Dicyemida
Dicyemida, also known as Rhombozoa, is a phylum of tiny parasites that live in the renal appendages of cephalopods.
Classification is controversial.[1] Traditionally, dicyemids have been grouped with the Orthonectida in the Mesozoa, and, as of 2017, molecular evidence[2] appears to confirm this.
However, other molecular phylogenies have placed the dicyemids more closely related to the roundworms.[3] Additional molecular evidence suggests that this phylum is derived from the Lophotrochozoa.[4][5]
The phylum is not divided in classes or orders, but contains three families, Conocyemidae, Dicyemidae, and Kantharellidae.[6]
Adult dicyemids range in length from 0.5 to 7 millimetres (0.020 to 0.276 in), and they can be easily viewed through a light microscope.[7] 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. Dicyemida lack respiratory, circulatory, excretory, digestive, and nervous systems.
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.[7]
