See text Orthonectida (/ˌɔːrθəˈnɛktɪdə, -θoʊ-/[3]) is a small phylum of poorly known parasites of marine invertebrates[4] that are among the simplest of multi-cellular organisms.

They swim freely within the bodies of their hosts, which include flatworms, polychaete worms, bivalve molluscs, and echinoderms.

[5][6] When they are ready to reproduce, adults leave the host, and sperm from the males penetrate the bodies of the females to achieve internal fertilisation.

[12] On the other hand, a 2022 study compensating for long-branch attraction has recovered the traditional grouping of Orthonectida with rhombozoans in a monophyletic Mesozoa placed close to Platyhelminthes or Gnathifera.

[13] This supports a previous study which found orthonectids and rhombozoans to make a monophyletic taxon Mesozoa and form a clade with Rouphozoa (platyhelminths and gastrotrichs).