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The Echinodermata is a phylum of exclusively marine invertebrates with a fossil record extending back to the Precambrian. The echinoderm clade is supported by a number of unique features, including:
- a water vascular system
- a system of endoskeletal elements (arising from mesoderm) composed of a calcareous meshwork called stereom
- basic 5-part symmetry (often modified later in ontogeny) expressed in the development of 5 primary lobes in the hydrocoel of the rudiment
- construction of ambulacra by the addition of new plates adjacent to the terminal (or ocular) plate that marks the ends of the radial canals
- a diffuse sub-epithelial nervous system