• Source: Myzostomida

The Myzostomida or Myzostomatida are an order of small marine worms, which are parasitic on echinoderms, mostly crinoids. These highly unusual and diverse annelids were first discovered by Friedrich Sigismund Leuckart in 1827.




Morphology



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.
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.




Biology


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.
Fridtjof Nansen wrote in 1885 the thesis Bidrag til myzostomernes anatomi og histologi on the Myzostomida.




Classification


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. However, another analysis in 2002 suggested that myzostomids are more closely related to flatworms or to rotifers and acanthocephales. They are now thought to be annelids, while their relationship to other annelids is unclear.
According to the World Register of Marine Species, these families and genera are accepted in this group:

Asteriomyzostomidae Jägersten, 1940
Asteriomyzostomum Jägersten, 1940
Asteromyzostomidae Wagin, 1954
Asteromyzostomum Wagin, 1954
Eenymeenymyzostomatidae Summers & Rouse, 2015
Eenymeenymyzostoma Summers & Rouse, 2015
Endomyzostomatidae Perrier, 1897
Endomyzostoma Perrier, 1897
Mycomyzostoma Eeckhaut, 1998
Myzostomatidae Benham, 1896
Cystimyzostomum Jagersten, 1940
Hypomyzostoma Perrier, 1897
Mesomyzostoma Remscheid, 1918
Myzostoma Leuckart, 1836
Notopharyngoides
Protomyzostomidae Stummer-Traunfels, 1926
Protomyzostomum Fedotov, 1912
Pulvinomyzostomidae Jägersten, 1940
Pulvinomyzostomum Jägersten, 1940
Myzostomida incertae sedis
Contramyzostoma Eeckhaut, Grygier & Deheyn, 1998
Cyclocirra Müller, 1841
Stelechopus Graff, 1884




References






Further reading



Bleidorn C. et al. 2009. On the phylogenetic position of Myzostomida: can 77 genes get it wrong? BMC Evolutionary Biology 2009, 9:150. doi:10.1186/1471-2148-9-150

Kata Kunci Pencarian:

• Platyzoa
• Myzostomida
• Annelid
• Pleistoannelida
• Sedentaria
• Platyzoa
• Polychaete
• Nemertea
• Errantia
• Myzostoma divisor
• Myzostoma