Asterozoans, or more commonly 'sea stars', 'brittle stars', and 'starfish', are echinoderms that have five arms that branch outwards. They are covered in hard calcitic plates, called ossicles, which create a protective external skeleton. Internally, they have a sophisticated hydraulic water-vascular system which power hundreds of tiny suction-cupped 'tube-feet' to move around. Asteroids are predators with a mouth on the bottom in the center of their body. They prey on molluscs and even other echinoids along the sea floor by eviscerating (spitting out) their stomach to dissolve prey with emzymes before slurping everything back in. Just like other echinoderms, they are exclusively marine.
Spencer, W.K. and Wright, C. W., 1966. Asterozoans in Moore, R. C., ed., Treatise on Invertebrate Paleontology, Part U, Echinodermata 3(1), Volume 1. The University of Kansas and Geological Society of America. 366 pp.