I have the lab exams graded. True starfish have no sharp separation between arms and central body, and they move using tube feet rather than wriggling movements of the whole arms.
Most starfish are predators, feeding on sessile or slow-moving prey such as mollusks and barnacles.
Many, but not all, starfish are able to turn a portion of their stomachs out through the mouth, and thus digest food outside of the body. Class Ophiuroidea Ophiuroids are commonly known brittle stars. Opiuroids differ from starfish in having a more circular central disc with the arms more sharply set off. They are the most active and motile members of this phylum and lack an ambulacral groove. The tube feet, although present, are not used in locomotion.