[Hydromedusae] Photoreceptor found in some Hydroidomedusae, most common in Anthomedusae, usually situated on the marginal bulbs in abaxial or adaxial position. Exteriorly they appear as round, oblong, or elongated black, brown, yellow or red spots. They consist of a small mass or cupule of pigmented cells associated with nerve cells, with or without a lens. Ocelli are of ectodermal origin, except in the Tiaropsidae (see ecto-endodermal ocelli).