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 on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has c ...

echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea ...

s, mostly

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

s. These highly unusual and diverse

annelids The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecolo ...

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 explo ...

in 1827.

Morphology

A typical myzostomid has a flattened, rounded shape, with a thin edge drawn out into delicate radiating hairs called

cirri Giovanni Battista Cirri (1 October 1724 – 11 June 1808) was an Italian cellist and composer in the 18th century. Biography Cirri was born in Forlì in the Emilia-Romagna Region of Italy. He had his first musical training with his brother ...

. The dorsal surface is smooth, with five pairs of

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

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

seta In biology, setae (singular seta ; from the Latin word for "bristle") are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Annelid setae are stiff bristles present on the body. Th ...

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 In animal anatomy, a cloaca ( ), plural cloacae ( or ), is the posterior orifice that serves as the only opening for the digestive, reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles and birds, a ...

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

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...

, from which the

esophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...

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 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 are both sympatheti ...

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

coelom The coelom (or celom) is the main body cavity in most 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, it r ...

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

hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many Taxonomy (biology), taxonomic groups of animals (mostly invertebrate ...

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'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The uter ...

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

rectum The rectum is the final straight portion of the large intestine in humans and some other mammals, and the Gastrointestinal tract, gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the s ...

Biology

Some species, such as ''

Myzostoma cirriferum ''Myzostoma'' is a genus of marine worms, which are parasitic on crinoids in the family Myzostomidae. Species The following species are classified in this genus: *'' Myzostoma abundans'' Graff, 1883 *'' Myzostoma adhaerens'' Remscheid, 1918 *' ...

'', move about on the host; others, such as ''

Myzostoma glabrum ''Myzostoma'' is a genus of marine worms, which are parasitic on crinoids in the family Myzostomidae. Species The following species are classified in this genus: *'' Myzostoma abundans'' Graff, 1883 *'' Myzostoma adhaerens'' Remscheid, 1918 *' ...

'', remain stationary with the

inserted in the mouth of the crinoid. ''

Myzostoma deformator ''Myzostoma'' is a genus of marine worms, which are parasitic on crinoids in the family Myzostomidae. Species The following species are classified in this genus: *'' Myzostoma abundans'' Graff, 1883 *'' Myzostoma adhaerens'' Remscheid, 1918 *' ...

'' 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, fungi, or animals. Plant galls are abnormal outgrowths of plant tissues, similar to be ...

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. Co ...

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

Myzostoma pulvinar ''Myzostoma'' is a genus of marine worms, which are parasitic on crinoids in the family Myzostomidae. Species The following species are classified in this genus: *'' Myzostoma abundans'' Graff, 1883 *'' Myzostoma adhaerens'' Remscheid, 1918 *' ...

'' lives in the alimentary canal of a species of ''

Antedon ''Antedon'' is a genus of free-swimming, stemless Crinoidea, crinoids. The genus first appeared in the fossil record in the Cretaceous period. Characteristics Members of this genus have no stems but have five pairs of feathery arms arising fro ...

''.

Fridtjof Nansen Fridtjof Wedel-Jarlsberg Nansen (; 10 October 186113 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, and humanitarian. He led the team t ...

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 of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is usually a snail. The definitive host ...

flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegment ...

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 Kleiner Wasserbär ...

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 The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegment ...

or to

rotifer The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John H ...

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 list of names of marine organisms. Content The content of the registry is edited and maintained by scientific specialist ...

, these

families Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideal ...

and

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

are accepted in this group: * Endomyzostomatidae **'' Mycomyzostoma'' *Myzostomida

incertae sedis ' () or ''problematica'' is a term used for a 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'' **'' Cyclocirra'' **'' Mesomyzostoma'' **'' Protomyzostomum'' **'' Stelechopus'' * Myzostomidae Benham, 1896 **'' Cystimyzostomum'' Jagersten, 1940 **'' Endomyzostoma'' Perrier, 1897 **'' Hypomyzostoma'' **''

Myzostoma ''Myzostoma'' is a genus of marine worms, which are parasitic on crinoids in the family Myzostomidae. Species The following species are classified in this genus: *'' Myzostoma abundans'' Graff, 1883 *'' Myzostoma adhaerens'' Remscheid, 1918 *'' ...

'' Leuckart, 1829 **'' Notopharyngoides'' **'' Protomyszostomum''

Morphology

Biology

Classification

References

Further reading