Echinoderms are marine invertebrates with radial symmetry and a water vascular system. There are 6 classes: sea stars, brittle stars, sea urchins, sea lilies, sea cucumbers, and sea daisies. They have tube feet for movement and respiration, and most are sessile or slow-moving. Reproduction involves releasing gametes into water. Characteristics include radial symmetry in adults and bilateral symmetry in larvae.