The Biology of Echinoderms Used with permission from BioMedia Associates and Environmental Media Corporation. The five classes of the phylum Echinodermata live exclusively in the sea. In sea stars, class Asteroidea, the body tapers into the arms. The tube feet and water vascular system functions are shown through animation, as is the digestive system, which relies largely on external digestion.
Sea stars are predators.
Nutrients from their prey are dispersed around the body through an open circulatory system. Gas exchange occurs directly with the sea through finger-like projections, called papilli, on the skin. Sea star reproduction usually involves broadcasting sperm and eggs directly into the sea. The larvae develop through early stages with bilateral symmetry before becoming radically symmetrical juveniles that settle on the sea floor. The snake-like arms of brittle stars, class Ophiuroidea, are narrow and distinctly attached to the central disk.