A phylum of around 125 known species of microscopic multicellular animals found as parasites inside the nephridia of cephalopod molluscs. They range in size from 0.5 to 7 mm and consist of a single axial cell surrounded by a jacket of ciliated cells, plus an anterior structure, the calotte, which attaches the organism to the host site. Genetic analysis has shown that dicyemids are members of the Lophotrochozoa clade and are related to the Orthonectida.