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SYNOPSIS. Understanding the evolutionary origin of novel metazoan body plans continues to be one of the most sought after answers in biology. Perhaps the most profound change that may have occurred in the Metazoa is the appearance of bilaterally symmetrical forms from a presumably radially symmetrical ancestor. The symmetry properties of bilaterally symmetrical larval and adult metazoans are generally set up during the cleavage period while most "radially" symmetrical cnidarians do not display a stereotyped cleavage program. Ctenophores display biradial symmetry and may represent one intermediate form in the transition to bilateral symmetry. The early development of cnidarians and ctenophores is com- pared with respect to the timing and mechanisms of axial determination. The origin of the dorsal-ventral axis, and indeed the relationships of the major longitudinal axes, in cnidarians, ctenophores, and bilaterian animals are far from certain. The realization that many of the molecular mechanisms of axial determination are conserved throughout the Bilateria allows one to formulate a set of predictions as to their possible role in the origins of bilaterian ancestors.

The Development of Radial and Biradial Symmetry: The Evolution of Bilaterality