Echinoderms have a skeleton composed of numerous plates of mineral calcium carbonate (calcite). Part of the body cavity, or coelom, is a water-vascular system, consisting of fluid-filled vessels that are pushed out from the body surface as tube feet, papillae, and other structures that are used in locomotion, feeding, respiration, and sensory perception. The conspicuous five-rayed, or pentamerous, radial symmetry of living echinoderms tends to obliterate their fundamental bilateral symmetry.
Many of the earliest echinoderms either lacked symmetry or were bilaterally symmetrical. Bilateral symmetry occurs in all living groups and is especially marked in the larval stages. A tendency toward …(100 of 8684 words)