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Rhombozoa, or Dicyemida, 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.

Traditionally, dicyemids have been grouped with the Orthonectida in the Mesozoa; however, molecular phylogenies indicate that dicyemids may be more closely related to the Platyhelminthes (flatworms).

Adult dicyemids range in length from 0.1-9.0 mm, and they can be easily viewed through a light microscope. Dicyemids 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 ciliated cells. The anterior region of the organism is termed a "calotte" and functions to attach the dicyemid to folds on the surface of its host's renal appendages.

Dicyemids exist in both asexual and sexual forms. The former predominate in juvenile and immature hosts, and the latter in mature hosts. The asexual stage is termed a nematogen; it produces vermiform larvae which mature through direct development to form more nematogens. Nematogens proliferate in young cephalopods, filling the kidneys.

As the infection ages, perhaps as the nematogens reach a certain density, vermiform larvae mature to form rhombogens, the sexual life stage, rather than more nematogens. This sort of density-responsive reproductive cycle is reminiscent of the asexual reproduction of sporocysts or rediae in larval trematode infections of snails. As with the trematode asexual stages, a few nematogens can usually be found in older hosts. Their function may be to increase the population of the parasite to keep up with the growth of the host.

Rhombogens contain hermaphroditic infusorigens which produce infusoriform larvae. These larvae possess a very distinctive morphology, swimming about with ciliated rings that resemble headlights. It has long been assumed that this sexually produced infusoriform, which is released when the host eliminates urine from the kidneys, is both the dispersal and the infectious stage. The mechanism of infection, however, remains unknown, as are the effects, if any, of dicyemids on their hosts.

Some part of the dicyemid life cycle may be tied to temperate benthic environments, where they occur in greatest abundance. While dicyemids have occasionally been found in the tropics, the infection rates are typically quite low, and many potential host species are not infected. Dicyemids have never been reported from truly oceanic cephalopods, who instead host a parasitic ciliate fauna. Most dicyemid species are recovered from only one or two host species. While not strictly host specific, most dicyemids are only found in a few closely related hosts.

Dicyemids are divided into two families, Conocyemidae and Dicyemidae.

Wikispecies has information related to:

Rhombozoa

• Furuya, H. & K. Tsuneki. 2003. Biology of Dicyemid Mesozoans. Zoological Science 20: 519-532.
• Furuya, H., F. G. Hochberg, & K. Tsuneki. 2003. Reproductive traits in dicyemids. Marine Biology 143: 693-706.
• Hochberg, F.G. 1982. The "kidneys" of cephalopods: a unique habitat for parasites. Malacologia 23: 121-134.
• McConnaughey, B.H. 1951. The life cycle of the dicyemid Mesozoa. University of California Publications in Zoology 55: 295-336.
• Pawlowski, J.I. et al. 1996. Origin of the Mesozoa inferred from 18S rRNA gene sequences. Molecular Biology and Evolution 13: 1128-1132.