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The following HTML text is provided to enhance online readability. Many aspects of typography translate only awkwardly to HTML. Please use the page image as the authoritative form to ensure accuracy. annelids was similar, with a hydrostatic hemocoel; these forms are probably among trace makers of the late Precambrian. Cell-phenotype numbers in living phyla, and a model of cell-phenotype number increase, suggest an origin of metazoans near 600 my ago, followed by a passive rise in body-plan complexity. Living phyla appearing during the Cambrian explosion have a Hox/HOM gene cluster, implying its presence in the common ancestral trace makers. The explosion required a repatterning of gene expression that mediated the development of novel body plans but evidently did not require an important, abrupt increase in genomic or morphologic complexity. I thank Dave Jablonski (University of Chicago) and Allen Collins and Chris Meyer (University of California, Berkeley) for valuable discussions and reviews of the manuscript, and Clint Turbeville for insights into coelom development. This research was based on work supported by Grant EAR-9196068, National Science Foundation, and by Faculty Research grants, University of California, Berkeley. ReferencesAkam, M. (1987) The molecular basis for metameric pattern in Drosophila embryos. Development 101, 1–22. Anderson, D. T. (1973) Embryology and Phylogeny of Annelids and Arthropods (Pergamon, Oxford). Backeljau, T., Winnepenninckx, B. & De Bruyn, L. (1993) Cladistic analysis of metazoan relationships: a reappraisal. Cladistics 9, 167–181. Ballard, J. W. O., Olsen, G. J., Faith, D. P., Odgers, W. A., Rowell, D. M. & Atkinson, P. W. (1992) Evidence from 12S ribosomal RNA sequences that onychophores are modified arthropods. Science 258, 1345–1348. Barnes, R. S. K., Callow, P. & Olive, P. J. W. (1993) The Invertebrates, 2nd. Ed. (Blackwell Sci., Oxford). Bartels, J. L., Murtha, M. T. & Ruddle, F. H. (1993) Multiple Hox/HOM -class homeobox genes in Platyhelminthes. Mol. Phylog. Evol. 2, 143–151. Bengtson, S. & Conway Morris, S. (1992) Early radiation of biomineralizing phyla. In Origin and Early Evolution of the Metazoa, eds. Lipps, J. H. & Signor, P. W. (Plenum, New York), pp. 448–481. Bengtson, S., Matthews, S. C. & Missarzhevsky, V. V. (1986) The Cambrian netlike fossil Microdictyon. In Problematic Fossil Taxa, Hoffman, A. & Nitecki, M. H. (Oxford Univ. Press, Oxford), pp. 97–113. Bergström, J. (1989) The origin of animal phyla and the new phylum Procoelomata. Lethaia 22, 259–269. Bonner, J. T. (1965) Size and Cycle (Princeton Univ. Press, Princeton, NJ). Bowring, S. A., Grotzinger, J. P., Isachsen, C. E., Knoll, A. H., Pelechaty, S. M. & Kolosov, P. (1993) Calibrating rates of Early Cambrian evolution. Science 261, 1293–1298. Briggs, D. E. G. & Collins, D. (1988) A Middle Cambrian chelicerate from Mount Stephen, British Columbia. Palaeontology 31, 779–798.
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