Acorn worms are marine, worm-like animals, most of which live in sand or mud burrows. Acorn worms are members of the phylum Hemichordata, which includes two classes—the Enteropneusta (the acorn worms) and the Pterobranchia (pterobranchs). The acorn worms include four genera, Balanoglossus, also known as tongue worms; Glossobalanus, Ptychodera, and Saccoglossus. Acorn worms generally vary in size from 1 to 39 in (1–100 cm) in length. The body of acorn worms consists of a proboscis, a collar, and a trunk. The proboscis is very muscular and primarily used for digging. Together with the collar it resembles an acorn, hence its name. The trunk is usually substantially longer than the proboscis and collar segments.
Embryos of hemichordates share similarities with both the phylum Echinodermata (starfish and sand dollars) and with the phylum Chordata (cephalochordates and vertebrates). The larvae of the subphylum Cephalochordata, which includes Amphioxus, closely resemble the larvae of Hemichordates. This suggests that the Hemichordata may have given rise to the Chordata and, therefore, vertebrates.
The phylum Chordata is characterized by a dorsal, hollow nerve cord, a notochord, pharyngeal gill slits or pouches, and a coelom, the fluid-filled main body cavity. Acorn worms resemble chordates in that these worms have pharyngeal gill slits, a nerve cord, and a coelom. A small structure in the anterior trunk was once thought to be a notochord, but it has been shown to be an extension of the gut.