Acorn worms are fragile tube worms that live in sand or mud burrows in the intertidal areas of the world's oceans. Acorn worms are members of the phylum Hemichordata, which includes two classes—the Enteropneusta (acorn worms) and the Pterobranchia (pterobranchs). Acorn worms, also known as tongue worms, belong to one of four genera, Balanoglossus, Glossobalanus, Ptychodera, and Saccoglossus. They are mostly burrowing animals that vary in size from 1 to 39 in (1 to 100 cm) in length (Balanoglossus gigas). The body of acorn worms consists of proboscis, collar, and trunk. The proboscis is a digging organ and together with the collar (and a lot of imagination) it resembles an acorn, hence its name.
The embryos of the hemichordates show affinities with both the phylum Echinodermata (starfish and sand dollars) and with the phylum Chordata (which includes the vertebrates). The relationships between these phyla are tenuous and are not demonstrable in all forms. The larvae of the Chordata subphylum Cephalochordata, which includes Amphioxus, resemble the larvae of the Hemichordata, indicating that the Hemichordata may have given rise to the Chordata, and therefore the 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.
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