Orthonectida

Orthonectida is a minor phylum of marine parasites that rank among the simplest multicellular life forms. Its members are called orthonectids. In their adult, sexual stage, they are microscopic and worm-like, built from a single outer layer of ciliated cells wrapped around a core of sex cells. They move freely inside their hosts, which include flatworms, polychaete worms, bivalve molluscs, echinoderms, ribbon worms, acoels, and Xenoturbellida. Most species have separate males and females, though a few are hermaphroditic.

When ready to reproduce, adults leave the host. Males release sperm that penetrates females, achieving internal fertilization. The resulting zygote becomes a ciliated larva that escapes the mother to find a new host. Once inside, the larva sheds its cilia and becomes a syncytial plasmodium, which then splits into many individual cells called agametes. These agametes grow into the next generation of adults.

The phylum contains about 20 known species, with *Rhopalura ophiocomae* being the most familiar. It is not divided into classes or orders, only two families. Originally described as a class by Édouard van Beneden in 1876 and later placed as an order within Mesozoa, a 1996 study suggested orthonectids differ significantly from rhombozoans, the other Mesozoa group. The genome of *Intoshia linei* has been sequenced, revealing these animals are simplified spiralians—a finding that matches earlier morphological evidence linking them to Spiralia. Their exact position in the spiralian tree is still uncertain. Some research connects them to Annelida, particularly Clitellata. However, a 2022 study that accounted for long-branch attraction revived the traditional grouping of Orthonectida with rhombozoans in a monophyletic Mesozoa, placed near Platyhelminthes or Gnathifera. This supports an earlier study that also found orthonectids and rhombozoans forming a clade with Rouphozoa (platyhelminths and gastrotrichs).