The Myzostomida or Myzostomatida are an order of small marine worms, which are

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...

echinoderm An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...

s, mostly

crinoid Crinoids are marine invertebrates that make up the class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or comatulids, are ...

s. These highly unusual and diverse

annelids The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to vario ...

were first discovered by

Friedrich Sigismund Leuckart Friedrich Andreas Sigismund Leuckart (26 August 1794 – 25 August 1843) was a German doctor and naturalist. He was born in Helmstedt in Lower Saxony and studied medicine at the University of Göttingen. From 1816 he made several voyages of explor ...

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 In invertebrates, the term parapodium ( Gr. ''para'', beyond or beside + ''podia'', feet; : parapodia) refers to lateral outgrowths or protrusions from the body. Parapodia are predominantly found in annelids, where they are paired, unjointed late ...

on the bottom surface. These parapodia are armed with supporting and hooked

seta In biology, setae (; seta ; ) are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae, ...

e, 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

cloaca A cloaca ( ), : cloacae ( or ), or vent, is the rear orifice that serves as the only opening for the digestive (rectum), reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles, birds, cartilagin ...

l opening are generally at opposite ends of the bottom surface. The former leads to a protrusible

pharynx The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...

, from which the

esophagus The esophagus (American English), oesophagus (British English), or œsophagus (Œ, archaic spelling) (American and British English spelling differences#ae and oe, see spelling difference) all ; : ((o)e)(œ)sophagi or ((o)e)(œ)sophaguses), c ...

opens into a wide intestinal chamber with branching lateral

diverticula In medicine or biology, a diverticulum is an outpouching of a hollow (or a fluid-filled) structure in the body. Depending upon which layers of the structure are involved, diverticula are described as being either true or false. In medicine, t ...

. 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 A ganglion (: ganglia) is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system, this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system, there a ...

. The dorsoventral and the parapodial muscles are much developed, while the

coelom The coelom (or celom) is the main body cavity in many animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, i ...

is reduced mostly to branched spaces in which the genital products ripen. Full-grown myzostomids are

hermaphrodite A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...

s. 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 The uterus (from Latin ''uterus'', : uteri or uteruses) or womb () is the hollow organ, organ in the reproductive system of most female mammals, including humans, that accommodates the embryonic development, embryonic and prenatal development, f ...

, from which the ripe ova (eggs) are discharged by a mediar dorsal pore into the end of the

rectum The rectum (: rectums or recta) is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. Before expulsion through the anus or cloaca, the rectum stores the feces temporarily. The adult ...

Biology

Some species, such as '' Myzostoma cirriferum'', move about on the host; others, such as '' Myzostoma glabrum'', remain stationary with the

inserted in the mouth of the crinoid. '' Myzostoma deformator'' gives rise to a

gall Galls (from the Latin , 'oak-apple') or ''cecidia'' (from the Greek , anything gushing out) are a kind of swelling growth on the external tissues of plants. Plant galls are abnormal outgrowths of plant tissues, similar to benign tumors or war ...

on the arm of the host, one joint of the

pinnule A leaflet (occasionally called foliole) in botany is a leaf-like part of a compound leaf. Though it resembles an entire leaf, a leaflet is not borne on a main plant stem or branch, as a leaf is, but rather on a petiole or a branch of the leaf. C ...

growing round the worm so as to enclose it in a cyst while '' Myzostoma pulvinar'' lives in the

alimentary canal The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. ...

of a species of ''

Antedon ''Antedon'' is a genus of sessile, stemless crinoids found in shallow tropical and temperate marine environments worldwide. The genus first appears in the fossil record during the Cretaceous period. Characteristics Members of this genus have no s ...

''.

Fridtjof Nansen Fridtjof Wedel-Jarlsberg Nansen (; 10 October 1861 – 13 May 1930) was a Norwegian polymath and Nobel Peace Prize laureate. He gained prominence at various points in his life as an explorer, scientist, diplomat, humanitarian and co-founded the ...

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

Classification

In the past Myzostomida have been regarded as close relatives of the

trematode Trematoda is a Class (biology), class of flatworms known as trematodes, and commonly as flukes. They are obligate parasite, obligate Endoparasites, internal parasites with a complex biological life cycle, life cycle requiring at least two Host ( ...

flatworm Platyhelminthes (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a Phylum (biology), phylum of relatively simple bilaterian, Segmentation (biology), ...

s or of the

tardigrade Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them . In 1776, th ...

s, 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 Platyhelminthes (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates commonly called ...

or to

rotifer The rotifers (, from Latin 'wheel' and 'bearing'), sometimes called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic Coelom#Pseudocoelomates, pseudocoelomate animals. They were first describ ...

s 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 The World Register of Marine Species (WoRMS) is a taxonomic database that aims to provide an authoritative and comprehensive catalogue and list of names of marine organisms. Content The content of the registry is edited and maintained by scien ...

, these

families Family (from ) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictability, structure, and safety as ...

and

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

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 Myzostomatidae is a family of polychaetes belonging to the order Myzostomida The Myzostomida or Myzostomatida are an order of small marine (ocean), marine worms, which are Parasitism, parasitic on echinoderms, mostly crinoids. These highly un ...

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 or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...

'' ** '' Contramyzostoma'' Eeckhaut, Grygier & Deheyn, 1998 ** '' Cyclocirra'' Müller, 1841 ** '' Stelechopus'' Graff, 1884

Morphology

Biology

Classification

References

Further reading