CEPHALOCHORDATE

also called Acrania, or lancelet, any member of the subphylum Cephalochordata of the phylum Chordata. Small, fishlike marine invertebrates, they probably are the closest living relatives of the vertebrates. There are about 20 species in two families, each with a single genus. Branchiostoma was formerly called Amphioxus, a name that is retained as an informal term. The other genus is Epigonichthys, also called Asymmetron. The genus Asymmetron is sometimes retained for some species. Additional reading E.J.W. Barrington, The Biology of Hemichordata and Protochordata (1965), is an account of the lower chordates and their evolution; N.J. Berrill, The Origin of Vertebrates (1955), argues the thesis that the urochordate larva represents the prototype from which cephalochordates and vertebrates are derived; A. Willey, Amphioxus and the Ancestry of the Vertebrates (1894), an early but good comprehensive account, presents the orthodox theory of chordate relationships; R.P.S. Jefferies, The Ancestry of the Vertebrates (1986), expounds an alternate theory of chordate origin; and Libbie H. Hyman, The Invertebrates, vol. 5, Smaller Coelomate Groups (1959), is a classic work treating the hemichordates in extensive detail. Later works include Charles K. Weichert and William Presch, Elements of Chordate Anatomy, 4th ed. (1975); and R. McNeill Alexander, The Chordates, 2nd ed. (1981); supplemented by Brian Bracegirdle and Patricia H. Miles, An Atlas of Chordate Structure (1978). N.J. Berrill, The Tunicata with an Account of the British Species (1950, reprinted 1968), a taxonomic survey with a useful section on tunicate biology; Pierre P. Grasse (ed.), Trait de zoologie: anatomie, systmatique, biologie, vol. 11, Echinodermes, stomocords, procords (1966), an advanced zoological treatise devoted to protochordates, with good illustrations; W.A. Herdman, Tunicata (Ascidians and Their Allies) and Cephalochordata, in The Cambridge Natural History, vol. 7, pp. 35138 (1904), an important general account; R.N. Millar, The Marine Fauna of New Zealand: Ascidiacea (1982), a morphological account of a single species of ascidian; Willard G. Van Name, The North and South American Ascidians, Bulletin of the American Museum of Natural History, vol. 84 (1945). For a later treatment, see Invertebrate Chordates: Tunicates and Lancelets, in Vicki Pearse et al., Living Invertebrates (1987). Michael T. Ghiselin