mesozoa

Mesozoa were once thought to be evolutionary intermediate forms between Protozoans and Metazoans, but now they are thought to be degenerate or simplified metazoa. Their ciliated larvae are similar to the miracidium of trematodes, and their internal multiplication is similar to what happens in the sporocysts of trematodes. Mesozoan DNA has a low GC-content (40%). This amount is similar to ciliates, but ciliates tend to be binucleate. Others relate mesozoa to a group including annelids, planarians and 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.{{cite journal | url=https://onlinelibrary.wiley.com/doi/abs/10.1111/azo.12279 | doi=10.1111/azo.12279 | title=The structure of the muscular and nervous systems of the male Intoshia linei (Orthonectida) | year=2019 | last1=Slyusarev | first1=George S. | last2=Nesterenko | first2=Maksim A. | last3=Starunov | first3=Viktor V. | journal=Acta Zoologica | volume=100 | issue=4 | pages=451–458 }}{{cite journal | pmc=6543705 | year=2019 | last1=Zverkov | first1=O. A. | last2=Mikhailov | first2=K. V. | last3=Isaev | first3=S. V. | last4=Rusin | first4=L. Y. | last5=Popova | first5=O. V. | last6=Logacheva | first6=M. D. | last7=Penin | first7=A. A. | last8=Moroz | first8=L. L. | last9=Panchin | first9=Y. V. | last10=Lyubetsky | first10=V. A. | last11=Aleoshin | first11=V. V. | title=Dicyemida and Orthonectida: Two Stories of Body Plan Simplification | journal=Frontiers in Genetics | volume=10 | page=443 | doi=10.3389/fgene.2019.00443 | pmid=31178892 | doi-access=free }}

The two main mesozoan groups are the Dicyemida and the Orthonectida. Other groups sometimes included in the Mesozoa are the Placozoa and the Monoblastozoa.

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.{{Cite report |type=Data set |first1=Philipp |last1=Schiffer |first2=Helen|last2=Robertson |first3=Maximilian |last3=Telford |date=2018-04-03 |title=Orthonectids are highly degenerate annelid worms |language=en |doi=10.5281/zenodo.1206208 |doi-access=free}}{{better source needed|date=February 2021|reason=shouldn't this clam be sourced to a secondary source and not to 'data'? perhaps doi:10.1016/j.cub.2018.04.088?}} As described, the animal had only a single layer of tissue.{{Cite journal |last1=de Meeûs |first1=Thierry |last2=Renaud |first2=François |date=July 2002 |title=Parasite within the new phylogeny of eukaryotes |url=https://www.researchgate.net/publication/11335429 |journal=Trends in Parasitology |volume=18 |issue=6 |pages=247–51 |doi=10.1016/S1471-4922(02)02269-9|pmid=12036736 }}

{{Main|Dicyemida}}

Rhombozoa, or dicyemid mesozoans, are found in the nephrid tracts of squid and octopuses.{{Cite journal |last=Hochberg |first=F. G. |date=30 June 1983 |url=https://www.biodiversitylibrary.org/item/130017#page/3/mode/1up |journal=Memoirs of the National Museum of Victoria |volume=44 |pages=109–145 |doi=10.24199/j.mmv.1983.44.10|title=The parasites on cephalopods: A review |doi-access=free }}

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.{{Cite journal |last1=Kobayash |first1=M. |last2=Furuya |first2=H. |last3=Wada |first3=H. |date=September–October 2009 |title=Molecular markers comparing the extremely simple body plan of dicyemids to that of lophotrochozoans: insight from the expression patterns of Hox, Otx, and brachyury|journal=Evolution & Development|volume=11 |issue=5 |pages=582–589 |doi=10.1111/j.1525-142X.2009.00364.x|pmid=19754714 |s2cid=6070504}}{{Cite journal |last1=Suzuki |first1=T. G. |last2=Ogino |first2=K. |last3=Tsuneki |first3=K. |last4=Furuya |first4=H. |s2cid=25877334 |date=June 2010 |title=Phylogenetic Analysis of Dicyemid Mesozoans (Phylum Dicyemida) From Innexin Amino Acid Sequences: Dicyemids Are Not Related to Platyhelminthes|journal=Journal of Parasitology |volume=96 |issue=3 |pages=614–625 |doi=10.1645/GE-2305.1|pmid=20557208 |doi-access=free}}

{{Main|Orthonectida}}

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.{{Cite book |title=Invertebrate Zoology |last= Barnes |first=Robert D. |publisher=Holt-Saunders International |year=1982 |isbn=978-0-03-056747-6 |location=Philadelphia, PA |pages=247–248}}

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 – Intoshia linei – has been sequenced.{{Cite journal |last1=Mikhailov |first1=K. V. |last2=Slyusarev |first2=G. S. |last3=Nikitin |first3=M. A. |last4=Penin |first4=A. A. |last5=Aleoshin |first5=V. V. |last6=Panchin |first6=Y. V. |s2cid=3917921 |date=11 July 2016 |title=The Genome of Intoshia linei Affirms Orthonectids as Highly Simplified Spiralians |journal=Curr Biol |volume=26 |issue=13 |pages=1768–74 |doi=10.1016/j.cub.2016.05.007|pmid=27374341 |doi-access=free |bibcode=2016CBio...26.1768M }} The Orthonectids may be degenerate annelid worms.{{Cite journal|last1=Telford|first1=Maximilian J.|last2=Robertson|first2=Helen E.|last3=Schiffer|first3=Philipp H.|s2cid=44166754|date=2018-06-18|title=Orthonectids Are Highly Degenerate Annelid Worms|journal=Current Biology|language=en|volume=28|issue=12|pages=1970–1974.e3|doi=10.1016/j.cub.2018.04.088|issn=0960-9822|pmid=29861137|doi-access=free|bibcode=2018CBio...28E1970S }}

{{Reflist}}

• {{Cite EB1911|wstitle=Mesozoa|short=x}}

{{Animalia}}

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Category:Parasitic animals

Category:Marine animals

Category:Lophotrochozoa