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BILATERAL SYMMETRY. "It is achieved by the intersection of two orthogonal axes of polarity: the anterior-posterior (A-P) axis and the dorsal-ventral (D-V) axis." https://lnkd.in/dph5vzr View in LinkedIn

AXIS GENES. "Hox genes play a conserved role in patterning the A-P axis of bilaterians. Hox genes are expressed in staggered axial domains along the oral-aboral axis of cnidarians, suggesting that Hox patterning of the primary body axis was already present in the cnidarian-bilaterian ancestor. Dpp plays a conserved role patterning the D-V axis of bilaterians." https://lnkd.in/dph5vzr View in LinkedIn

EVIDENCE FOR INVERSION. "The "dorsal-ventral inversion" hypothesis failed to gain broad acceptance until 1995, when homologous D-V patterning mechanisms were found to be operating in vertebrates and insects." https://lnkd.in/dph5vzr View in LinkedIn

DORSAL-VENTRAL INVERSION HYPOTHESIS. "This reversal in the relative location of the CNS led Geoffroy St-Hilaire to propose in 1822 that the dorsal-ventral axes of vertebrates and arthropods are homologous but reversed in their typical orientation to the substratum. In other words, dorsal in vertebrates corresponds to ventral in arthropods." https://lnkd.in/dph5vzr View in LinkedIn

CNS ASYMMETRY. "A hallmark of dorsal-ventral polarity in many bilaterians is the asymmetrical location of the central nervous system. In vertebrates, the CNS is located dorsally with the gut and major vessels located more ventrally. In arthropods the CNS is located ventrally." https://lnkd.in/dph5vzr View in LinkedIn

SYMMETRY FLAW. "Many sea anemones (class Anthozoa, order Actinaria) exhibit bilaterality. Like Hydra, the external appearance of a sea anemone is that of a simple cylinder with a single central axis of polarity, the oral-aboral axis. The external radial symmetry is nearly perfect, but it is disrupted by the slit-shaped mouth and external openings on the oral disk." https://lnkd.in/dph5vzr View in LinkedIn

Modern sword in the stone. View in LinkedIn

ASSUMPTION BUSTED. "However, careful morphological examination reveals instances of asymmetry, biradiality, tetraradiality, or even bilaterality in the internal anatomy of cnidarians." https://lnkd.in/dph5vzr View in LinkedIn

CNIDARIAN RADIAL ASSUMPTION. "Since Cuvier erected the “Radiata” as one of four metazoan “embranchments” cnidarians have generally been described as radially-symmetrical animals. Indeed, many cnidarians exhibit radial symmetry. The body plan of Hydra is a simple cylinder, radially symmetrical about the primary body axis, the oral-aboral axis." https://lnkd.in/dph5vzr View in LinkedIn

SEPARABLE MULTIGENIC AXES. "The bilateral symmetry of Bilateria is a complex multigenic trait achieved by the intersection of two axes of polarity: a primary body axis (the anterior-posterior axis) and a secondary, orthogonal axis (the dorsal-ventral axis). These two axes are logically and evolutionarily separable." https://lnkd.in/dph5vzr View in LinkedIn