Shortly into the lab one student spotted a sea star (starfish) enjoying a meal of a mussel. Sea stars are voracious predators, and they're some of the most feared animals in the intertidal zone. Simply placing a severed arm of certain species of sea stars into a tide pool will cause nearly everything in that pool to go running for cover. However, sea star mouths are pretty small, often less than half the width of their central disk. By examining the picture it becomes clear that the sea star cannot possibly fit the mussel into its mouth.
Sea stars get around this size problem by digesting prey outside their body. Sea stars have two stomachs, a cardiac and a pyloric stomach, and can push their cardiac stomach outside their body so that it surrounds a prey item. In the case of a bivalve, such as a mussel, they use their arms to pull open the bivalve's shell wide enough to push their stomach inside, and then digest the bivalve in its own shell. Once the food is partially digested by the cardiac stomach it is moved to the pyloric stomach to finish digestion.
Sea stars are echinoderms, as are sea urchins, sand dollars, sea cucumbers, feather stars, sea lilies, and brittle stars. Echinoderms, along with hemichordates (acorn worms and pterobranchs), are humans' closest evolutionary relatives outside phylum Chordata. Why? Besides molecular data, echinoderms, hemichordates, and chordates share many characteristics of their embryonic development, and thus are classified as deuterostomes, while most invertebrate phyla are protostomes (e.g. mollusks, arthropods, and annelids).