Phylum of ‘spiny-skinned’ invertebrate animals which are entirely marine. They are characterized by an internal skeleton of porous calcite plates; a pentameral symmetry (although a bilateral symmetry is often superimposed upon this radial plan, especially in many modern Echinoidea); and the presence of a water-based vascular system, a complex internal apparatus of fluid-containing tubes and bladders which pass through pores in the skeleton and are seen from the outside as tube feet. The phylum is varied, and includes Ophiuroidea (brittle stars), Asteroidea (starfish), Echinoidea (sea urchins), Holothuroidea (sea cucumbers), and Crinoidea (sea lilies); and the extinct Edrioasteroidea, Blastoidea, Cystoidea, and members of the subphylum Homalozoa (carpoids). Tribrachidium from the late Precambrian of Australia is probably an echinoderm, close to the stock from which the other groups evolved in the Cambrian and Ordovician. Echinoderms first appeared in the Lower Cambrian, but of the 20 classes known in the Palaeozoic, only six survive into the Mesozoic and on to the present day.