| Myzostomida | |
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| Myzostoma fuscomaculatum on its host the crinoidTropiometra carinata | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Annelida |
| Class: | Polychaeta |
| Order: | Myzostomida von Graff, 1877 |
TheMyzostomida orMyzostomatida are an order of smallmarine worms, which areparasitic onechinoderms, mostlycrinoids. These highly unusual and diverseannelids[1] were first discovered byFriedrich Sigismund Leuckart in 1827.
A typical myzostomid has a flattened, rounded shape, with a thin edge drawn out into delicate radiating hairs calledcirri. The dorsal surface is smooth, with five pairs ofparapodia on the bottom surface. These parapodia are armed with supporting and hookedsetae, 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 andcloacal opening are generally at opposite ends of the bottom surface. The former leads to a protrusiblepharynx, from which theesophagus opens into a wide intestinal chamber with branching lateraldiverticula. 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 fusedganglia. The dorsoventral and the parapodial muscles are much developed, while thecoelom is reduced mostly to branched spaces in which the genital products ripen.[2]
Full-grown myzostomids arehermaphrodites. 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 theuterus, from which the ripe ova (eggs) are discharged by a mediar dorsal pore into the end of therectum.[2]
Some species, such asMyzostoma cirriferum, move about on the host; others, such asMyzostoma glabrum, remain stationary with thepharynx inserted in the mouth of the crinoid.Myzostoma deformator gives rise to agall on the arm of the host, one joint of thepinnule growing round the worm so as to enclose it in a cyst whileMyzostoma pulvinar lives in thealimentary canal of a species ofAntedon.[2]
Fridtjof Nansen wrote in 1885 the thesisBidrag til myzostomernes anatomi og histologi[3] on the Myzostomida.
In the past Myzostomida have been regarded as close relatives of thetrematodeflatworms or of thetardigrades, 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 toflatworms or torotifers andacanthocephales.[5] They are now thought to be annelids, while their relationship to other annelids is unclear.[1]
According to theWorld Register of Marine Species, thesefamilies andgenera are accepted in this group:[6]
One or more of the preceding sentences incorporates text from a publication now in thepublic domain: Goodrich, Edwin Stephen (1911). "Myzostomida". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 19 (11th ed.). Cambridge University Press. pp. 144–145.
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