| Myzostomida | |
|---|---|
 | |
| Myzostoma fuscomaculatum on its host the crinoid Tropiometra carinata | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Annelida |
| Class: | Polychaeta |
| Order: | Myzostomida von Graff, 1877 |
The Myzostomida or Myzostomatida are an order of small marine worms, which are parasitic on echinoderms, mostly crinoids. These highly unusual and diverse annelids [1] were first discovered by Friedrich Sigismund Leuckart in 1827.
A typical myzostomid has a flattened, rounded shape, with a thin edge drawn out into delicate radiating hairs called cirri. The dorsal surface is smooth, with five pairs of parapodia on the bottom surface. These parapodia are armed with supporting and hooked setae, by means of which the worm adheres to its host. Beyond the parapodia are four pairs of organs, often called suckers. These organs are probably of sensory nature, and are comparable to the lateral sense organs of capitellids. The mouth and cloacal opening are generally at opposite ends of the bottom surface. The former leads to a protrusible pharynx, from which the esophagus opens into a wide intestinal chamber with branching lateral diverticula. There appears to be no vascular system. The nervous system consists of a circumoesophageal nerve, with scarcely differentiated brain, joining below a large ganglionic mass, no doubt representing many fused ganglia. The dorsoventral and the parapodial muscles are much developed, while the coelom is reduced mostly to branched spaces in which the genital products ripen. [2]
Full-grown myzostomids are hermaphrodites. Their internal organs consist of a branched sac opening to the exterior or each side. The paired ovaries discharge their eggs into a median chamber with side branches, often called the uterus, from which the ripe ova (eggs) are discharged by a mediar dorsal pore into the end of the rectum. [2]
Some species, such as Myzostoma cirriferum , move about on the host; others, such as Myzostoma glabrum , remain stationary with the pharynx inserted in the mouth of the crinoid. Myzostoma deformator gives rise to a gall on the arm of the host, one joint of the pinnule growing round the worm so as to enclose it in a cyst while Myzostoma pulvinar lives in the alimentary canal of a species of Antedon . [2]
Fridtjof Nansen wrote in 1885 the thesis Bidrag til myzostomernes anatomi og histologi [3] on the Myzostomida.
In the past Myzostomida have been regarded as close relatives of the trematode flatworms or of the tardigrades, but in 1998 it was suggested that they are a sub-group of polychaetes. [4] However, another analysis in 2002 suggested that myzostomids are more closely related to flatworms or to rotifers and acanthocephales. [5] They are now thought to be annelids, while their relationship to other annelids is unclear. [1]
According to the World Register of Marine Species, these families and genera are accepted in this group: [6]
Polychaeta is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. More than 10,000 species are described in this class. Common representatives include the lugworm and the sandworm or clam worm Alitta.
The Caryophyllideans are a group of tapeworms that infect fish and annelids with a simple scolex or "head." Worms in this order only have one proglottid, which is believed to be the primitive condition for tapeworms. They are generally less than 10 centimetres long. Caryophillideans represent a unique type of tapeworm, such that they possess a monzoic, unsegmented, body, with only a single set of reproductive organs.
The Terebellidae is a marine family of polychaete worms, of which the type taxon is Terebella, described by Carl Linnaeus in his 1767 12th edition of Systema Naturae.
Sabellidae, or feather duster worms, are a family of marine polychaete tube worms characterized by protruding feathery branchiae. Sabellids build tubes out of a tough, parchment-like exudate, strengthened with sand and bits of shell. Unlike the other sabellids, the genus Glomerula secretes a tube of calcium carbonate instead. Sabellidae can be found in subtidal habitats around the world. Their oldest fossils are known from the Early Jurassic.
The Vermetidae, the worm snails or worm shells, are a taxonomic family of small to medium-sized sea snails, marine gastropod molluscs in the clade Littorinimorpha. The shells of species in the family Vermetidae are extremely irregular, and do not resemble the average snail shell, hence the common name "worm shells" or "worm snails".
Cryptocelidae is a family of flatworms.
Palpata is a subclass of polychaete worm. Members of this subclass are mostly deposit feeders on marine detritus or filter feeders. Palpata has become superfluous with the elevation of Canalipalpata to subclass.
Spionida is an order of marine polychaete worms in the infraclass Canalipalpata. Spionids are cosmopolitan and live in soft substrates in the littoral or neritic zones.
Acanthozoon is a genus of polyclad flatworms belonging to the family Pseudocerotidae.
Myzostoma is a genus of marine worms in the family Myzostomatidae. They are parasitic on crinoids.
Myzostoma fuscomaculatum, the crinoid worm, is a species of marine worm in the family Myzostomatidae.
The elegant feather star is a species of crinoid in the family Tropiometridae.
The annelids, also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecologies – some in marine environments as distinct as tidal zones and hydrothermal vents, others in fresh water, and yet others in moist terrestrial environments.
Phyllodocidae is a family of polychaete worms. Worms in this family live on the seabed and may burrow under the sediment.
Eulagisca gigantea is a species of scale worm. This species is specifically found in the deep-sea in cold waters like the Antarctic Ocean. The scale worms are named for the elytra on their surface that look like scales
Promachocrinus kerguelensis is a species of free-swimming, stemless crinoids. It was the only member of its genus until several species were discovered in 2023. P. keruguelensis a coldwater crinoid which is found in the seas around Antarctica and surrounding island groups, including under the sea ice.
Fabriciidae is a family of annelid worm in the class Polychaeta.
Myzostomatidae is a family of polychaetes belonging to the order Myzostomida.
Pleistoannelida is a group of annelid worms that comprises the vast majority of the diversity in phylum Annelida. Discovered through phylogenetic analyses, it is the largest clade of annelids, comprised by the last common ancestor of the highly diverse sister groups Errantia and Sedentaria and all the descendants of that ancestor. Most groups in the Clade find their ancestors within the Cambrian explosion when Annelid diversity expanded dramatically. The Pleistoannelida clade covers a variety of traits. However, the evolution of simple to complex eyes, developed papillae for burrowing, and for some specialized radioles for feeding can be seen universally across every species. New findings have discovered the range of Annelid diversity have led to uncertainty if groups with developed ancestral traits should remain within the clade. Furthermore There's been a lack of recently discovered Annelid traits being used in the categorization of groups within the clade, leading to many hypothesis on how to do so and which should remain within the clade. Currently three smaller clades that were originally a part of the groups Errantia and Sedentaria have been proven to fall outside while still being connected to the basal groups.
Myzostoma divisor is a species of parasitic marine polychaete in the order Myzostomida.
| Authority control databases: National |
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