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Dicyemida, or Rhombozoa, is a phylum of tiny parasites that live in the renal appendages of cephalopods. Although the name "Dicyemida" precedes "Rhombozoa" in usage, and is preferred by most contemporary authors, "Rhombozoa" still enjoys much popular support.
Classification is controversial (Aruga et al. 2007). Traditionally, dicyemids have been grouped with the Orthonectida in the Mesozoa; however, molecular phylogenies indicate that dicyemids may be more closely related to the roundworms (Pawlowski et al. 1996). The phylum is not divided in classes, but contains three families, Conocyemidae, Dicyemidae, and Kantharellidae (ITIS 2010). Molecular evidence suggests that this phylum are derived from the Lophotrochozoa (Kobayashi et al. 2009, Suzuki et al. 2010).
Adult dicyemids range in length from 0.5 to 7 millimetres (0.020 to 0.28 in), and they can be easily viewed through a light microscope (Barnes 1982). They display eutely, a condition in which each adult individual of a given species has the same number of cells, making cell number a useful identifying character.
The organism's structure is simple: a single axial cell is surrounded by a jacket of twenty to thirty ciliated cells. The anterior region of the organism is termed a calotte and functions to attach the parasite to folds on the surface of its host's renal appendages (Barnes 1982).