Many molluscs spend their whole life in the plankton, like the beautiful pteropods and heteropods. Also, many bottom dwelling snails and bivalves have a free-swimming planktonic larval stage. The larvae swims with a modified foot covered in rows of tiny hairs called cilia. Shellfish can discharge their eggs and sperm into the water where the eggs are fertilised, but many species lay their eggs in protective cases so larvae can develop sheltered from predation.