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Skeleton components arranged efficiently: starfish


Sand dollars at Monterey Aquarium / Brandon Fick / LicenseCC-by - Attribution

The skeletons of some echinoderms arrange their calcium carbonate plates efficiently using pentaradial symmetry.

"Starfish have five arms; sand dollars have five radial food grooves on their undersides--this arrangement of five elements radiating from a center point ('pentaradial symmetry') is widespread among the echinoderms but unknown elsewhere in nature…Early echinoderms were covered with a skeleton made up of discrete plates of calcium carbonate. Now one can pave a floor with triangles, squares, or hexagons, but using pentagons alone inevitably leaves gaps. One can't make an array of squares close on itself to form a hollow solid unless at eight special locations the apices of three rather than four squares touch, a distinct complication. And one can't make any array solely of hexagons close on itself at all. Conversely one can get a closed, space-enclosing structure from triangles (tetrahedrons are the simplest, but others such as twenty-sided icosahedrons are possible) and pentagons (the simplest being the twelve-sided dodecahedron). Among the pentagons (fig. 4.13) hexagons can be intercalated practically without limit, but twelve basic pentagons must remain. In the most symmetrical arrangement, these pentagons are in six pairs with members of a pair at the opposite extremities of the solid. If we run an axis between members of one pair, the ten other pentagons then arrange themselves in two nearly equatorial rings. If enough hexagons are intercalated, these can form the key elements of five arms. And a look at any book treating the paleontology of echinoderms reveals a host of hexagonal plates. Perhaps a pentaradial symmetry is, in fact, a 'natural' or easy way to organize a radially symmetrical creature built of a shell of little solid elements!" (Vogel 2003:87-88)
About the inspiring organism
Med_2101950173_f05f7ac9bd_b Echinodermata

Learn more at
Organism/taxonomy data provided by:
Species 2000 & ITIS Catalogue of Life: 2008 Annual Checklist

Bioinspired products and application ideas

Application Ideas: Materials or packaging that incorporate the 'soccer ball' structure.

Industrial Sector(s) interested in this strategy: Materials science, packaging

Vogel S. 2003. Comparative Biomechanics: Life's Physical World. Princeton: Princeton University Press. 580 p.
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