The order Mollusca is divided into 5 classes.
1. Caudofovita: They are 2–140 mm long. Body covered with chitinous cuticle. They are marine.
2. Polyplacophora: They are 8 mm to 33 cm long. Body covered by chitin.
3. Gastropods: Their bodies are soft, fleshy and unsegmented. Body covered by mantle.
4. Bivalvia: They have hemolymph fluid. There are two shells made of calcium carbonate.
5. Cephalopoda: These are marine animals with eight arms. There is no hard shell on the body. Has eight legs (cephalopods). They are nocturnal.