Defining characteristics: • A complex series of fluid-filled canals (the water vascular system) derived from a pair of coelomic compartments and which service numerous flexible feeding and locomotory appendages (tube feet). • Pentamerous radial symmetry • Calcaraneous ossicles derived from mesodermal tissue forming an endoskeleton. • Connective tissue is mutable: its stiffness and fluidity can be rapidly and dramatically altered by the nervous system
General characteristics: • Exclusively Marine • Body unsegmented • Adults with pentamerous secondary radial symmetry • Larvae bilaterally symmetrical • With unique water vascular system • Types include sea stars, sea urchins, sea cucumbers, sand dollars.
The Body Plan of Echinoderms • During their free-living larval stage, they are bilaterally symetrical. • In their adult stage, echinoderm species show a form of radial symmetry, called pentaradial symmetry, in which body is arranged into five or multiple of five. • As radially symmetrical animals, they lack cephalization. Thus, adult echinoderms do not have anterior and posterior ends. Instead, body surfaces are designated as being oral (bearing the mouth) or aboral (not bearing the mouth).
The endoskeleton • Most members of the echinoderm posses a well-develop internal skeleton (endoskeleton) composed largely (up to 95%) of calcium carbonate, with smaller amounts of magnesium carbonate (up to 15%), even lesser amounts of other salts and trace metals, and a small amount