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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