|Two different female Orthonectids|
Giard, 1877 
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 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
- H. Furuya & J. van der Land (2010). "Orthonectida".
- "Orthonectida Giard, 1877".
- Ben Hanelt, David Van Schyndel, Coen M. Adema, Louise A. Lewis & Eric S. Loker (November 1996). (Orthonectida) based on 18S ribosomal DNA sequence analysis"Rhopalura ophiocomae"The phylogenetic position of .
- Robert D. Barnes (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 247–248.