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Echinodermata is an entirely marine taxon, occurring throughout the world’s oceans and includes sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers, and sea lilies. Echinoderms are tricoelomate deuterostomes, with a simple hemal/excretory system, decentralized nervous system, and (in most species) separate sexes, with planktonic larvae settling to become benthic adults.
1. Pentamerous symmetry in adults, though larvae are bilaterally symmetrical.
2. Hard endoskeleton of calcareous ossicles, either loosely connected (e.g., brittle stars) or tightly bound (e.g., sea urchins).
3. Water vascular system involved in feeding, locomotion, and respiration.
4. Mutable dermis and connective tissue that, under the control of the nervous system, can fluctuate between extremely rigid and essentially fluid.