HEMICHORDA, or Hemichordata, a zoological term introduced by W. Bateson in 1884, without special definition, as equivalent to Enteropneusta, which then included the single genus Balanoglossus, and now generally employed to cover a group of marine worm-like animals believed by many zoologists to be related to the lower vertebrates and so to represent the invertebrate stock from which Vertebrates have been derived. Vertebrates, or as they are sometimes termed Chordates, are distinguished from other animals by several important features. The chief of these is the presence of an elastic rod, the notochord, which forms the longitudinal axis of the body, and which persists throughout life in some of the lowest forms, but which appears only in the embryo of the higher forms, being replaced by the jointed backbone or vertebral column. A second feature is the development of outgrowths of the pharynx which unite with the skin of the neck and form a series of perforations leading to the exterior. These structures are the gill-slits, which in fishes are lined with vascular tufts, but which in terrestrial breathing animals appear only in the embryo. The third feature of importance is the position of structure of the central nervous system, which in all the Chordates lies dorsally to the alimentary canal and is formed by the sinking in of a longitudinal media dorsal groove. Of these structures the Vertebrata or Craniata possess all three in a typical form; the Cephalochordata (see Amphioxus) also possess them, but the notochord extends throughout the whole length of the body to the extreme tip of the snout; the Urochordata (see Tunicata) possess them in a larval condition, but the notochord is present only in the tail, whilst in the adult the notochord disappears and the nervous system becomes profoundly modified; in the Hemichorda, the respiratory organs very closely resemble gill-slits, and structures comparable with the notochord and the tubular dorsal nervous system are present.
The Hemichorda include three orders, the Phoronidea (q.v.), the Pterobranchia (q.v.) and the Enteropneusta (see Balanoglossus), but the relationship to the Chordata expressed in the designation Hemichordata cannot be regarded as more than an attractive theory with certain arguments in its favour. (P. C. M.)