Rhopalura ophiocomae is a member of a very small, very obscure, and very enigmatic group of parasites that are part of a phylum called Orthonectida. They are multicellular organisms but lack both gastrointestinal tracts and nervous systems. This species infects brittlestars and will invade the gonads and castrate their hosts. The larvae are ciliated and invade new brittlestars via entering the genital clefts of those echinoderms. Inside this host, they essentially disintegrate to form an amoeboid structure called a plasmodium. A phylogenetic analysis based on the 18S genes of this species and several multicellular organisms and protists showed that these parasites were more closely related to triploblast organisms, but were not allied with anything else in the tree. Clearly someone needs to do some more work on this group!The image comes from this site.
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