Echinoderms are diverse, commonly fossilized, marine animals. Living echinoderms include starfish, sea cucumbers, and sea urchins. Fossil forms are even more disparate. Echinoderms possess a number of distinctive features.
1 Water vascular system: this unique internal mechanism controls most echinoderm activity. Sea water is transported within the animal through a system of radial canals. Tube feet are the main organs for locomotion. They are also adapted for food collection, respiration, and can act as sensory tentacles.
2 Endoskeleton: echinoderms have an internal skeleton (the test) formed of small porous plates (ossicles). Each ossicle is a single crystal of high-magnesium calcite formed from a complex three-dimensional framework of rods (stereom). Spaces within this lattice are filled with soft tissue. Echinoderm ossicles are easy to identify in thin section as the presence of the soft tissue gives them a speckled appearance, and the single crystals of calcite forming individual ossicles have uniform optical properties. Ossicles are joined by living tissue to form the test, allowing the animal to grow without molting.
Fossils at a Glance, 2nd edition. By Clare Milsom and Sue Rigby. Published 2010 by Blackwell Publishing, ISBN 978-1-4051-9336-8 (pb)
3 Symmetry: many echinoderms have fivefold symmetry (Fig. 7.1), although some groups have an imposed, secondary bilateral symmetry. This radial symmetry is usually associated with animals that have evolved from sessile groups that collect food from all sides. Why echinoderms evolved a five-rayed (pentaradiate) symmetry is not known.
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