Superficially resembling bivalve molluscs, brachiopods (Phylum Brachiopoda) have a few fundamental differences. Their two shells are not symmetrical, and they have an upper and lower valve compared to the left/right valves of bivalves. They are also mostly sessile, attaching to the substrate by a stalk or "pedicle". Brachiopods are filter feeders, taking in food particles from the water.