HEMICHORDA or HEMICHORDATA, a zoological term introduced by W. Bateson in 1884 as equivalent to Entero pneusta, which then included the single genus Balanoglossus, and now generally employed to cover a group of marine worm like animals probably related to the lower vertebrates. Verte brates, or Chordates, are distinguished by the presence of a notochord, which forms the longitudinal axis of the body; by the gill-slits in the pharynx; and by the hollow dorsal central nervous system (see VERTEBRATA). In the Hemichorda, the respiratory organs closely resemble gill-slits, and structures comparable with the notochord and the tubular dorsal nervous system are present.
The Hemichorda include two orders, the Pterobranchia (q.v.) and the Enteropneusta (see BALANOGLOSSUS), and are now gen erally regarded as a sub-phylum of the Chordata.