A phylum of marine invertebrates that includes the sea urchins, starfish, brittlestars, and sea cucumbers. Echinoderms have an exoskeleton (test) of calcareous plates embedded in the skin. In many species (e.g. sea urchins) spines protrude from the test. A system of water-filled canals (the water vascular system) provides hydraulic power for thousands of tube feet: saclike protrusions of the body wall used for locomotion, feeding, and respiration. Echinoderms have a long history: fossils of primitive echinoderms are known from rocks over 500 million years old. Like chordates, they are deuterostomes. Extant classes include the Crinoidea (sea lilies), Holothuroidea (sea cucumbers), Echinoidea (sea urchins), and Stelleroidea (starfish and brittlestars).
http://tolweb.org/Echinodermata Overview of echinoderm characteristics and phylogeny from the Tree of Life project
Subjects: Biological Sciences.