A small phylum (about 25 named species) of microscopic animals that parasitize a wide variety of marine invertebrates: turbellarians, nemerteans, polychaete annelids, gastropod and bivalve mollusks, brittle stars, and ascidians. Four genera of orthonectids are now recognized. The name Orthonectida (“straight-swimming”) is based on an early observation that, when apparently mature orthonectids are released prematurely by rupturing the body wall of the host, they swim in a nearly straight line. Males and females escaping naturally from their host, however, typically swim with a spiral motion.