Myzostomida
Encyclopedia
Myzostomida are a remarkable taxonomic
group of small marine
worm
s, which are parasitic
on crinoid
s or "sea lilies", a kind of echinoderm
. These unusual parasitic Lophotrochozoa
were first discovered by Friedrich Sigismund Leuckart
in 1827.
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 seta
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
l 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 hermaphrodite
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
, from which the ripe ova (eggs) are discharged by a mediar dorsal pore into the end of the rectum
.
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 Myzostomidae.
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
group of small marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...
worm
Worm
The term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...
s, which are parasitic
Parasitism
Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...
on crinoid
Crinoid
Crinoids are marine animals that make up the class Crinoidea of the echinoderms . Crinoidea comes from the Greek word krinon, "a lily", and eidos, "form". They live both in shallow water and in depths as great as 6,000 meters. Sea lilies refer to the crinoids which, in their adult form, are...
s or "sea lilies", a kind of echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
. These unusual parasitic Lophotrochozoa
Lophotrochozoa
The Lophotrochozoa are a major grouping of protostome animals. The taxon was discovered based on molecular data. Molecular evidence such as a result of studies of the evolution of small-subunit ribosomal RNA supports the monophyly of the phyla listed in the infobox shown at right.-Terminology:The...
were first discovered by Friedrich Sigismund Leuckart
Friedrich Sigismund Leuckart
Friedrich Andreas Sigismund Leuckart was a German doctor and naturalist.He was born in Helmstedt in Lower Saxony and studied medicine at the University of Göttingen...
in 1827.
A typical myzostomid has a flattened rounded shape, with a thin edge drawn out into delicate radiating hairs called cirri
Cilium
A cilium is an organelle found in eukaryotic cells. Cilia are slender protuberances that project from the much larger cell body....
. The dorsal surface is smooth, with five pairs of parapodia
Parapodium
Parapodia , singular parapodium, are paired, un-jointed lateral outgrowths from the bodies of two different invertebrate groups, which are primarily marine in habitat...
on the bottom surface. These parapodia are armed with supporting and hooked seta
Seta
Seta is a biological term derived from the Latin word for "bristle". It refers to a number of different bristle- or hair-like structures on living organisms.-Animal setae:In zoology, most "setae" occur in invertebrates....
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
Cloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...
l opening are generally at opposite ends of the bottom surface. The former leads to a protrusible pharynx, from which the esophagus
Esophagus
The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...
opens into a wide intestinal chamber with branching lateral diverticula
Diverticulum
A diverticulum is medical or biological term for an outpouching of a hollow structure in the body. Depending upon which layers of the structure are involved, they are described as being either true or false....
. 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
Ganglion
In anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
. The dorsoventral and the parapodial muscles are much developed, while the coelom
Coelom
The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
is reduced mostly to branched spaces in which the genital products ripen.
Full-grown myzostomids are hermaphrodite
Hermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...
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
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
, from which the ripe ova (eggs) are discharged by a mediar dorsal pore into the end of the rectum
Rectum
The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...
.
Some species, such as Myzostoma cirriferum, move about on the host; others, such as Myzostoma glabrum, remain stationary with the pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
inserted in the mouth of the crinoid. Myzostoma deformator gives rise to a gall
Gall
Galls or cecidia are outgrowths on the surface of lifeforms caused by invasion by other lifeforms, such as parasites or bacterial infection. Plant galls are abnormal outgrowths of plant tissues and can be caused by various parasites, from fungi and bacteria, to insects and mites...
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
Antedon
Antedon is a genus of free-swimming, stemless 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 from a central concave disc...
.
Fridtjof Nansen
Fridtjof Nansen
Fridtjof Wedel-Jarlsberg Nansen was a Norwegian explorer, scientist, diplomat, humanitarian and Nobel Peace Prize laureate. In his youth a champion skier and ice skater, he led the team that made the first crossing of the Greenland interior in 1888, and won international fame after reaching a...
wrote in 1885 the thesis Bidrag til myzostomernes anatomi og histologi on the Myzostomidae.
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.