Mollusks, or mollusks. It is a triploblastic, bilateral symmetry, with a real body cavity. The real body cavity of a mollusk is formed by the split cavity method, which is the body cavity formed by the mesoderm. However, the real body cavity of the mollusk is not developed, only in the pericardial cavity and the gonadal cavity. Molluscs vary greatly in morphology, but can be divided into four parts: head, foot, viscera and mantle.