Abstract
ONE of the many puzzling features of the Echinodermata is their pentameral symmetry. The theory put forward by Nichols1 goes far towards accounting for the origin of pentamerism. Taking as his starting point a young echinoderm with its aboral surface covered only by an anal plate and a surrounding ring of plates, Nichols reasons that a strong outer covering (test) would be best developed if there were as few plates as possible in the circum-anal ring, with short sutures between each plate. An even number of plates would have a long suture of weakness cutting across the resultant test. With three plates, the sutures would be at 120°, again a weak arrangement. With five plates of uniform size, however, the sutures are few and short and each suture is opposite a solid plate.
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References
Nichols, D., Echinoderms (Hutchinson, 1962).
Raup, D. M., J. Geol., 67 (b), 661 (1959).
Raup, D. M., J. Paleont., 34, 5, 1041 (1960).
Berry, L. G., and Mason, B., Mineralogy (Freeman, 1959).
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COCKBAIN, A. Pentamerism in Echinoderms and the Calcite Skeleton. Nature 212, 740–741 (1966). https://doi.org/10.1038/212740a0
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DOI: https://doi.org/10.1038/212740a0
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