Current situation of systematic studies on the Mesozoa is briefly reviewed. The phylum Mesozoa was proposed by van Beneden to represent an evolutionary stage between Protozoa and Metazoa. He originally established the phylum for dicyemids and made orthonectids belong into it. Additional multicellular organisms, not assignable to any metazoan phyla, Amoebophrya, Buddenbrockia, Haplozoon, Lohmannella, Salinella, Trichoplax, and Treptoplax, were included in this phylum. Thus the phylum Mesozoa was served as a sort of catchall for a variety of enigmatic organisms. Subsequent research on these organisms revealed the true systematic position. Amoebophrya, Haplozoon, and Lohmannella moved to the phylum Dinophyta. Trichoplax and Treptoplax were united to form a new phylum Placozoa. Buddenbrockia is possibly a member of myxozoans. Dicyemids and orthonectids are placed into the different phyla, Dicyemida and Orthonectida, respectively. The systematic position of Salinella remains enigmatic.