ECHINODERMATA Description: Ossicles are not solid, but have a sponge-like microstructure called stereom that is unique to the phylum. Embryologically, echinoderm ossicles are a true endoskeleton, since they are produced by mesenchymal cells and are usually covered by epidermis. Water vascular system, mutable collagenous tissue.Pentaradial body organization in adults (radially symmetrical).