Open main menu

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.

Myzostoma fuscomaculatum at Percys Hole detail.jpg
Myzostoma fuscomaculatum on its host the crinoid Tropiometra carinata
Scientific classification


Myzostoma anatomy, showing cirri (c); the pharynx (p), and anus (a)
Diversity of myzostomid body shapes

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.[6]

According to the World Register of Marine Species, these families and genera are accepted in this group:[7]


  1. ^ Summers, Mindi M.; Rouse, Greg W. (2014). "Phylogeny of Myzostomida (Annelida) and their relationships with echinoderm hosts". BMC Evolutionary Biology. 14: 170. doi:10.1186/s12862-014-0170-7. PMC 4160548. PMID 25164680.
  2. ^ a b c   One or more of the preceding sentences incorporates text from a publication now in the public domainGoodrich, Edwin Stephen (1911). "Myzostomida". In Chisholm, Hugh (ed.). Encyclopædia Britannica. 19 (11th ed.). Cambridge University Press. pp. 144–145.
  3. ^ [1] Bidrag til myzostomernes anatomi og histologi
  4. ^ Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D.T. (ed.). Invertebrate Zoology. Oxford University Press. pp. 179–183. ISBN 978-0-19-551368-4.
  5. ^ Halanych, K.M.; Dahlgren, T.G.; McHugh, D. (2002). "Unsegmented Annelids? Possible Origins of Four Lophotrochozoan Worm Taxa". Integrative and Comparative Biology. 42 (3): 678–684. doi:10.1093/icb/42.3.678. PMID 21708764.
  6. ^ Summers, Mindi M.; Rouse, Greg W. (2014). "Phylogeny of Myzostomida (Annelida) and their relationships with echinoderm hosts". BMC Evolutionary Biology. 14: 170. doi:10.1186/s12862-014-0170-7. PMC 4160548. PMID 25164680.
  7. ^ accessed 13 September 2013
  8. ^ accessed 13 September 2013
  9. ^ accessed 13 September 2013
  10. ^ accessed 13 September 2013

Further readingEdit