|Two different female Orthonectids|
Giard, 1877 
Orthonectida (/, /) is a small phylum of poorly known parasites of marine invertebrates that are among the simplest of multi-cellular organisms. Members of this phylum are known as orthonectids.
The adults are microscopic wormlike animals, consisting of a single layer of ciliated outer cells surrounding a mass of sex cells. They swim freely within the bodies of their hosts, which include flatworms, polychaete worms, bivalve molluscs, and echinoderms. They are gonochoristic, with separate male and female individuals.
When they are ready to reproduce, the adults leave the host, and sperm from the males penetrate the bodies of the females to achieve internal fertilisation. The resulting zygote develops into a ciliated larva that escapes from the mother to seek out new hosts. Once it finds a host, the larva loses its cilia and develops into a syncytial plasmodium larva. This, in turn, breaks up into numerous individual cells that become the next generation of adults.
The genome of one of these species - Intoshia linei -has been sequenced. These animals are simplified spiralians. Their position in the phyogentic tree has yet to be determined.
The phylum consists of about 20 known species, of which Rhopalura ophiocomae is the best-known. The phylum is not divided into classes or orders, and contains just two families.
Although originally described in 1877 as a class, and sometimes characterized as an order of the phylum Mesozoa, recent study shows that orthonectids are quite different from the rhombozoans, the other group in Mesozoa.
- Family Rhopaluridae Stunkard, 1937
- Ciliocincta akkeshiensis Tajika, 1979 – Hokkaido, Japan; in flatworms (Turbellaria)
- Ciliocincta julini (Caullery and Mesnil, 1899) – E North Atlantic, in polychaetes
- Ciliocincta sabellariae Kozloff, 1965 – San Juan Islands, WA (USA); in polychaete (Neosabellaria cementarium)
- Intoshia leptoplanae Giard, 1877 – E North Atlantic, in flatworms (Leptoplana)
- Intoshia linei Giard, 1877 – E North Atlantic, in nemertines (Lineus) = Rhopalura linei
- Intoshia major Shtein, 1953 – Arctic Ocean; in gastropods (Lepeta, Natica, Solariella) = Rhopalura major
- Intoshia metchnikovi (Caullery & Mesnil, 1899) – E North Atlantic, in polychaetes and nemertines
- Intoshia paraphanostomae (Westblad, 1942) – E North Atlantic, in flatworms (Acoela)
- Intoshia variabili (Alexandrov & Sljusarev, 1992) – Arctic Ocean, in flatworms (Macrorhynchus)
- Rhopalura elongata Shtein, 1953 – Arctic Ocean, in bivalves (Astarte)
- Rhopalura gigas (Giard, 1877)
- Rhopalura granosa Atkins, 1933 – E North Atlantic, in bivalves (Pododesmus)
- Rhopalura intoshi Metchnikoff – Mediterranean, in nemertines
- Rhopalura litoralis Shtein, 1953 – Arctic Ocean, in gastropods (Lepeta, Natica, Solariella)
- Rhopalura major Shtein, 1953
- Rhopalura murmanica Shtein, 1953 – Arctic Ocean, in gastropods (Rissoa, Columbella)
- Rhopalura ophiocomae Giard, 1877 – E North Atlantic, in ophiuroids (usually Amphipholis)
- Rhopalura pelseneeri Caullery & Mesnil, 1901 – E North Atlantic, polychaetes and nemertines
- Rhopalura philinae Lang, 1954 – E North Atlantic, in gastropods
- Rhopalura pterocirri de Saint-Joseph, 1896 – E North Atlantic, in polychaetes
- Rhopalura vermiculicola
- Stoecharthrum burresoni Kozloff, 1993
- Stoecharthrum fosterae Kozloff, 1993
- Stoecharthrum giardi Caullery & Mesnil, 1899 – E North Atlantic, in polychaetes
- Stoecharthrum monnati Kozloff, 1993 – E North Atlantic, in molluscs
- Family Pelmatosphaeridae Stunkard, 1937
- Pelmatosphaera polycirri Caullery and Mesnil, 1904 – E North Atlantic, in polychaetes and nemertines
|Wikispecies has information related to: Orthonectida|
- H. Furuya & J. van der Land (2010). "Orthonectida". World Register of Marine Species. Retrieved January 12, 2011.
- "Orthonectida Giard, 1877". Integrated Taxonomic Information System. Retrieved January 12, 2011.
- "Orthonectida". Oxford Dictionaries. Oxford University Press. Retrieved 2016-01-21.
- Ben Hanelt, David Van Schyndel, Coen M. Adema, Louise A. Lewis & Eric S. Loker (November 1996). "The phylogenetic position of Rhopalura ophiocomae (Orthonectida) based on 18S ribosomal DNA sequence analysis". Molecular Biology and Evolution 13 (9): 1187–1191. doi:10.1093/oxfordjournals.molbev.a025683. PMID 8896370.
- Robert D. Barnes (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 247–248. ISBN 0-03-056747-5.
- Mikhailov KV, Slyusarev GS, Nikitin MA, Logacheva MD, Penin AA, Aleoshin VV, Panchin YV (2016) The genome of Intoshia linei affirms Orthonectids as highly simplified spiralians. pii: S0960-9822(16)30458-4. doi: 10.1016/j.cub.2016.05.007
- Alfred Mathieu Giard (1877). "Sur les Orthonectida, classe nouvelle d'animaux parasites des Échinodermes et des Turbellariés". Comptes Rendus (in French) 85 (18): 812–814.