Orthonectida is a phylum consisting of small, parasitic marine invertebrates that lack body segmentation. Some synonyms for this group include Orthonectoidea, Orthonectids, and Orthonectidans. These terms are often used interchangeably to refer to the same phylum, but they may also be used to describe different species within the group. Other synonyms for Orthonectida include Rhopalurida and Dicyemida, which are similar groups of parasitic invertebrates. While Orthonectida may be a less well-known phylum compared to others, it has been the subject of research due to its unique life cycle and potential medical applications.