Starfish digestion is carried out in two stomachs: the cardiac stomach and the pyloric stomach. The cardiac stomach is a sack like stomach located at the center of the body and may be everted out of the organism's body to engulf and digest food. Some species are able to use their water vascular systems to force open the shells of bivalve mollusks such as clams and mussels by injecting their stomachs into the shells. With the stomach inserted inside the shell, the starfish is able to digest the mollusk in place. The cardiac stomach is then brought back inside the body, and the partially digested food is moved to the pyloric stomach. Further digestion occurs in the intestine.
https://en.wikipedia.org/wiki/Starfish