Usualy, as a starfish finds a snail on a cliff, or a bivalve mollusc in a sand it sucks to the shell with those orange ambulacrals, strains folds to a millimeter (or pulls the shell off the surface in case of a snail), and drops inside the shell it's elastic stomach, that starts digesting everything inside the shell.

I had a chance to admire the stomach of this starfish before it was pulled back inside: it looked like a pretty greenish-yellow transparent bubble with a slightly visible lines of veins at it's inner side. Dining with Patella has been almost finished.