This suggests that the body cavity (coelum) evolved before the lophophore. Protostomes are one of the groups of animals having a true body cavity and are believed to share a common ancestor. They include annelids, mollusks, and arthropods. Lophophore is an organ characteristic of aquatic invertebrates (protostomes) that functions in filter feeding, it consists of a ridge of hollow tentacles bearing cilia, which waft food particles into the mouth.