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The arch 0-hypothesis. A caricature of a segmented worm-like ancestor, as described by de Beer (1937). c, cartilaginous gill support; Evo, evolution; m, mouth; s, gill slit; T, trabecula, supporting the brain and evolving from the arch-0 cartilage. More anatomically detailed drawings of the hypothetical ancestor also are available from other sources, notably Bjerring 1977; Jollie 1984; Kuratani et al. 1997. These drawings differ substantially from one another.
