Flexi Says: Although they look quite different on the outside, mollusks and annelids share a more recent common ancestor than roundworms and annelids. Along with their close relatives the annelids, mollusks are the first true coelomates. In mollusks, however, the coelom is often reduced to a small cavity surrounding the heart. Annelids generally have a well-developed coelom. Another major difference between mollusks and annelids is that annelids have segmented bodies and mollusks do not.