Mesozoa

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Mesozoa
Dicyema macrocephalum
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Clade:	ParaHoxozoa
Clade:	Bilateria
Clade:	Nephrozoa
(unranked):	Protostomia
(unranked):	Spiralia
Clade:	Platytrochozoa
(unranked):	Mesozoa van Beneden, 1876
Phyla
Dicyemida ?Monoblastozoa ?Orthonectida

The Mesozoa are minuscule,

cells surrounding one or more reproductive cells.

A recent study recovered Mesozoa as a monophyletic group that emerged in the

Some workers previously classified Mesozoa as the sole

In the 19th century, the Mesozoa were a

Evolution

Mesozoa were once thought to be

nemerteans

.

Orthonectida have a very reduced muscular and nervous system, only consisting of a few cells, but so far no muscle cells or neurons have been found in Dicyemida.[4]^[5]

Groupings

The two main mesozoan groups are the

.

Monoblastozoans consist of a single description written in the 19th century of a species that has not been seen since. As such, many workers doubt that they are a real group.^[6] As described, the animal had only a single layer of tissue.^[7]

Rhombozoan mesozoans

Rhombozoa, or dicyemid mesozoans, are found in the

They range from a few millimeters long with twenty to thirty cells that include anterior attachment cells and a long central reproductive cell called an axial cell. This axial cell may develop asexually into vermiform juveniles or it may produce eggs and sperm that self-fertilize to produce a ciliated infusiform larva.

There are three genera: Dicyema, Pseudicyema and Dicyemennea.

Molecular evidence suggests that this phylum are derived from the Lophotrochozoa.^[9][10]

Orthonectid mesozoans

Orthonectida are found in the body spaces of various marine invertebrates including tissue spaces, gonads, genitorespiratory bursae. This pathogen causes host castration of different species.^[11]

The best known of Orthonectida is the parasite of brittle stars. The multinucleate syncytial stage lives within tissues and spaces of the gonad but can spread into arms. It causes the destruction of starfish ovary and eggs to cause castration (the male gonads are usually unaffected). The stages of the plasmodium develop into more plasmodia by simple fragmentation; at some point, they decide to go sexual. The syncytia are dioecious (either male or female), but young syncytia can fuse to produce both male and female. The males are ciliated and smaller than the females. The females and the males leave the starfish and mate in the sea. Tailed sperm enters the female and fertilizes the numerous oocytes. Each oocyst produces a small ciliated larva which makes its way to another star.

The genome of one of these species –

References

External links

• "Mesozoa" . Encyclopædia Britannica (11th ed.). 1911.