Orthonectida

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.

Related Topics:
Phylum - Parasite

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Multinucleate plasmodia give rise to sexual males and females, which are free-swimming by means of cilia. They are composed of a single layer of ciliated outer cells surrounding a mass of sex cells. When a female is inseminated, the eggs develop into ciliated larvae that seek out new hosts and then break up into individual cells that become the next generation of plasmodia.

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Parasitized organisms include flatworms, polychaete worms, bivalve molluscs, and echinoderms.

Related Topics:
Flatworm - Polychaete - Bivalve - Echinoderm

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The phylum consists of about 20 known species, of which Rhopalura ophiocomae is the best-known.

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Originally described in 1880 as a class, and sometimes characterized as an order of Mesozoa, recent study shows that orthonectids are indeed quite different from rhombozoans, the other group in Mesozoa.

Related Topics:
1880 - Mesozoa - Rhombozoa

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Known species:

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• Family Rhopaluridae
• Ciliocincta julini (Caullery and Mesnil, 1899) - E North Atlantic, in polychaetes
• 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 paraphanostomae (Westblad, 1942) - E North Atlantic, in flatworms (Acoela)
• 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, 1954) - Arctic Ocean, in gastropods (Lepeta, Natica, Solariella)
• Rhopalura metschnikowi (Caullery and Mesnil, 1901) - E North Atlantic, in polychaetes and nemertines
• Rhopalura murmanica (Shtein, 1953) - Arctic Ocean, in gastropods (Rissoa, Columbella)
• Rhopalura ophiocomae (Giard, 1877) - E North Atlantic, in ophiuroids (usually Amphipholis)
• Rhopalura pelseeneri (Caullery and Mesnil, 1901) - E North Atlantic, polychaetes and nemertines
• Rhopalura philinae (Lang, 1951) - E North Atlantic, in gastropods
• Rhopalura pterocirri (de Saint-Joseph, 1896) E North Atlantic, in polychaetes
• Rhopalura variabili (Alexandrov and Sljusarev, 1992) - Arctic Ocean, in flatworms (Macrorhynchus)
• Stoecharthrum giardi (Caullery and Mesnil, 1899) - E North Atlantic, in polychaetes
• Stoecharthrum monnati (Kozloff, 1993) - E North Atlantic, in molluscs
• Family Pelmatosphaeridae
• Pelmatosphaera polycirri (Caullery and Mesnil, 1904) - E North Atlantic, in polychaetes and nemertines
 
Orthonectida: References ►