Editing Mesozoa (section)

===Orthonectid mesozoans===
{{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.<ref name="IZ">{{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}}</ref>

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.<ref name="Mikhailov2016">{{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 }}</ref> The Orthonectids may be degenerate [[annelid worm]]s.<ref name=":0" /><ref>{{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 }}</ref>