average 10 cm (4 in) in length, although some species may be up to 1.5 m (up to 5 ft) long. They construct burrows, commonly U-shaped, in sand of shallow seafloors using an extendable, muscular proboscis attached to a thick collar that resembles an acorn—hence the name. They secrete a slime t