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Myzostomida (Myzostomids) Facts

Evolution and systematics

The identify “myzostomid” comes from the Greek myzo, which means “to suck,” and stoma, which means “mouth.” Leuckart described the primary myzostomid, Myzostoma parasiticum, in 1827 and 1836. Since that discovery, the phylogenetic place of myzostomids throughout the metazoans has been a topic of controversy. The early assignments of myzostomids to Trematoda, then to Crustacea or Stelechopoda (a taxon grouping myzostomids with Tardigrada and Pentastomida), are now not thought of legitimate. Because myzostomids exhibit characters equivalent to parapodia with chaetae, trochophore-type larvae, and segmentation (though incomplete), they're now categorized in textbooks and encyclopedias as a household of Polychaeta or as a category of Annelida (the latter classification is adopted right here). However, phylogenetic analyses, together with that of myzostomid DNA sequences, strongly help the view that they don't seem to be annelids however a phylum intently associated to the flatworms. The class Myzostomida is split in two orders and eight households: the order Proboscidea (Myzostomatidae); and the order Pharyngidea (Protomyzostomatidae, Asteromyzostomatidae, Asteriomyzostomatidae, Endomyzostomatidae, Stelechopidae, Pulvinomyzostomatidae, and Mesomyzostomatidae). There are about 170 species.

Physical traits

The physique of most myzostomids consists of an anterior cylindrical introvert (additionally known as a proboscis) and a flat, disklike trunk. The introvert is prolonged when the person feeds, however it's retracted into an anteroventral pouch throughout the trunk more often than not. The trunk ranges in size from 0.118 in (Three mm) to 1.181 in (Three cm) for the biggest species. There are normally 5 pairs of parapodia, placed lateroventrally in two rows; every parapodium accommodates a protrusible hook, some alternative hooks, and a help rod, or aciculum. Most species have 4 pairs of slitlike or disklike lateroventral sense organs, generally named lateral organs, and the trunk margin typically bears needlelike cirri (greater than 100 in some species). In a number of species, humplike cirri happen on the base of every parapodium. Two male gonopores are placed on the degree of the third pair of parapodia; the feminine gonopore opens shut to the anus, posteroventrally. When myzostomids are parasitic, their physique is usually extremely modified. The introvert, exterior appendages, and sensory organs are normally decreased or have disappeared. According to the placement of myzostomids within the host, their trunk could also be pleated dorsally, mushroom formed, or irregular in form.


Myzostomids are present in all oceans from shallow waters to depths of over 9,840 ft (3,000 m). Most happen in tropical waters, however a number of are discovered within the Arctic and Antarctic Oceans.


These marine animals stay in affiliation with echinoderms.


All myzostomids stay in affiliation with echinoderms. Most species (90% of myzostomids) are ectocommensals of crinoids, however a number of are parasites of crinoids, asteroids, or ophiuroids, infesting their gonads, coelom, integumental, or digestive programs. The affiliation between myzostomids and echinoderms is historical, and indicators of parasitic actions, comparable to these induced by extant parasitic myzostomids, are present in fossilized crinoid skeletons relationship again to the Carboniferous interval.

Feeding ecology and food regimen

Very few myzostomids feed on host tissues or coelomic fluid, most feed on particles that they divert from the host utilizing their introvert. Crinoids, the primary hosts, are suspension feeders, catching food particles within the water column with podia on either side of the ciliated grooves that run alongside their arms. When myzostomids need to feed, they insert their introvert into these ciliated grooves and suck up water and food particles into their mouth.

Reproductive biology

Most myzostomids are hermaphroditic. They are sometimes practical simultaneous hermaphrodites, although the male genital system develops a bit sooner than the feminine genital system throughout organogenesis. In some species, each men and women happen, and are sometimes interpreted as the 2 levels of protandrous hermaphroditic species, the dwarf male apparently remodeling right into a feminine as soon as it lives alone. Reproduction in myzostomids takes place by the emission of spermatophores adopted by the intradermic penetration of sperm cells. In ectocommensals, mating entails two mature people that contact one another, one particular person ejecting one spermatophore that attaches to the integument of the opposite. Contact between the 2 people may be very transient they usually separate shortly after mating. Spermatophores usually connect to the again of the receiver, however they are often emitted efficiently onto any physique half. Depending on the species, the emitted spermatophores could also be white V-shaped, club-shaped, or ball-shaped baskets. After attachment, they pierce the integument and release all of the sperm cells into it. Penetration will be noticed, thanks to the presence of the white trails produced by the spermatophore contents extending into the translucent physique of the receiver. These trails seem from 10 to 30 minutes after attachment of a spermatophore, and after from one to 5 hours, the spermatophores are decreased to an empty matrix. Fertilization of mature oocytes is inner, and eggs are amassed right into a uterus earlier than being laid. Division is spiral, occurring within the water column, and offers rise to free-swimming trochophore larvae.

Conservation standing

No species of myzostomids are listed by the IUCN.

Significance to people

No parasitic myzostomids infest people or animals reared by people. They don't have any impression on human health.