Orthonectidans: Orthonectida

Orthonectidans (or-thuh-NEK-tih-duhns) are parasites (PAIR-uh-sites) that live in the tissues of sea-dwelling invertebrates (in-VER-teh-brehts). A parasite is an animal or plant that lives in or on another animal or plant, called a host, without helping it and usually harming it. Invertebrates are animals without a backbone. Depending on the species of orthonectidan, either the sexes are separate or both male and female reproductive organs are present in the same animal. Adult orthonectidans are
0.002 to 0.031 inch (50 to 800 micrometers) long. The body of an adult orthonectidan consists of a jacket of body cells arranged in rings around an internal mass. Some of these body cells have hairlike fibers and some do not. Contracting muscle cells in the sex organ run in the lengthwise, circular, and oblique directions.