Echinoderm skeleton. There is a fascinating tension in this animal pattern, as evolution has pulled the shape first to 5-symmetry (as with most of the phylum, including starfish and sea-lilies), then seemingly to roundness and then towards bilateral symmetry (2-sidedness, like most animals). Why? What are the roots of these patterns? Roundness seems best for things that do not move; 2-sidedness for things that have a definite direction of movement - a head and a tail end - and 5-symmetry perhaps for strength of interlocking plates in things that move slowly and in any direction. The reasons must be ancient: sea-lilies are found in Jurassic rocks.
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