Echinodermata
Echinoderms
Phylum recognized by
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General Description
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Pressure allows movement: echinoderms
Source and Additional Information
Legs and tubes in echinoderms such as starfish allow movement and feeding by use of hydrostatic pressure.
"Their [echinoderms'] bodies work by unique exploitation of hydrostatic principles. Feet, each a thin tube ending in a sucker and kept firm by the pressure of water within, wave and curl in rows along the arms. The water for this system circulates quite separately from that in the body cavity. It is drawn through a pore into a channel surrounding the mouth and circulated throughout the body and into the myriads of tube feet. When a drifting particle of food touches an arm, tube feet fasten on to it and pass it on from one to another until it reaches the gutter that runs down the upper surface of the arm to the mouth at the centre." (Attenborough 1979:49)
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References
- Attenborough, David. 1979. Life on Earth. Boston, MA: Little, Brown and Company. 319 p.
Tube feet attach in marine environment: echinoderms
Source and Additional Information
The tube feet of echinoderms attach to surfaces via suction adhesion.
"Besides mollusks, echinoderm tube feet make use of suction adhesion, as do a wide variety of other aquatic systems--either as the only attachment mechanism or in combination with others. Among terrestrial systems one thinks first of wet ones--frogs for instance. But the mechanism finds use even in arboreal mammals." (Vogel 2003:427)
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References
- Steven Vogel. 2003. Comparative Biomechanics: Life's Physical World. Princeton: Princeton University Press. 580 p.
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Skeleton components arranged efficiently: starfish
Source and Additional Information
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)
Learn more about this functional adaptation.
References
- Steven Vogel. 2003. Comparative Biomechanics: Life's Physical World. Princeton: Princeton University Press. 580 p.
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Diversity
Source and Additional Information
Echinodermata has approximately 7000 described living species and about 13,000 extinct species known from the fossil record. This phylum is the largest without any freshwater or terrestrial forms. (Brusca and Brusca, 2003; Waggoner, 1999)
References
- Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc..
- Waggoner, B. 1999. "Introduction to the Echinodermata" (On-line). Accessed January 16, 2005 at http://www.ucmp.berkeley.edu/echinodermata/echinodermata.html .
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"Echinodermata". Encyclopedia of Life, available from "http://www.eol.org/pages/1926". Accessed
21 Mar 2010.
