Dicyemid mesozoans (phylum Dicyemida) are endoparasites that are typically found in the renal sac of benthic cephalopod molluscs. Dicyemid bodies are very simply organized, consisting of only 8 to 40 cells, which is the fewest number of cells in metazoans. The current paper gives an overview of the morphology of dicyemids and reviews current status of the taxonomy of dicyemids. The renal sac of cephalopods is a unique environment providing habitat for a diversity of dicyemids. The current paper also reviews recent advances in several biological aspects, such as prevalence, zoogeography, and host specificity. The degree of host specificity differs among different species of dicyemids, although the recent study reveals narrow host specificity. Most previous descriptions were based on the character of vermiform stages, and several species seem to be incorrectly identified. The current study suggests that the cellular composition and cell types of infusoriform embryos are significant characters used to help identify dicyemids species. In addition, the type host specimen for a new species of parasite is briefly discussed The accurate identification of a host organism is an important component in the taxonomic recognition of a new species of parasite.
